the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Oct 2021
Was there a glacial outburst flood in the Torngat Mountains during Marine Isotope Stage 3?
Abstract. We report previously unpublished evidence for a Marine Isotope Stage 3 (MIS 3; 60–26 ka) glacial outburst flood in the Torngat Mountains (northern Quebec/Labrador, Canada). We present 10Be cosmogenic exposure ages from legacy fieldwork for a glacial lake shoreline with evidence for outburst flooding in the Torngat Mountains, with a minimum age of 36 ± 3 ka (we consider the most likely age, corrected for burial, to be ~56 ± 3 ka). This shoreline position and age can potentially constrain the Laurentide Ice Sheet margin in the Torngat Mountains. This region, considered a site of glacial inception, has no published dated geologic constraints for high-elevation MIS 3 ice margins. We estimate the freshwater flux associated with the inferred glacial outburst flood using high-resolution digital elevation maps corrected for glacial isostatic adjustment. Using assumptions about the ice-dammed locations we find that a freshwater flood volume of 1.14 × 1012 m3 could have entered the Hudson Strait. This glacial outburst flood volume could have contributed to surface ocean freshening to cause a measurable meltwater signal in δ18O records, but would not necessarily have been associated with substantial ice rafted debris. Future work is required to refine estimates of the size and timing of such a glacial outburst flood. Nevertheless, we outline testable hypotheses about the Laurentide Ice Sheet and glacial outburst floods, including possible implications for Heinrich events and glacial inception in North America, that can be assessed with additional fieldwork and cosmogenic measurements.
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RC1: 'Comment on cp-2021-132', Anonymous Referee #1, 29 Oct 2021
The manuscript by Pico et al. reports evidence of a pre-LGM (Last Glacial Maximum), ice-dammed proglacial lake in the Torngat Mountains in Labrador, with two cosmogenic exposure ages of a wave-cut platform at the lake shoreline. The record, albeit incomplete, provides important evidence of ice-free conditions in the Torngat Mountains during MIS 3, which is contrary to many previous reconstructions of pre-LGM ice sheets and assumptions of the location of ice-sheet inception prior to the LGM. The data are worth publishing as they represent an important limit on ice-sheet extent prior to the LGM. The manuscript is written well, will be of broad interest in the paleoclimate community, and is appropriate for Climate of the Past. The following comments are suggestions for providing more context and perhaps more geologic information regarding the existence of the pre-LGM lake, which would help to motivate future work on the geologic record of the area as the authors repeatedly state is necessary.
While the geomorphology of the lake shoreline and the cosmogenic ages of the wave-cut platforms are well supported, the evidence of one or more outburst floods from the lake is relatively unclear in the manuscript. Most ice-dammed lakes are unstable and are known to drain and refill, the authors do not report any direct geologic evidence of outburst flooding. This may be a result of the subsequent glaciation in the area and the removal of any geomorphic or sedimentary evidence of outburst floods through lake outlets, which are more abundant (and perhaps better preserved) for ice-dammed lakes that existed in Canada during deglaciation. The authors should provide more geologic evidence for outburst floods from the lake, or a more detailed explanation of the why they believe the lake drained via outburst floods.
Finally, the volume of the lake should include some additional considerations of uncertainty given the unclear shape of the damming ice margins during MIS 3. The reported lake volume is based on some detailed consideration of the effects of isostatic rebound, but does not report the uncertainty of the lake volume calculation based on the presumed ice margin positions. This should be taken into account in paragraphs from lines 301-318 to more appropriately limit the potentially contribution to Heinrich Events during MIS 3.
Line by line comments:
Line 67: reference for the marine oxygen isotope data?
Line 82 (and elsewhere): should the names of glacial lakes be capitalized, as in Glacial Lake Koroc? They appear to considered proper nouns in other literature and are capitalized in the paragraph on lines 180-191.
Lines 129 and 130: use of “constraints” here and elsewhere. Editors may suggest replacing this with “limits”.
Lines 133: change “previous unpublished” to “new”?
Line 134: this is the first appearance of “pre-LGM glacial lake Koroc” (unless my PDF reader is mistaken). Two considerations: (1) add the name of the lake to the abstract, (2) perhaps give the lake a unique name? It appears to be distinct from the younger Glacial Lake Koroc, with a much higher shoreline elevation and presumably much deeper, and perhaps should have a unique name.
Figure 3 - The caption indicates that the boxed area with a white dashed outline is the assumed area of the lake, but it is unclear what the label “assumed glacial lake boundaries” means.
Lines 196-200: Is it possible to provide more geologic information about the lake shorelines? If so, considering showing a map with the location of the landforms and sediments representing flow into the lake, or a map that traces out the distribution of shoreline deposits and/or landforms representing the lake. The existence of the lake would be strengthened by more details about its record.
Lines 203-206: the relative location of the two samples is unclear. This should be explained here. Any photos of the sample sites would be worth including in the paper or in the supplement. The photos in Figure 4 show examples of the wave-cut benches but do not specify the sample locations.
Line 205: is the channel in Figure 4A an inlet or outlet channel? If an inlet, the authors should elaborate on how this channel and any associated deposits are indicative of an outburst flood.
Line 220: should the reference be “Willenbring and Staiger, 2005”?
Lines 232-234: More information is needed to justify the assumed duration of ice cover. It would be good to include at least a few sentences about the basis of the “apparent exposure age” given on lines 232-234. This is warranted given the potential uncertainty of the duration of ice cover, as it is the only limit on it. Additionally, the preservation of wave-cut platforms suggests cold-based ice at the sample site, but the authors do not explicitly state this. Consider doing this in the Methods paragraphs, as there appears to be sufficient evidence for it and it alleviates at least one concern about the exposure history of the site.
Line 260: change “presently” to “at present” or “currently”
Lines 255-268: Provide some estimate of uncertainty in the lake volume calculation given the unknown ice margin positions.
Line 301: This would be an appropriate place to add some explanation of why the lake drained as outburst floods (see general comment above).
Line 308: would it be possible to add to Figure 3A or 3B the flow direction of water when the lake drained?
Line 407, 409: it would be more appropriate to state “the first direct on-land evidence for a proglacial lake”, because the evidence of an outburst flood not reported in the paper.
Line 420: change “tease out” to “resolve”?
Citation: https://doi.org/10.5194/cp-2021-132-RC1 -
AC1: 'Reply on RC1', Tamara Pico, 10 Jan 2022
Reviewer 1
The manuscript by Pico et al. reports evidence of a pre-LGM (Last Glacial Maximum), ice-dammed proglacial lake in the Torngat Mountains in Labrador, with two cosmogenic exposure ages of a wave-cut platform at the lake shoreline. The record, albeit incomplete, provides important evidence of ice-free conditions in the Torngat Mountains during MIS 3, which is contrary to many previous reconstructions of pre-LGM ice sheets and assumptions of the location of ice-sheet inception prior to the LGM. The data are worth publishing as they represent an important limit on ice-sheet extent prior to the LGM. The manuscript is written well, will be of broad interest in the paleoclimate community, and is appropriate for Climate of the Past. The following comments are suggestions for providing more context and perhaps more geologic information regarding the existence of the pre-LGM lake, which would help to motivate future work on the geologic record of the area as the authors repeatedly state is necessary.
We thank the reviewer for these positive comments.
While the geomorphology of the lake shoreline and the cosmogenic ages of the wave-cut platforms are well supported, the evidence of one or more outburst floods from the lake is relatively unclear in the manuscript. Most ice-dammed lakes are unstable and are known to drain and refill, the authors do not report any direct geologic evidence of outburst flooding. This may be a result of the subsequent glaciation in the area and the removal of any geomorphic or sedimentary evidence of outburst floods through lake outlets, which are more abundant (and perhaps better preserved) for ice-dammed lakes that existed in Canada during deglaciation. The authors should provide more geologic evidence for outburst floods from the lake, or a more detailed explanation of the why they believe the lake drained via outburst floods.
We agree with the reviewer that the manuscript should be more specific about outburst flooding evidence to contextualize our findings for the readers.
In response to this comment and similar comments by Reviewer 2 we have revised the text to avoid overextending our data interpretation. We have replaced wording about “glacial outburst flooding” in the title and abstract with “glacial lake”, to be specific that our data supports evidence of a glacial lake more strongly than evidence for outburst flooding.
In addition, we have highlighted the evidence for outburst flooding by adding the following sentence:
“Evidence for outburst flooding is based on the rounded, imbricated cobbles in inlet channels. We found these deposits in multiple inlet channels leading to the sample site, which we interpreted as a lake shoreline. We hypothesize that the lake level may have fallen rapidly because there is no evidence for bands of lake shorelines at progressively lower elevations. Nevertheless, future work is required to verify such evidence for glacial outburst flooding”
Finally, the volume of the lake should include some additional considerations of uncertainty given the unclear shape of the damming ice margins during MIS 3. The reported lake volume is based on some detailed consideration of the effects of isostatic rebound, but does not report the uncertainty of the lake volume calculation based on the presumed ice margin positions. This should be taken into account in paragraphs from lines 301-318 to more appropriately limit the potentially contribution to Heinrich Events during MIS 3.
We agree with the reviewer that additional exploration of possible ice margins would contextualize the uncertainty on lake volume estimates for readers. In response to these comments, we performed additional calculations that vary ice margin location to explore the uncertainty on estimated lake volume.
The estimated total lake volume is highly sensitive to our choice of assumed ice margins. We shifted the assumed ice margin boundary northward from 58 N to 58.5 N (~56 km), and found that this reduced the estimated lake volume from 1.14x1012 m3 to 5.42x1011 m3 (reduced by ~50%). Shifting the assumed ice margin boundary northward from 58 N to 58.25 N (~28 km) reduces the estimated lake volume from 1.14x1012 m3 to 7.62x1011 m3 (reduced by ~30%). Shifting the assumed ice margin boundary southward from 58 N to 57.75 N increases the estimated lake volume 1.14x1012 m3 to 1.44x1012 m3.
To address the uncertainty on lake volume resulting from our ice margin assumptions, we will add the following text (line 318):
The total lake volume depends strongly on the assumed ice margin positions that bound the glacial lake. Given the uncertainty on the ice-dammed locations of this lake, we explored three possible scenarios varying the assumed southern position of the ice margin that bound pre-LGM Glacial Lake Koroc. For the intermediate scenario, the southern ice margin boundary is at 58 N, resulting in an estimated lake volume of 1.14x1012 m3. For the smaller lake scenario, we shift the assumed ice margin boundary northward from 58 N to 58.25 N (~28 km), which reduces the estimated lake volume from 1.14x1012 m3 to 7.62x1011 m3 (reduced by ~30%). For the larger lake scenario, shifting the assumed ice margin boundary southward from 58 N to 57.75 N increases the estimated lake volume 1.14x1012 m3 to 1.44x1012 m3 (increased by ~25%).
Line by line comments:
Line 67: reference for the marine oxygen isotope data?
Thank you for catching this. We now reference Channell et al., 2012 here.
Line 82 (and elsewhere): should the names of glacial lakes be capitalized, as in Glacial Lake Koroc? They appear to considered proper nouns in other literature and are capitalized in the paragraph on lines 180-191.
We have edited the text to read “Glacial Lake Koroc” throughout the manuscript.
Lines 129 and 130: use of “constraints” here and elsewhere. Editors may suggest replacing this with “limits”.
Done!
Lines 133: change “previous unpublished” to “new”?
We have left this wording as is since we are reporting legacy data (therefore not new).
Line 134: this is the first appearance of “pre-LGM glacial lake Koroc” (unless my PDF reader is mistaken). Two considerations: (1) add the name of the lake to the abstract, (2) perhaps give the lake a unique name? It appears to be distinct from the younger Glacial Lake Koroc, with a much higher shoreline elevation and presumably much deeper, and perhaps should have a unique name.
We appreciate this suggestion. We have edited the abstract to include the name “pre-LGM Glacial Lake Koroc”
Figure 3 - The caption indicates that the boxed area with a white dashed outline is the assumed area of the lake, but it is unclear what the label “assumed glacial lake boundaries” means.
We have edited this text to read “assumed location of ice margins bounding glacial lake” to clarify that these represent assumed ice margin limits.
Lines 196-200: Is it possible to provide more geologic information about the lake shorelines? If so, considering showing a map with the location of the landforms and sediments representing flow into the lake, or a map that traces out the distribution of shoreline deposits and/or landforms representing the lake. The existence of the lake would be strengthened by more details about its record.
Unfortunately, our study relies on legacy data from a 2003 field expedition, and we do not have data from additional shoreline sites. Attaining such information will be the focus of future field studies.
Lines 203-206: the relative location of the two samples is unclear. This should be explained here. Any photos of the sample sites would be worth including in the paper or in the supplement. The photos in Figure 4 show examples of the wave-cut benches but do not specify the sample locations.
The location of the two samples is at the site marked by the white circle in Figure 3. There is only one location, which is shown in Figure 4b.
We now include a schematic figure in Figure 4 that shows the sample location on this set of wave-cut benches.
We have added text to the manuscript to read “The two samples are at the same location shown in Figure 4C”
Line 205: is the channel in Figure 4A an inlet or outlet channel? If an inlet, the authors should elaborate on how this channel and any associated deposits are indicative of an outburst flood.
The channel in the forefront of Figure 4A on the interpreted wave-cut bench is an inlet channel leading into the glacial lake. The inlet channel would have served as a conduit for water to enter the glacial lake, which can be considered base level for the inlet. Therefore, the inlet channel itself is not indicative of an outburst flood.
We now note in the text:
Evidence for outburst flooding is based on the rounded, imbricated cobbles in inlet channels. We found these deposits in multiple inlet channels leading to the sample site, which we interpreted as a lake shoreline. We hypothesize that the lake level may have fallen rapidly because there is no evidence for bands of lake shorelines at progressively lower elevations. Nevertheless, future work is required to verify such evidence for glacial outburst flooding
Line 220: should the reference be “Willenbring and Staiger, 2005”?
No, the reference is Jane Willenbring’s PhD thesis. This is the same person (co-author of study).
Lines 232-234: More information is needed to justify the assumed duration of ice cover. It would be good to include at least a few sentences about the basis of the “apparent exposure age” given on lines 232-234. This is warranted given the potential uncertainty of the duration of ice cover, as it is the only limit on it. Additionally, the preservation of wave-cut platforms suggests cold-based ice at the sample site, but the authors do not explicitly state this. Consider doing this in the Methods paragraphs, as there appears to be sufficient evidence for it and it alleviates at least one concern about the exposure history of the site.
We will add the following text to the Methods section to note the likelihood of cold based ice and a discussion about the assumed duration of ice cover (line 235):
“The sample site was likely covered by cold-based ice for some duration (Staiger et al., 2005).”
Line 260: change “presently” to “at present” or “currently”
Done!
Lines 255-268: Provide some estimate of uncertainty in the lake volume calculation given the unknown ice margin positions.
We have now included a new calculation in response to these comments. We will add the following text:
The total lake volume depends strongly on the assumed ice margin positions that bound the glacial lake. Given the uncertainty on the ice-dammed locations of this lake, we explored three possible scenarios varying the assumed southern position of the ice margin that bound pre-LGM Glacial Lake Koroc. For the intermediate scenario, the southern ice margin boundary is at 58 N, resulting in an estimated lake volume of 1.14x1012 m3. For the smaller lake scenario, we shift the assumed ice margin boundary northward from 58 N to 58.25 N (~28 km), which reduces the estimated lake volume from 1.14x1012 m3 to 7.62x1011 m3 (reduced by ~30%). For the larger lake scenario, shifting the assumed ice margin boundary southward from 58 N to 57.75 N increases the estimated lake volume 1.14x1012 m3 to 1.44x1012 m3 (increased by ~25%).
Line 301: This would be an appropriate place to add some explanation of why the lake drained as outburst floods (see general comment above).
As discussed above, in response to this comment we have added the following text:
Evidence for outburst flooding is based on the rounded, imbricated cobbles in inlet channels. We found these deposits in multiple inlet channels leading to the sample site, which we interpreted as a lake shoreline. We hypothesize that the lake level may have fallen rapidly because there is no evidence for bands of lake shorelines at progressively lower elevations. Nevertheless, future work is required to verify such evidence for glacial outburst flooding
Line 308: would it be possible to add to Figure 3A or 3B the flow direction of water when the lake drained?
Thank you for this suggestion. We will add the potential flow route lines to this figure.
Line 407, 409: it would be more appropriate to state “the first direct on-land evidence for a proglacial lake”, because the evidence of an outburst flood not reported in the paper.
Yes, we agree with this suggestion. We will update this wording accordingly.
Line 420: change “tease out” to “resolve”?
Done!
Citation: https://doi.org/10.5194/cp-2021-132-AC1
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AC1: 'Reply on RC1', Tamara Pico, 10 Jan 2022
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CC1: 'Comment on cp-2021-132', Nicolas Young, 01 Nov 2021
Pico et al present an interesting manuscript that tackles two outstanding questions in Quaternary glacial geology and paleoclimatology: what were the dimensions of the Laurentide Ice Sheet during Marine Isotope Stage (MIS) 3 and what triggered Heinrich events? Pico et al suggest that the formation and sudden discharge of a proglacial ice-dammed lake in the Torngat Mountains (Labrador, Canada) sometime during MIS 3 could have served as a triggering mechanism for a MIS 3 Heinrich event(s). Indeed, the authors present an impressive reconstruction of the region’s paleotopography and potential paleolake dimensions and volume that hypothetically acted as a trigger for a MIS 3 Henrich event. This argument relies heavily on 1) field evidence for the existence of a paleo ice-dammed lake with wave action eroding bedrock platforms, and 2) the age of the lake dating to MIS 3 based on interpretation of two 10Be ages from a bedrock surface near the proposed shoreline. Based on our experience measuring 10Be in glaciated landscapes exhibiting weathering zones and spatial patterns of ice-sheet erosion, we think there are more likely, alternative interpretations of an ice-margin history that would result in the apparent 10Be ages reported by Pico et al.
10Be measurements in bedrock
The authors report two statistically indistinguishable 10Be ages from bedrock surfaces of 35.9 ± 1.1 ka and 36.0 ± 1.1 ka, although it is not clear whether the samples are from the arrows in Fig. 4 or elsewhere. The authors recognize that this landscape was likely covered by ice during the Last Glacial Maximum/MIS 2 and assume that the most recent episode of deglaciation occurred ca. 10 ka. The authors suggest that the apparent 10Be ages represent ~10 ka of recent surface exposure leading up to the year of sample collection, and ~25 ka of exposure (or exposure equivalent) occurred prior to the last deglaciation but fail to evaluate the range of possibilities for the exposure interval(s) represented by the residual 10Be inventory.
To make sense of the excess 10Be in these samples and to consider the sample locations’ full exposure/burial history, the authors assume that ice advanced over the sample sites at 30 ka and remained over these sites until 10 ka (20 kyr of ice cover). Therefore, there is an additional 20 ka of ice-cover history that is not reflected in the apparent 10Be ages (i.e., no nuclide production under ice). The authors use this 20-kyr interval of burial to suggest that the likely exposure age is 56 ka (36 ka measured + 20 ka assumed burial), which also implies the excess 10Be accumulated between 56 ka and 30 ka. In turn this “likely” age of 56 ka is used to date the drainage of an ice-dammed lake and further suggest that this region underwent an episode of deglaciation at this time.
We believe that alternative explanations are equally, if not more plausible.
The excess 10Be in the surfaces (the ~25 kyr worth of 10Be) could have accumulated in the samples just about any time in the last million or more years. Without 26Al data, we are essentially “blind” to when it may have accumulated. With 26Al data demonstrating no isotopic disequilibrium, one could then only reduce the time during which the 10Be could have accumulated to the last ~100-200 kyr. We also cannot know if the excess 10Be accumulated during a single episode of pre-LGM surface exposure, multiple episodes of pre-LGM surface exposure, single episode of subsurface isotope accumulation, or multiple episodes of subsurface isotope accumulation. Among all of these options, none of which are uniquely constrainable with available data, what support is there for the interpretation of a single period of MIS 3 exposure that the authors put forward? For the chosen interpretation to be supported, a fuller discussion and ruling out of alternative interpretations is required.
We offer two other possibilities that are more likely:
1) At 890 m asl in a landscape classified as “intermediate weathering,” studies show that bedrock surfaces almost always contain inheritance, usually 10s of kyr of inheritance in age equivalent terms. We did not see any text confronting – and ruling out – a likely scenario that wave action from a lake, or meltwater flow, during the last deglaciation stripped surficial debris off of the sampled outcrops leaving planar bedrock surfaces with inherited 10Be that lacks a unique temporal explanation. We think this is a likely explanation for the measured inventories.
2) We are unaware of studies where an apparent 10Be age that is known to be influenced by isotopic inheritance (as Pico et al show here) is interpreted literally because, again, there is no way to determine when exactly inherited 10Be accumulates in a rock sample (see above). Here, the authors are prescribing that the inherited 10Be accumulated between 56 ka and 30 ka. In fact, if the excess 10Be accumulated at any other time(s), then the paleo ice-dammed lake likely cannot be MIS 3 in age. There are, however, some reasonable exposure/burial histories in this region that would result in measurable amounts of isotopic inheritance in bedrock. An alternative scenario, in addition to the one we mention above, is that the currently unglaciated terrain where these bedrock samples were collected was also ice free during the Last Interglacial (MIS 5e) when temperatures likely exceeded those of the Holocene. In this scenario, bedrock exposure during MIS 5e would result in the accumulation of 10Be and during post-MIS 5e glaciation across the region, the Laurentide Ice Sheet did not erode through the required 2-3 m of bedrock needed to reset the cosmogenic clock. We think this is particularly likely in a region that was susceptible to non- or minimally erosive cold-based to polythermal ice; 20+ years of 10Be measurements in Canadian Arctic and sub-Arctic have revealed that bedrock is often influenced by isotopic inheritance. By Pico et al’s own conclusion, this region likely experienced times of cold-based ice and they invoke this phenomenon to explain the preservation of paleoshorelines. Thus, the measured 10Be inventory presented by the authors could simply reflect 10 ka of recent exposure combined with 10Be accumulated during MIS 5e or perhaps even leftover 10Be from even earlier interglacials (e.g., MIS 11), minus the amount of 10Be that was stripped from the bedrock as a result of the depth of subglacial erosion that occurred during post MIS 5e ice cover. Slightly more subglacial erosion (but not exceeding 2-3 m) during the interval of ice cover would result in younger apparent 10Be ages, while slightly less erosion would result in older apparent 10Be ages, but both end members would yield apparent 10Be ages influenced by isotopic inheritance.
It may be tempting to consider the statistically indistinguishable apparent 10Be ages presented here as evidence of a true exposure signal that are not influenced by erosion. To fully evaluate this possibility, however, full descriptions of the sample locations are needed; it is presently unclear where these bedrock samples are from (e.g., listed sample coordinates plot closer to Ungava Bay than the site shown in the accompanying figure, and only at 140 m asl). For example, if these samples are close to one another then the two bedrock surfaces almost certainly experienced the same long-term exposure-burial histories and therefore the similar apparent 10Be ages could be a function of residing under the same amount of till cover that was later stripped away (shielding; 10Be production at depth). Additionally, it is not unreasonable to suspect that similar 10Be ages are a function of the same magnitude of subglacial erosion through the 10Be production-depth profile. In this latter scenario, the full exposure-burial history of the sample locations would be greater than ~36 ka, but the excess 10Be that could potentially explain this full history has been eroded away.
The claim that this region, a suspected LIS inception point that therefore spends most of its history under ice, deglaciated during MIS 3 is a fairly extraordinary suggestion. We suggest this claim requires firm chronological constraints. Because it is highly likely this region, like today, was ice free during MIS 5e, we think the authors should provide compelling evidence for why the inherited 10Be is not simply a relic from MIS 5e (or earlier interglacials) that has been preserved under a minimally erosive Laurentide Ice Sheet.
Geomorphic evidence for a paleo ice-dammed lake
Considering the importance of this suspected paleo ice-dammed lake, the geomorphic evidence pointing to this lake’s existence needs to be better described. Pico et al provide one figure (Fig. 4) with two pictures meant to document paleo-shorelines attributed to this ice-dammed lake. We do not see anything definitive in these pictures that suggests the presence of a large ice-dammed lake nor clear evidence of wave-cut shorelines. In particular, in Figure 4b, there appears to be extensive till and/or regolith cover with a few bedrock surfaces nearby. Without further information, this appears to be till and/or regolith cover that was washed aside as meltwater flowed across the landscape during a period of deglaciation, not necessarily associated with an ice-dammed lake. To be clear, it is possible that a large ice-dammed lake at ~890 m asl existed, but the evidence for this needs to be more thoroughly documented.
Lastly, if there was a large ice-dammed lake and the 10Be samples are from wave-cut benches, the interpretation of the apparent 10Be ages and lake chronology presented by Pico et al would require that wave action on the edge of the lake remove 2-3 m of bedrock to reset the cosmogenic clock. This magnitude of wave-based erosion might be possible at the outlet channel hosting a catastrophic outburst flood, but 2-3 m of wave-cut erosion by an ephemeral ice-dammed lake into crystalline granitic terrain is unlikely. Moreover, if wave action only removed a few 10s of cm or ~1 meter, instead of 2-3 m, then the measured 10Be inventories reflect a component of 10Be produced at depth and the full exposure history of the bedrock surface is more, but this evidence has been eroded away. We note that the text speculates that there was outburst flooding, but the evidence provided for this is also consistent with a variety of smaller scale glaciofluvial processes. Rather than a large ice-dammed lake that suddenly drained, we wonder why this cannot be a case of terrain that was ice-covered during the LGM and during landscape deglaciation, the likely significant amounts of meltwater runoff simply washed away till and/or regolith exposing bare bedrock surfaces that contain 10Be from a previous period of exposure. Note this would be consistent with the alternative interpretation of the 10Be inventories that we mention above.
We accept that it is possible, although unlikely, that these bedrock surfaces were exposed during MIS 3. To support their interpretation, the authors would need to provide strong evidence as to why they prefer that inherited 10Be in their samples was from MIS 3 exposure instead of any number of alternative scenarios, including but not limited to some of those we discuss above.
Nicolás E. Young (Lamont-Doherty Earth Observatory, Columbia University)
Jason P. Briner (Dept. of Geology, University at Buffalo)
Gifford H. Miller (INSTAAR & Dept. of Geological Sciences, University of Colorado Boulder)
Citation: https://doi.org/10.5194/cp-2021-132-CC1 -
AC3: 'Reply on CC1', Tamara Pico, 10 Jan 2022
Community Comment # 1
Pico et al present an interesting manuscript that tackles two outstanding questions in Quaternary glacial geology and paleoclimatology: what were the dimensions of the Laurentide Ice Sheet during Marine Isotope Stage (MIS) 3 and what triggered Heinrich events? Pico et al suggest that the formation and sudden discharge of a proglacial ice-dammed lake in the Torngat Mountains (Labrador, Canada) sometime during MIS 3 could have served as a triggering mechanism for a MIS 3 Heinrich event(s). Indeed, the authors present an impressive reconstruction of the region’s paleotopography and potential paleolake dimensions and volume that hypothetically acted as a trigger for a MIS 3 Henrich event. This argument relies heavily on 1) field evidence for the existence of a paleo ice-dammed lake with wave action eroding bedrock platforms, and 2) the age of the lake dating to MIS 3 based on interpretation of two 10Be ages from a bedrock surface near the proposed shoreline. Based on our experience measuring 10Be in glaciated landscapes exhibiting weathering zones and spatial patterns of ice-sheet erosion, we think there are more likely, alternative interpretations of an ice-margin history that would result in the apparent 10Be ages reported by Pico et al.
We thank the authors of this comment for their positive comments on our manuscript and for their constructive feedback.
10Be measurements in bedrock
The authors report two statistically indistinguishable 10Be ages from bedrock surfaces of 35.9 ± 1.1 ka and 36.0 ± 1.1 ka, although it is not clear whether the samples are from the arrows in Fig. 4 or elsewhere. The authors recognize that this landscape was likely covered by ice during the Last Glacial Maximum/MIS 2 and assume that the most recent episode of deglaciation occurred ca. 10 ka. The authors suggest that the apparent 10Be ages represent ~10 ka of recent surface exposure leading up to the year of sample collection, and ~25 ka of exposure (or exposure equivalent) occurred prior to the last deglaciation but fail to evaluate the range of possibilities for the exposure interval(s) represented by the residual 10Be inventory.
To make sense of the excess 10Be in these samples and to consider the sample locations’ full exposure/burial history, the authors assume that ice advanced over the sample sites at 30 ka and remained over these sites until 10 ka (20 kyr of ice cover). Therefore, there is an additional 20 ka of ice-cover history that is not reflected in the apparent 10Be ages (i.e., no nuclide production under ice). The authors use this 20-kyr interval of burial to suggest that the likely exposure age is 56 ka (36 ka measured + 20 ka assumed burial), which also implies the excess 10Be accumulated between 56 ka and 30 ka. In turn this “likely” age of 56 ka is used to date the drainage of an ice-dammed lake and further suggest that this region underwent an episode of deglaciation at this time.
We believe that alternative explanations are equally, if not more plausible.
The excess 10Be in the surfaces (the ~25 kyr worth of 10Be) could have accumulated in the samples just about any time in the last million or more years. Without 26Al data, we are essentially “blind” to when it may have accumulated. With 26Al data demonstrating no isotopic disequilibrium, one could then only reduce the time during which the 10Be could have accumulated to the last ~100-200 kyr. We also cannot know if the excess 10Be accumulated during a single episode of pre-LGM surface exposure, multiple episodes of pre-LGM surface exposure, single episode of subsurface isotope accumulation, or multiple episodes of subsurface isotope accumulation. Among all of these options, none of which are uniquely constrainable with available data, what support is there for the interpretation of a single period of MIS 3 exposure that the authors put forward? For the chosen interpretation to be supported, a fuller discussion and ruling out of alternative interpretations is required.
We appreciate the perspectives raised in this comment and respond below to how we will incorporate such alternate interpretations in the revision of the manuscript.
We offer two other possibilities that are more likely:
1) At 890 m asl in a landscape classified as “intermediate weathering,” studies show that bedrock surfaces almost always contain inheritance, usually 10s of kyr of inheritance in age equivalent terms. We did not see any text confronting – and ruling out – a likely scenario that wave action from a lake, or meltwater flow, during the last deglaciation stripped surficial debris off of the sampled outcrops leaving planar bedrock surfaces with inherited 10Be that lacks a unique temporal explanation. We think this is a likely explanation for the measured inventories.
The authors of this comment are correct to note that we cannot rule out the possibility of till cover. This cover would make the ages older since the till would shield the bedrock surface.
We will add the following the text to the manuscript to note the possibility of till cover:
“Till cover would cause the age assigned to the glacial lake shoreline to be older”
2) We are unaware of studies where an apparent 10Be age that is known to be influenced by isotopic inheritance (as Pico et al show here) is interpreted literally because, again, there is no way to determine when exactly inherited 10Be accumulates in a rock sample (see above). Here, the authors are prescribing that the inherited 10Be accumulated between 56 ka and 30 ka. In fact, if the excess 10Be accumulated at any other time(s), then the paleo ice-dammed lake likely cannot be MIS 3 in age. There are, however, some reasonable exposure/burial histories in this region that would result in measurable amounts of isotopic inheritance in bedrock. An alternative scenario, in addition to the one we mention above, is that the currently unglaciated terrain where these bedrock samples were collected was also ice free during the Last Interglacial (MIS 5e) when temperatures likely exceeded those of the Holocene. In this scenario, bedrock exposure during MIS 5e would result in the accumulation of 10Be and during post-MIS 5e glaciation across the region, the Laurentide Ice Sheet did not erode through the required 2-3 m of bedrock needed to reset the cosmogenic clock. We think this is particularly likely in a region that was susceptible to non- or minimally erosive cold-based to polythermal ice; 20+ years of 10Be measurements in Canadian Arctic and sub-Arctic have revealed that bedrock is often influenced by isotopic inheritance. By Pico et al’s own conclusion, this region likely experienced times of cold-based ice and they invoke this phenomenon to explain the preservation of paleoshorelines. Thus, the measured 10Be inventory presented by the authors could simply reflect 10 ka of recent exposure combined with 10Be accumulated during MIS 5e or perhaps even leftover 10Be from even earlier interglacials (e.g., MIS 11), minus the amount of 10Be that was stripped from the bedrock as a result of the depth of subglacial erosion that occurred during post MIS 5e ice cover. Slightly more subglacial erosion (but not exceeding 2-3 m) during the interval of ice cover would result in younger apparent 10Be ages, while slightly less erosion would result in older apparent 10Be ages, but both end members would yield apparent 10Be ages influenced by isotopic inheritance.
It may be tempting to consider the statistically indistinguishable apparent 10Be ages presented here as evidence of a true exposure signal that are not influenced by erosion. To fully evaluate this possibility, however, full descriptions of the sample locations are needed; it is presently unclear where these bedrock samples are from (e.g., listed sample coordinates plot closer to Ungava Bay than the site shown in the accompanying figure, and only at 140 m asl). For example, if these samples are close to one another then the two bedrock surfaces almost certainly experienced the same long-term exposure-burial histories and therefore the similar apparent 10Be ages could be a function of residing under the same amount of till cover that was later stripped away (shielding; 10Be production at depth). Additionally, it is not unreasonable to suspect that similar 10Be ages are a function of the same magnitude of subglacial erosion through the 10Be production-depth profile. In this latter scenario, the full exposure-burial history of the sample locations would be greater than ~36 ka, but the excess 10Be that could potentially explain this full history has been eroded away.
The claim that this region, a suspected LIS inception point that therefore spends most of its history under ice, deglaciated during MIS 3 is a fairly extraordinary suggestion. We suggest this claim requires firm chronological constraints. Because it is highly likely this region, like today, was ice free during MIS 5e, we think the authors should provide compelling evidence for why the inherited 10Be is not simply a relic from MIS 5e (or earlier interglacials) that has been preserved under a minimally erosive Laurentide Ice Sheet.
We appreciate the comment author’s careful consideration of our manuscript and ideas for alternate hypotheses. We agree that there is considerable age uncertainty on the data presented in our study, and we hope that we have been clear that we view this dataset as preliminary evidence for future detailed fieldwork. Alternate hypotheses about the age of samples are certainly plausible. Such findings will be compelling in their own right since there is little data on pre-LGM glacial lakes in this region.
To note the possibility of alternate ages, we will include the following text on line 291:
Should an improved understanding of the ice burial history suggest a longer period of ice sheet cover, this glacial lake shoreline may be assigned older ages than those presented here. For example, depending on the history of ice cover and erosion, the lake shoreline may correspond to earlier periods of retracted ice such as MIS 5a or MIS 5e. Nevertheless, given the preservation of this lake shoreline feature, increasingly older ages are less likely.
Geomorphic evidence for a paleo ice-dammed lake
Considering the importance of this suspected paleo ice-dammed lake, the geomorphic evidence pointing to this lake’s existence needs to be better described. Pico et al provide one figure (Fig. 4) with two pictures meant to document paleo-shorelines attributed to this ice-dammed lake. We do not see anything definitive in these pictures that suggests the presence of a large ice-dammed lake nor clear evidence of wave-cut shorelines. In particular, in Figure 4b, there appears to be extensive till and/or regolith cover with a few bedrock surfaces nearby. Without further information, this appears to be till and/or regolith cover that was washed aside as meltwater flowed across the landscape during a period of deglaciation, not necessarily associated with an ice-dammed lake. To be clear, it is possible that a large ice-dammed lake at ~890 m asl existed, but the evidence for this needs to be more thoroughly documented.
The noted shoreline is not in line with any fracture plane, and because it is eroded into bedrock it cannot be a solifluction or gelifluction lobe. For other glacial erosion processes we would not expect to see such a planar surface. Therefore, we interpret this geomorphic feature as a shoreline platform.
Lastly, if there was a large ice-dammed lake and the 10Be samples are from wave-cut benches, the interpretation of the apparent 10Be ages and lake chronology presented by Pico et al would require that wave action on the edge of the lake remove 2-3 m of bedrock to reset the cosmogenic clock. This magnitude of wave-based erosion might be possible at the outlet channel hosting a catastrophic outburst flood, but 2-3 m of wave-cut erosion by an ephemeral ice-dammed lake into crystalline granitic terrain is unlikely. Moreover, if wave action only removed a few 10s of cm or ~1 meter, instead of 2-3 m, then the measured 10Be inventories reflect a component of 10Be produced at depth and the full exposure history of the bedrock surface is more, but this evidence has been eroded away. We note that the text speculates that there was outburst flooding, but the evidence provided for this is also consistent with a variety of smaller scale glaciofluvial processes. Rather than a large ice-dammed lake that suddenly drained, we wonder why this cannot be a case of terrain that was ice-covered during the LGM and during landscape deglaciation, the likely significant amounts of meltwater runoff simply washed away till and/or regolith exposing bare bedrock surfaces that contain 10Be from a previous period of exposure. Note this would be consistent with the alternative interpretation of the 10Be inventories that we mention above.
Wave-cut benches in granite do exist, for example, on Sharp Island, Hong Kong (Berry & Ruxton, 1957).
Although the bedrock in our study region is gneiss, it is highly fractured rock. Frost action and ice wedging acts on preexisting pervasive fractures, resulting in a more easily weathered rock. Studies based on glaciated gneiss terrains suggest that such fractured rock results in plucking-dominated erosion, which is likely for our study area (Krabbendam & Bradwell, 2014)
- Berry, B. P. Ruxton, Structure and Form of Cheung Chau Island, Hong Kong. Geol. Mag. XCIV (1957).
- Krabbendam, T. Bradwell, Quaternary evolution of glaciated gneiss terrains : pre-glacial weathering vs . glacial erosion. Quat. Sci. Rev. 95, 20–42 (2014).
We accept that it is possible, although unlikely, that these bedrock surfaces were exposed during MIS 3. To support their interpretation, the authors would need to provide strong evidence as to why they prefer that inherited 10Be in their samples was from MIS 3 exposure instead of any number of alternative scenarios, including but not limited to some of those we discuss above.
Our study relies on legacy data from a 2003 field expedition, and we do not have data from additional shoreline sites. We nevertheless pushed forward with publishing these data to draw attention to this work, and to highlight the exciting potential for several facets of Quaternary work, including constraining the configuration of pre-LGM ice sheets, understanding glacial inception, as well as quantifying the timing and amount of freshwater release into the North Atlantic. We have stressed in our current study that attaining such information will be the focus of future field studies.
We appreciate the ideas raised in this comment. In response, we will edit the manuscript to include discussion of some of the alternate hypotheses raised by the authors of this comment.
Citation: https://doi.org/10.5194/cp-2021-132-AC3
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AC3: 'Reply on CC1', Tamara Pico, 10 Jan 2022
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RC2: 'Comment on cp-2021-132', Anonymous Referee #2, 05 Nov 2021
Manuscript Number: cp-2021-132
Manuscript Type: Research Article
Title: Was there a glacial outburst flood in the Torngat Mountains during Marine Isotope Stage 3?
Authors: Tamara Pico, Jane Willenbring, April S. Dalton and Sidney HemmingComments to Authors:
Pico et al’s manuscript provides evidence of an outburst flood from Glacial Lake Koroc in the Torngat Mountain of northern Quebec and Labrador, Canada. Using two 10Be cosmogenic exposure ages they suggest this outburst occurred during MIS 3 and the flood volume could have contributed to surface ocean freshening and measurable meltwater signal in ð¿18O records. The manuscript is, for the most part, clearly written, and is well supported by figures. However, I have a number of concerns that need to be addressed before this manuscript is ready for publication.
1). The authors constrain the age of Glacial Lake Koroc using two 10Be cosmogenic exposure ages. My main concern is the small sample size and the possibility of inheritance in these samples. The presence of a cold-based ice sheet covering the study area is highlighted in the text (lines 293-296). It, therefore, seems equally plausible that the high elevation shorelines dated in this study could be one of the earliest stages of a post-LGM lake in the region and samples simply appear older due to inherited 10Be concentrations. The authors explore the possibility of erosion and burial of the sample site but no mention of inheritance in the samples is made.
2). More detail is needed regarding the geomorphic/sedimentological evidence for pre-LGM Glacial Lake Koroc. No details regarding the mechanism of failure and evidence for a spillway that drained the lake are discussed, despite ‘outburst flood’ being in the title. During LGM ice recession, multiple glacial lakes occupied the Torngat Mountains. Could this not have been the case for pre-LGM and if so, what is the likelihood that this lake didn’t drain into another lower elevation lake nearby? Is it likely that the whole lake drained during the outburst event? Without the elevation of the spillway, this cannot be assessed.
3). The main conclusion drawn in this study is the contribution an outburst flood from this lake could have had to surface ocean freshening and possible implications this may have had for Heinrich events.
‘A freshwater volume of 1.14x1012 m3, associated with the glacial lake outburst described in this
study could contribute to the large ð¿18O recorded for MIS 3 Heinrich events (minimum volume
required = 1.4x1013 -2.3 x1014 m3 ; Hemming, 2004).’
The minimum volume stated is from Hemming, 2004 is ‘a value assuming a volume the area of the Heinrich layers, and the thickness of the mixed layer is mixed one time with enough ice and water to make the d18O excursion’. However, my understanding of the paper is that 0.6 -1.9 Sv of water over 1 yr to 500 yrs is needed to explain the observed ð¿18O excursion. The estimate presented in this manuscript is 0.004x 106 over 3 days. This seems to be significantly less water than is needed to contribute to the ð¿18O excursion observed during a ~500 yrs of a typical Heinrich Event.
Line edits:
Line 24: Provide a value for the magnitude of freshwater flux in the abstract. Consider also adding the lake name to the abstract
Line 27: Present freshwater flood volume in km3 rather than m3
Line 123: Delete space
Figure 3: Please add the drainage route to panel B. It would also be useful to add the location of mapped shorelines to this figure
Line 206: Evidence for outlet needs to be clearly stated
Line 232/233: More information is needed regarding the duration of ice cover during the LGM. The uncertainty surrounding the 20 kyr ice cover is very briefly mentioned and needs to be more clearly stated
Line 232: Add space ‘concentration( Gosse and Phillips, 2001)’
Line 235: Add space ‘calculation(Jones et al., 2019)’
Line 230: Add a brief statement to highlight that this is a minimum age
Line 234: You describe the impact of GIA on your ages however no age GIA correct age is available to the reader. Consider adding these ages to the text and Table S2.
Line 206: ‘The shoreline is an erosional feature, and there is a small inlet channel at this elevation with rounded imbricated cobbles, suggestive of outburst flooding.’ Imbricated sediment can be produced by outburst events. However, is these are within a small inlet channel how do they suggest outburst flooding?
Line 263: Should glacial not be capitalized in ‘Pre-LGM glacial Lake Koroc”?
Supplementary Material Line 18: Table S2 seems to be identical to the table above
Citation: https://doi.org/10.5194/cp-2021-132-RC2 -
AC2: 'Reply on RC2', Tamara Pico, 10 Jan 2022
Reviewer 2
Pico et al’s manuscript provides evidence of an outburst flood from Glacial Lake Koroc in the Torngat Mountain of northern Quebec and Labrador, Canada. Using two 10Be cosmogenic exposure ages they suggest this outburst occurred during MIS 3 and the flood volume could have contributed to surface ocean freshening and measurable meltwater signal in ð¿18O records. The manuscript is, for the most part, clearly written, and is well supported by figures. However, I have a number of concerns that need to be addressed before this manuscript is ready for publication.
We thank the reviewer for this positive appraisal of our manuscript. We have addressed the concerns of Reviewer 2 below.
1). The authors constrain the age of Glacial Lake Koroc using two 10Be cosmogenic exposure ages. My main concern is the small sample size and the possibility of inheritance in these samples. The presence of a cold-based ice sheet covering the study area is highlighted in the text (lines 293-296). It, therefore, seems equally plausible that the high elevation shorelines dated in this study could be one of the earliest stages of a post-LGM lake in the region and samples simply appear older due to inherited 10Be concentrations. The authors explore the possibility of erosion and burial of the sample site but no mention of inheritance in the samples is made.
We agree that the possibility of inheritance is an important question to address in our manuscript for the reader. We will add the following text to the methods section of the manuscript:
The lowest elevation wave-cut bench was sampled to avoid possible inheritance of cosmogenic nuclides. We believe that substantial inheritance is unlikely because the sampled wave-cut bench is 3 meters below the highest observed platform elevation (Figure 4C). Because there is a relationship between cosmogenic nuclide concentration and depth, we can calculate the expected inheritance at 3 meters depth.
We use the following equation,
,
where N is cosmogenic nuclide concentration, x is depth below the surface, is rock density, and is attenuation length. We assume density is 2.9 g/cm3 and attenuation length is 165 cm.
If we consider the maximum inheritance expected at the surface in the Torngat Mountains region to be 5.7x106 atoms/g quartz (340 ky equivalent) (Staiger et al., 2005), the maximum concentration at 3 m depth below surface elevation should be near 2.8x104 atoms/g quartz (4.5 ky exposure). If we do not use the maximum concentration found but use a more realistic value found for similar altitudes 1x106 atoms/g (0.9 ky exposure), the possible inheritance is within our external age uncertainty of 3 ky.
We have added an additional figure to show the sampled location on the wave-cut platforms and benches, which will now be included in Figure 4.
2). More detail is needed regarding the geomorphic/sedimentological evidence for pre-LGM Glacial Lake Koroc. No details regarding the mechanism of failure and evidence for a spillway that drained the lake are discussed, despite ‘outburst flood’ being in the title. During LGM ice recession, multiple glacial lakes occupied the Torngat Mountains. Could this not have been the case for pre-LGM and if so, what is the likelihood that this lake didn’t drain into another lower elevation lake nearby? Is it likely that the whole lake drained during the outburst event? Without the elevation of the spillway, this cannot be assessed.
We agree with the Reviewer that more detail is needed regarding the geomorphic sedimentological evidence for pre-LGM Glacial Lake Koroc. However, our study relies on legacy data from a 2003 field expedition, and we do not have data from additional shoreline sites. Attaining such information will be the focus of future field studies.
We agree that this point should be clarified in the text. We have added the following text:
There is no evidence that the lakes drained to lower elevation regions, as no lower elevation shorelines were identified. However partial or complete draining of the lake to lower elevation locations is possible. Future work focused on obtaining sediment cores in the valley could allow us to learn whether multiple lake levels occurred.
In response to this comment, and similar comments by reviewer 1, we have changed the wording in the title to “glacial lake” instead of “outburst flood” to be more cautious in our interpretation of evidence.
3). The main conclusion drawn in this study is the contribution an outburst flood from this lake could have had to surface ocean freshening and possible implications this may have had for Heinrich events.
‘A freshwater volume of 1.14x1012 m3, associated with the glacial lake outburst described in this
study could contribute to the large ð¿18O recorded for MIS 3 Heinrich events (minimum volume
required = 1.4x1013 -2.3 x1014 m3 ; Hemming, 2004).’
The minimum volume stated is from Hemming, 2004 is ‘a value assuming a volume the area of the Heinrich layers, and the thickness of the mixed layer is mixed one time with enough ice and water to make the d18O excursion’. However, my understanding of the paper is that 0.6 -1.9 Sv of water over 1 yr to 500 yrs is needed to explain the observed ð¿18O excursion. The estimate presented in this manuscript is 0.004x 106 over 3 days. This seems to be significantly less water than is needed to contribute to the ð¿18O excursion observed during a ~500 yrs of a typical Heinrich Event.
In response to these comments, we will edit the text to include the following explanation: “Pre-LGM Glacial Lake Koroc represents one freshwater source, which, in addition to other freshwater sources from other glacial lakes that may have coexisted in the region, could constitute a substantial freshwater input to the ocean.”
Line edits:
Line 24: Provide a value for the magnitude of freshwater flux in the abstract. Consider also adding the lake name to the abstract
According to this suggestion and that of reviewer 1, we have edited the abstract to include the name “pre-LGM glacial lake Koroc”. We will also add the estimate of freshwater flux to the abstract.
Line 27: Present freshwater flood volume in km3 rather than m3
Done!
Line 123: Delete space
Done!
Figure 3: Please add the drainage route to panel B. It would also be useful to add the location of mapped shorelines to this figure
Thank you for this suggestion. We have added the potential drainage route to this figure. We also note that the mapped shoreline site is shown by the white circle in Figure 3.
Line 206: Evidence for outlet needs to be clearly stated
To clarify evidence for the glacial lake inlet we will add the following text:
Evidence for outburst flooding is based on the rounded, imbricated cobbles in inlet channels. We found these deposits in multiple inlet channels leading to the sample site, which we interpreted as a lake shoreline. We hypothesize that the lake level may have fallen rapidly because there is no evidence for bands of lake shorelines at progressively lower elevations. Nevertheless, future work is required to verify such evidence for glacial outburst flooding.
Line 232/233: More information is needed regarding the duration of ice cover during the LGM. The uncertainty surrounding the 20 kyr ice cover is very briefly mentioned and needs to be more clearly stated
The issue of ice cover duration is addressed on lines 281 to 284:
Although the duration of ice cover at this site is unknown, there is evidence for ice cover at the LGM (Staiger et al., 2005), and this region likely deglaciated between 11 and 8 ka (Dalton et al., 2020). The age correction for ice burial depends on the timing of glaciation, which occurred after the existence of pre-LGM glacial lake Koroc. For example, if ice advanced over our site at 30 ka and the entire area was glaciated until 10 ka, then there would have been 20 ky of ice cover, which would shift the age to 56 ka. We therefore consider 36 3 ka to represent a minimum age, and 56 3 ka a likely age for the identified pre-LGM glacial lake Koroc.
We have added the following sentence in include a recent publication reviewing possible ice margin configurations during MIS 3:
“Although poorly constrained, Dalton et al., 2022 suggest the Laurentide Ice Sheet margin may have been to the east of Ungava Bay in the Torngat Mountains during MIS 3 (Dalton et al., 2022).”
- S. Dalton, C. R. Stokes, C. L. Batchelor, Evolution of the Laurentide and Innuitian ice sheets prior to the Last Glacial Maximum ( 115 ka to 25 ka ). Earth-Science Rev. 224, 103875 (2022).
To flag the question of ice cover duration in the methods section, we will add the following text:
“Uncertainty on ice cover duration will impact sample age constraints (see Discussion)”
Line 232: Add space ‘concentration( Gosse and Phillips, 2001)’
Done!
Line 235: Add space ‘calculation(Jones et al., 2019)’
Done!
Line 230: Add a brief statement to highlight that this is a minimum age
We have incorporated this suggestion now by adding the following text to line 230: “and thus represent a minimum age”.
Line 234: You describe the impact of GIA on your ages however no age GIA correct age is available to the reader. Consider adding these ages to the text and Table S2.
In the text we have explained that we do not account for GIA in the age correction since it is smaller than the uncertainty (line 235).
Line 206: ‘The shoreline is an erosional feature, and there is a small inlet channel at this elevation with rounded imbricated cobbles, suggestive of outburst flooding.’ Imbricated sediment can be produced by outburst events. However, is these are within a small inlet channel how do they suggest outburst flooding?
The reviewer is correct that the rounded, imbricated cobbles are not necessarily indicative of outburst flooding. We found these deposits in multiple inlet channels leading to the sample site and at other parts of the discontinuous eroded notch, which we interpreted in the field as a lake shoreline. We hypothesize that the lake level fell quite rapidly because we don’t see bands of lake shorelines at progressively lower elevations. Indeed, it is somewhat hard to imagine a lake margin at 890 m elevation dammed from the ocean by ice ending in any way other than a geologically near-instantaneous event. It is possible that these were simply not preserved. However, in the field, we believed that we saw spillway features similar in morphology to those at lower elevations in the region (those at lower elevation date to the ~8-9 ka) lake draining events that have been documented by (Dube-Loubert et al., 2018).
We hope that this report inspires further investigation of the features in this important field area. Unfortunately, we ran out of time for further investigations in 2003 and don’t want the fact that these features exist and have curious ages to be lost to the glacial geologic history. This discussion paper serves the purpose of making these features known so that the community can investigate in the future.
Line 263: Should glacial not be capitalized in ‘Pre-LGM glacial Lake Koroc”?
We will now capitalize this term throughout the manuscript.
Supplementary Material Line 18: Table S2 seems to be identical to the table above
The top table in Table S2 is repeated from Table S1c to include this information alongside the location information.
Citation: https://doi.org/10.5194/cp-2021-132-AC2 - AC8: 'Reply on RC2', Tamara Pico, 13 Jan 2022
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AC2: 'Reply on RC2', Tamara Pico, 10 Jan 2022
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CC2: 'Comment on cp-2021-132', Martin Roy, 10 Nov 2021
The manuscript of Pico et al. presents evidence for a glacial lake that developed before the LGM in the Koroc River valley and adjoining basins, to the west of the Torngat Mountains (NE Quebec-Labrador). It is then hypothesized that the drainage of this lake into the Labrador Sea may have contributed to climate perturbations (Heinrich events) during Marine Isotope Stage 3 – an interval of the last glacial cycle for which the authors claim to be characterized by a significantly reduced ice cover in the region formerly covered by the Laurentide ice sheet (LIS).
As glacial geologists working since several years on the deglaciation of the greater Ungava Bay region, we were extremely enthusiastic about this new piece of information. Indeed, the Labrador-Ungava region is a key area for the study of the evolution of the LIS during the last glacial cycle. Previous work in this region has showed the occurrence of several ice-dammed lakes that developed during the last deglaciation and their location near a critical sector of the North Atlantic makes freshwater forcings from this region highly relevant to paleoclimate studies. It is also a remote area difficult to access and as such, many aspects of its geological record remain to be studied.
The conclusions drawn in this study are mainly based on the assumption that a glacial lake existed and that it drained during MIS 3. The geological evidence presented in support to this MIS 3 ice-dammed lake and flood are:
1) a wave-cut bench (c.f., shoreline) at an elevation of 890 m used to reconstruct a high-elevation ice-dammed lake in the Koroc River valley;
2) an inlet channel filled with cobbles recording the abrupt drainage of the lake;
3) two cosmogenic ages from bedrock surfaces exposed by lakeshore erosion indicating an age of ~36 ka (MIS 3).
It appears that these landforms and 10Be ages come from a single site (as suggested by the geographic coordinates in the Table). Also, no geological evidence is presented for the damming mechanism of the lake, which is based on the presumed occurrence of the LIS margin in the lower reach of the Koroc River and valley glaciers in its upper reach.
Careful examination of the manuscript and data therein, and consideration of available geological maps (see below), leads us to conclude that, at this stage, the geomorphological evidence and geochronological data reported are unfortunately too fragmentary to support the existence of this glacial lake and outburst flood during MIS 3. More importantly, we believe that the interpretation of the geomorphological features reported may be incorrect, while the proposed ice margin configuration for the LIS and valley glaciers appears inconsistent with the reduced ice cover scenario presented for MIS 3 in the study.
We further expand these points in the paragraphs below, in addition to comment on geochronological approach used.
Geomorphological evidence for a pre-LGM glacial Lake Koroc and outburst flood
The evidence supporting the existence of this glacial lake is essentially enclosed in the two photographs presented in Figure 4. In Fig. 4A, it is not clear to what geomorphological feature(s) the authors refer to when “interpolating” a former lake level running up high across the valley. Despite the low resolution of the photo, an apparent trim line can be seen and from this distance, it seems to separate permafrost terrains (frost-shattered bedrock and felsenmeer) present on the high mountain top from the lower freshly glacially-eroded valley walls. In the foreground of this photo, it is also difficult to identify the geomorphological feature supposed to represent a former shoreline, but the action of freeze-thaw processes on the bedrock and deposits is clearly present with slopes partly covered by solifluction lobes dissected by meltwater channels.
The photo in 4B is supposed to show a wave-cut bench and bedrock surfaces exposed by lakeshore erosion. However, the entire feature lacks the continuity that would normally characterize a shoreline. Based on this photo, this setting could be interpreted as a frost-shattered erosional bedrock surface (felsenmeer), while the presence of the smoother rock surfaces on the gently sloping terrain may reflect erosion (block removal) by meltwater. Erosion of Archean gneissic rocks by lakeshore wave-action would require a lot of time, which is generally not the case with the short-lived ice-dammed lakes of Ungava-Labrador.
Given the importance of this glacial lake in the study, and considering the associated implications for paleogeographic reconstructions, the manuscript should better document the characteristics of the shorelines reported. An unanswered question concerns the number of sites with shorelines that have been documented? The manuscript reports currently one, although it is mentioned that “this glacial lake was identified based on geomorphological mapping of shorelines, spillways, and drainage channels” (Lines199-200). Indeed, these features form key indicators for identifying a glacial lake and the results from this mapping should be clearly presented to constrain the areal extent of the lake and build a proper case for its existence. A glacial lake of this importance should have left several geomorphological evidence of its presence in the study area, even in the context of preservation by a non-erosive cold-based ice.
Similarly, the claim of the “first direct evidence on land for an MIS 3 jokulhaup” (Lines 363 and 407) can be questioned. This outburst flood was identified on the basis of “a small inlet channel at this elevation with rounded, imbricated cobbles” (c.f. Fig. 4). Again, the geological support for this outburst flood should be better documented and the feature reported here should be better detailed, especially regarding its spatial relationship with respect to the lake configuration (location in the study area?).
As mentioned in the manuscript, the drainage of glacial lakes through outburst floods occurred during the last deglaciation in the Ungava-Labrador region. The nearby Lake Naskaupi indeed drained through outburst flooding events that left behind a wealth of large-scale geomorphological deposits and landforms, such as km-size ripples and other features like multi-tens of meters thick alluvial bars (Dubé-Loubert and Roy, 2017; Dubé-Loubert et al., 2018). Even if we assume that part of these features was eroded by the ice advance of the last glaciation (MIS 2), evidence for the outburst drainage of a lake the size of the hypothetical pre-LGM Lake Koroc should still be present in the geomorphological and sedimentary records of this region.
We note that insights on whether a pre-LGM glacial lake existed in the Koroc River valley may gained in part from two detailed Quaternary geology maps produced by the Geological Survey of Canada that cover the whole Koroc River drainage basin (Paradis and Parent, 2002a, 2002b). These maps do show evidence for glacial lake(s) at lower elevations in the Koroc River valley, but these glacial lake deposits and landforms (deltas and shorelines) are part of a morpho-sedimentary assemblage typical of the early Holocene deglaciation. Small and scarce occurrences of glacial lake deposits are found in places at higher elevations, but they likely relate to ephemeral lakes that were dammed by small retreating glacier tongues during the last deglaciation, as indicated by belts of minor moraine crests in these minor valleys. The maps also show that the high-elevation terrains of the study area consist essentially of bedrock exposures that are commonly blanketed and flanked by block fields (felsenmeer) produced by frost action, with abundant solifluction lobes – a setting reminiscent of the features presented in the photos of Figure 4. Given the ease of access to these maps (references below), this critical information should have been taken into consideration when attempting to reconstruct such a lake.
Configuration of the ice masses damming the lake
Accepting the premise that a pre-LGM glacial lake occupied the Koroc River valley also raises other important questions regarding the configuration and origin of the ice masses that dammed the lake at that time. Given the lack of geological data constraining ice-margin positions during MIS 3 in Labrador-Ungava, the authors use a series of assumptions about the ice dam locations.
Some of these are coherent with paleogeographic reconstructions of the last deglaciation, when the lower reach of the rivers flowing into southeastern Ungava Bay was dammed by the margin of the retreating Labrador Ice Dome – a damming mechanism that resulted in the accumulation of large amount of meltwater in river valleys such as the Koroc River. Although this scenario is consistent with the last deglaciation, it nonetheless involves a substantial ice mass located in north-central Quebec in order to have an ice margin extending down into the Ungava Bay lowlands. This ice margin would also have to be very thick and voluminous to hold the large meltwater body (high hydrostatic pressure) associated with a pre-LGM Lake Koroc with a surface-elevation at ~900 m. These considerations appear counterintuitive in the context of a drastically reduced ice cover for MIS 3 that is presented in the manuscript.
Damming a high-elevation glacial lake (~900 m) in this region also requires the presence of ice masses in the upper reaches of the Koroc River drainage watershed, without which, the lake would drain via different low-elevation outlets. To circumvent this problem, it is stipulated that several valley glaciers occupied low-lying topographic depressions to prevent excess meltwater to flow eastward across the continental divide separating the Ungava Bay and Labrador Sea watersheds. Unfortunately, no geological evidence is presented to support the existence of these valley glaciers. The manuscript refers to the work of Ives to support the existence of these valley glaciers in the Torngats, although it does not relate to old valley glaciations. Accordingly, the manuscript should build a better case for this aspect of the reconstruction.
Again, drawing comparisons based on the geological maps reported above, the deglacial (Holocene) geomorphology of the numerous secondary valleys of this region shows a record of small-scale moraines and ice-contact deposits that tend to suggest that these valley glaciers were perhaps not large enough to hold such a large lake. For these valley glaciers to be efficient for the damming of the pre-LGM Lake Koroc would imply larger valley glaciers and ice caps, which in turn calls for a large amount of ice above the Torngats at that time – a scenario inconsistent with the hypothesis of a significantly reduced ice cover presented for this time interval.
Geochronological constraints
An important limitation regarding the conclusions reached by the study relates to the fact the MIS 3 age assigned to the old glacial Lake Koroc is based on only two cosmogenic ages. This represents an extremely low number of geochronological constraints considering the method employed here and given the complexity and variety of terrains present in formerly glaciated environments.
For instance, several studies have documented complex arrangements of subglacial thermal conditions in the Labrador-Ungava region, which show evidence that high-elevation regions were likely characterized by at least a partial cold-based ice cover (Marquette et al., 2004; Staiger et al., 2005; Rice et al., JQS 2019; Dubé-Loubert et al., Geomorph. 2021). Although this provides a setting for a non-erosive ice cover and thus the preservation of old geomorphological features, it does warrant caution in the interpretation of cosmogenic ages, which may be affected by inheritance signal from previous exposures and result with age overestimates.
However, the manuscript presents a different interpretation for the two pre-deglaciation 10Be ages obtained. The favored age of ~56 ± 3 ka derives from a correction applied to the two samples to take into account a period of burial by ice; a plausible choice of calculation and consistent with the hypothesis presented in the study. Still, considering the inheritance problem inherent to high elevation terrains of this region, it would have been essential to carry out paired isotope (Al/Be) measurements to characterize the history of ice cover of this site – an approach that would greatly facilitate the interpretation of the results. In addition, as for the geomorphological evidence for the lake, it would have been important to present 10Be ages from more than one site in order to support the deductions made on the basis of the two geochronological constraints.
Summary
We believe the manuscript presents an interesting hypothesis regarding the possibility of a pre-LGM glacial lake to the east of the Torngat Mountains and the potential contribution of its drainage to the forcing mechanisms responsible for the climate variability (Heinrich events) of MIS 3. Unfortunately, the geomorphological evidence and geochronological data supporting the existence of a glacial Lake Koroc during MIS 3 are too fragmentary and unconvincing at the moment, while the proposed configuration of the damming LIS margins and valley glaciers appears inconsistent with the scenario of significantly reduced ice cover presented. As stated at numerous places in the manuscript, we concur with the authors that further fieldwork is required to convincingly show that old (relict) geomorphological features are present on high-elevation terrains of Labrador-Ungava. At this point we would simply urge caution in using such a small amount of geological evidence to claim the existence of a glacial lake and outburst flood, especially when using the reconstruction to constrain the ice margin of the Laurentide ice sheet during MIS 3 in one of its core sectors.
Martin Roy – Department of Earth and atmospheric sciences / Geotop Research Center, University of Quebec at Montreal, Montréal (QC), Canada.
Hugo Dubé-Loubert – Quebec Ministry of Energy and Natural Resources, Val d’Or (QC), Canada.
References for the maps by the Geological Survey of Canada.
Quaternary geology of the Koroc River in Quebec-Newfoundland and Labrador (eastern and western map sheets 2013A and 2013A, scale 1:125,000).
Paradis, SJ and Parent, M, 2002. Géologie des formations en surface, Rivière Koroc (moitié est), Québec-Terre-Neuve-et-Labrador, Commission Géologique du Canada, Carte 2013A, échelle 1 :125,000. (available on https://geoscan.nrcan.gc.ca)
Paradis, SJ and Parent, M, 2002. Géologie des formations en surface, Rivière Koroc (moitié ouest), Québec-Terre-Neuve-et-Labrador, Commission Géologique du Canada, Carte 2014A, échelle 1 :125,000). (available on https://geoscan.nrcan.gc.ca)
Citation: https://doi.org/10.5194/cp-2021-132-CC2 -
AC4: 'Reply on CC2', Tamara Pico, 10 Jan 2022
Community Comment # 2
The manuscript of Pico et al. presents evidence for a glacial lake that developed before the LGM in the Koroc River valley and adjoining basins, to the west of the Torngat Mountains (NE Quebec-Labrador). It is then hypothesized that the drainage of this lake into the Labrador Sea may have contributed to climate perturbations (Heinrich events) during Marine Isotope Stage 3 – an interval of the last glacial cycle for which the authors claim to be characterized by a significantly reduced ice cover in the region formerly covered by the Laurentide ice sheet (LIS).
As glacial geologists working since several years on the deglaciation of the greater Ungava Bay region, we were extremely enthusiastic about this new piece of information. Indeed, the Labrador-Ungava region is a key area for the study of the evolution of the LIS during the last glacial cycle. Previous work in this region has showed the occurrence of several ice-dammed lakes that developed during the last deglaciation and their location near a critical sector of the North Atlantic makes freshwater forcings from this region highly relevant to paleoclimate studies. It is also a remote area difficult to access and as such, many aspects of its geological record remain to be studied.
The conclusions drawn in this study are mainly based on the assumption that a glacial lake existed and that it drained during MIS 3. The geological evidence presented in support to this MIS 3 ice-dammed lake and flood are:
1) a wave-cut bench (c.f., shoreline) at an elevation of 890 m used to reconstruct a high-elevation ice-dammed lake in the Koroc River valley;
2) an inlet channel filled with cobbles recording the abrupt drainage of the lake;
3) two cosmogenic ages from bedrock surfaces exposed by lakeshore erosion indicating an age of ~36 ka (MIS 3).
It appears that these landforms and 10Be ages come from a single site (as suggested by the geographic coordinates in the Table). Also, no geological evidence is presented for the damming mechanism of the lake, which is based on the presumed occurrence of the LIS margin in the lower reach of the Koroc River and valley glaciers in its upper reach.
Careful examination of the manuscript and data therein, and consideration of available geological maps (see below), leads us to conclude that, at this stage, the geomorphological evidence and geochronological data reported are unfortunately too fragmentary to support the existence of this glacial lake and outburst flood during MIS 3. More importantly, we believe that the interpretation of the geomorphological features reported may be incorrect, while the proposed ice margin configuration for the LIS and valley glaciers appears inconsistent with the reduced ice cover scenario presented for MIS 3 in the study.
We further expand these points in the paragraphs below, in addition to comment on geochronological approach used.
We thank the comment’s authors for their constructive feedback on our manuscript.
Geomorphological evidence for a pre-LGM glacial Lake Koroc and outburst flood
The evidence supporting the existence of this glacial lake is essentially enclosed in the two photographs presented in Figure 4. In Fig. 4A, it is not clear to what geomorphological feature(s) the authors refer to when “interpolating” a former lake level running up high across the valley. Despite the low resolution of the photo, an apparent trim line can be seen and from this distance, it seems to separate permafrost terrains (frost-shattered bedrock and felsenmeer) present on the high mountain top from the lower freshly glacially-eroded valley walls. In the foreground of this photo, it is also difficult to identify the geomorphological feature supposed to represent a former shoreline, but the action of freeze-thaw processes on the bedrock and deposits is clearly present with slopes partly covered by solifluction lobes dissected by meltwater channels.
There are solifluction lobes in this region, however these tend to be present in the valleys where there is abundant unconsolidated material. These are not present in the forefront of the photo of the wave-cut shoreline. The noted shoreline is not in line with any fracture plane, and because it is eroded into bedrock it cannot be a solifluction or gelifluction lobe. For other glacial erosion processes we would not expect to see such a planar surface. Therefore, we interpret this geomorphic feature as a shoreline platform.
The photo in 4B is supposed to show a wave-cut bench and bedrock surfaces exposed by lakeshore erosion. However, the entire feature lacks the continuity that would normally characterize a shoreline. Based on this photo, this setting could be interpreted as a frost-shattered erosional bedrock surface (felsenmeer), while the presence of the smoother rock surfaces on the gently sloping terrain may reflect erosion (block removal) by meltwater. Erosion of Archean gneissic rocks by lakeshore wave-action would require a lot of time, which is generally not the case with the short-lived ice-dammed lakes of Ungava-Labrador.
Although the bedrock in our study region is gneiss, it is highly fractured rock. Frost action and ice wedging acts on preexisting pervasive fractures, resulting in a more easily weathered rock. Studies based on glaciated gneiss terrains suggest that such fractured rock results in plucking-dominated erosion, which is likely for our study area (Krabbendam & Bradwell, 2014)
- Krabbendam, T. Bradwell, Quaternary evolution of glaciated gneiss terrains : pre-glacial weathering vs . glacial erosion. Quat. Sci. Rev. 95, 20–42 (2014).
Given the importance of this glacial lake in the study, and considering the associated implications for paleogeographic reconstructions, the manuscript should better document the characteristics of the shorelines reported. An unanswered question concerns the number of sites with shorelines that have been documented? The manuscript reports currently one, although it is mentioned that “this glacial lake was identified based on geomorphological mapping of shorelines, spillways, and drainage channels” (Lines199-200). Indeed, these features form key indicators for identifying a glacial lake and the results from this mapping should be clearly presented to constrain the areal extent of the lake and build a proper case for its existence. A glacial lake of this importance should have left several geomorphological evidence of its presence in the study area, even in the context of preservation by a non-erosive cold-based ice.
Future fieldwork should be aimed at assessing the areal extent of this lake. We agree that there are likely other localities where this glacial lake shoreline can be documented, and such investigation will be the subject of future inquiry.
Similarly, the claim of the “first direct evidence on land for an MIS 3 jokulhaup” (Lines 363 and 407) can be questioned. This outburst flood was identified on the basis of “a small inlet channel at this elevation with rounded, imbricated cobbles” (c.f. Fig. 4). Again, the geological support for this outburst flood should be better documented and the feature reported here should be better detailed, especially regarding its spatial relationship with respect to the lake configuration (location in the study area?).
In response to this comment, and similar comments by reviewers, we will edit the text to emphasize evidence for a glacial lake shoreline, rather than an outburst flood.
As mentioned in the manuscript, the drainage of glacial lakes through outburst floods occurred during the last deglaciation in the Ungava-Labrador region. The nearby Lake Naskaupi indeed drained through outburst flooding events that left behind a wealth of large-scale geomorphological deposits and landforms, such as km-size ripples and other features like multi-tens of meters thick alluvial bars (Dubé-Loubert and Roy, 2017; Dubé-Loubert et al., 2018). Even if we assume that part of these features was eroded by the ice advance of the last glaciation (MIS 2), evidence for the outburst drainage of a lake the size of the hypothetical pre-LGM Lake Koroc should still be present in the geomorphological and sedimentary records of this region.
Future work could focus on assessing the nature of pre-LGM glacial lakes and potential outburst floods in this region. For example, sediment cores sampling the Koroc valley may provide clues for the history of past lake levels.
In response to this comment and that of Reviewer 1, we have added the following text:
There is no evidence that the lakes drained to lower elevation regions, as no lower elevation shorelines were identified. However partial or complete draining of the lake to lower elevation locations is possible. Future work focused on obtaining sediment cores in the valley could allow us to learn whether multiple lake levels occurred.
We note that insights on whether a pre-LGM glacial lake existed in the Koroc River valley may gained in part from two detailed Quaternary geology maps produced by the Geological Survey of Canada that cover the whole Koroc River drainage basin (Paradis and Parent, 2002a, 2002b). These maps do show evidence for glacial lake(s) at lower elevations in the Koroc River valley, but these glacial lake deposits and landforms (deltas and shorelines) are part of a morpho-sedimentary assemblage typical of the early Holocene deglaciation. Small and scarce occurrences of glacial lake deposits are found in places at higher elevations, but they likely relate to ephemeral lakes that were dammed by small retreating glacier tongues during the last deglaciation, as indicated by belts of minor moraine crests in these minor valleys. The maps also show that the high-elevation terrains of the study area consist essentially of bedrock exposures that are commonly blanketed and flanked by block fields (felsenmeer) produced by frost action, with abundant solifluction lobes – a setting reminiscent of the features presented in the photos of Figure 4. Given the ease of access to these maps (references below), this critical information should have been taken into consideration when attempting to reconstruct such a lake.
We are aware of these geologic maps, and these maps were available for earlier studies in this region conducted by one of the coauthors. For these higher elevation regions, where the study site is located, these geologic maps are largely based on analyzing airphotos. The features are hard to see in airphotos. Ground truthing through field mapping could help to better document these features in published geologic maps.
Configuration of the ice masses damming the lake
Accepting the premise that a pre-LGM glacial lake occupied the Koroc River valley also raises other important questions regarding the configuration and origin of the ice masses that dammed the lake at that time. Given the lack of geological data constraining ice-margin positions during MIS 3 in Labrador-Ungava, the authors use a series of assumptions about the ice dam locations.
Some of these are coherent with paleogeographic reconstructions of the last deglaciation, when the lower reach of the rivers flowing into southeastern Ungava Bay was dammed by the margin of the retreating Labrador Ice Dome – a damming mechanism that resulted in the accumulation of large amount of meltwater in river valleys such as the Koroc River. Although this scenario is consistent with the last deglaciation, it nonetheless involves a substantial ice mass located in north-central Quebec in order to have an ice margin extending down into the Ungava Bay lowlands. This ice margin would also have to be very thick and voluminous to hold the large meltwater body (high hydrostatic pressure) associated with a pre-LGM Lake Koroc with a surface-elevation at ~900 m. These considerations appear counterintuitive in the context of a drastically reduced ice cover for MIS 3 that is presented in the manuscript.
Damming a high-elevation glacial lake (~900 m) in this region also requires the presence of ice masses in the upper reaches of the Koroc River drainage watershed, without which, the lake would drain via different low-elevation outlets. To circumvent this problem, it is stipulated that several valley glaciers occupied low-lying topographic depressions to prevent excess meltwater to flow eastward across the continental divide separating the Ungava Bay and Labrador Sea watersheds. Unfortunately, no geological evidence is presented to support the existence of these valley glaciers. The manuscript refers to the work of Ives to support the existence of these valley glaciers in the Torngats, although it does not relate to old valley glaciations. Accordingly, the manuscript should build a better case for this aspect of the reconstruction.
Again, drawing comparisons based on the geological maps reported above, the deglacial (Holocene) geomorphology of the numerous secondary valleys of this region shows a record of small-scale moraines and ice-contact deposits that tend to suggest that these valley glaciers were perhaps not large enough to hold such a large lake. For these valley glaciers to be efficient for the damming of the pre-LGM Lake Koroc would imply larger valley glaciers and ice caps, which in turn calls for a large amount of ice above the Torngats at that time – a scenario inconsistent with the hypothesis of a significantly reduced ice cover presented for this time interval.
We agree with the comment authors that MIS 3 ice margin locations in this region are poorly constrained. It is plausible that evidence from pre-LGM glacial lakes in this region could appear quite differently in the geological record compared with deglacial lake records. Unlike pre-LGM features, deglacial geomorphic features are more likely to be preserved as they would not have been eroded by advancing and retreating ice cover.
While we do not report evidence for ice margin locations damming pre-LGM Glacial Lake Koroc, future fieldwork could be tasked with mapping out the extent of pre-LGM glacial lake shorelines in this region, which would help bound the ice-dammed locations of this lake.
Since we are unable to make a stronger case for the existence of valley glaciations (or other ice margin locations) barring additional field data collection, we make assumptions for the locations of ice-dammed glacial lake boundaries.
In response to these comments, and those by Reviewer 1, we performed additional calculations that vary ice margin location to explore the uncertainty on estimated lake volume, and we will add the following text:
The total lake volume depends strongly on the assumed ice margin positions that bound the glacial lake. Given the uncertainty on the ice-dammed locations of this lake, we explored three possible scenarios varying the assumed southern position of the ice margin that bound pre-LGM Glacial Lake Koroc. For the intermediate scenario, the southern ice margin boundary is at 58 N, resulting in an estimated lake volume of 1.14x1012 m3. For the smaller lake scenario, we shift the assumed ice margin boundary northward from 58 N to 58.25 N (~28 km), which reduces the estimated lake volume from 1.14x1012 m3 to 7.62x1011 m3 (reduced by ~30%). For the larger lake scenario, shifting the assumed ice margin boundary southward from 58 N to 57.75 N increases the estimated lake volume 1.14x1012 m3 to 1.44x1012 m3 (increased by ~25%).
Geochronological constraints
An important limitation regarding the conclusions reached by the study relates to the fact the MIS 3 age assigned to the old glacial Lake Koroc is based on only two cosmogenic ages. This represents an extremely low number of geochronological constraints considering the method employed here and given the complexity and variety of terrains present in formerly glaciated environments.
For instance, several studies have documented complex arrangements of subglacial thermal conditions in the Labrador-Ungava region, which show evidence that high-elevation regions were likely characterized by at least a partial cold-based ice cover (Marquette et al., 2004; Staiger et al., 2005; Rice et al., JQS 2019; Dubé-Loubert et al., Geomorph. 2021). Although this provides a setting for a non-erosive ice cover and thus the preservation of old geomorphological features, it does warrant caution in the interpretation of cosmogenic ages, which may be affected by inheritance signal from previous exposures and result with age overestimates.
However, the manuscript presents a different interpretation for the two pre-deglaciation 10Be ages obtained. The favored age of ~56 ± 3 ka derives from a correction applied to the two samples to take into account a period of burial by ice; a plausible choice of calculation and consistent with the hypothesis presented in the study. Still, considering the inheritance problem inherent to high elevation terrains of this region, it would have been essential to carry out paired isotope (Al/Be) measurements to characterize the history of ice cover of this site – an approach that would greatly facilitate the interpretation of the results. In addition, as for the geomorphological evidence for the lake, it would have been important to present 10Be ages from more than one site in order to support the deductions made on the basis of the two geochronological constraints.
We are in full agreement with the authors of this comment that an accurate age for this glacial lake shoreline feature will require future analysis that incorporates paired cosmogenic isotope systems to derive a burial history for this site.
Summary
We believe the manuscript presents an interesting hypothesis regarding the possibility of a pre-LGM glacial lake to the east of the Torngat Mountains and the potential contribution of its drainage to the forcing mechanisms responsible for the climate variability (Heinrich events) of MIS 3. Unfortunately, the geomorphological evidence and geochronological data supporting the existence of a glacial Lake Koroc during MIS 3 are too fragmentary and unconvincing at the moment, while the proposed configuration of the damming LIS margins and valley glaciers appears inconsistent with the scenario of significantly reduced ice cover presented. As stated at numerous places in the manuscript, we concur with the authors that further fieldwork is required to convincingly show that old (relict) geomorphological features are present on high-elevation terrains of Labrador-Ungava. At this point we would simply urge caution in using such a small amount of geological evidence to claim the existence of a glacial lake and outburst flood, especially when using the reconstruction to constrain the ice margin of the Laurentide ice sheet during MIS 3 in one of its core sectors.
We thank the authors of this comment for their positive remarks and constructive comments. We hope that this report inspires further investigation of the features in this important field area. Unfortunately, we ran out of time for further investigations in 2003 and don’t want the fact that these features exist to be lost to the glacial geologic history. This discussion paper serves the purpose of making these features known so that the community can investigate in the future.
Citation: https://doi.org/10.5194/cp-2021-132-AC4
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AC4: 'Reply on CC2', Tamara Pico, 10 Jan 2022
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CC3: 'Comment on cp-2021-132', Jessey Rice, 24 Nov 2021
Pico et al. present an interesting discussion regarding a glacial lake reconstruction and outburst flood they attribute to a pre-LGM (MIS3) period. This reconstruction is supported by 10Be age calcualations from a bedrock upland and glacial lake volume calculations. As previous commenters have mentioned this region is of critical importance for freshwater forcing and represents a region where many outstanding questions regarding ice sheet dynamics and deglaciation persist.
As other commenters have presented detailed disucssions, I will keep my comment brief, however, I do share the concerns of the previous commenters and support the issues they have highlighted. Specifically, there needs to be more evidence to convince the reader that the wave-cut bench shorelines are indeed shorelines and that the rounded, imbricated cobbles are indeed the result of an outburst flood and not simply from subglacial meltwater flow, which could have removed any till cover on the bedrock (i.e., a vanished protector: Veillette and Roy, 1995; https://doi.org/10.4095/202923) which would affect the inheritance of the bedrock sample. Additionally, as previously mentioned, a more discussion on how the glacial lake was reconstructed and possible inheritance from the 10Be should be explored as other suggested hypotheses seem plausible.
I can appreciate the limited dataset available to the authors, which they fully acknowledge, calling for additional fieldwork in the region. However, until additional data is available, either other hypotheses should be explored, or additional discussions to address the issues presented by other commenters is needed. Again, I fully appreciate the difficulty of this type of investigation and I do hope this discussion sparks future work in this remote region of Canada where little fieldwork has been conducted.
Jessey Rice- Geological Survey of Canada, Ottawa
Citation: https://doi.org/10.5194/cp-2021-132-CC3 -
AC5: 'Reply on CC3', Tamara Pico, 10 Jan 2022
Community Comment # 3
Pico et al. present an interesting discussion regarding a glacial lake reconstruction and outburst flood they attribute to a pre-LGM (MIS3) period. This reconstruction is supported by 10Be age calculations from a bedrock upland and glacial lake volume calculations. As previous commenters have mentioned this region is of critical importance for freshwater forcing and represents a region where many outstanding questions regarding ice sheet dynamics and deglaciation persist.
As other commenters have presented detailed disucssions, I will keep my comment brief, however, I do share the concerns of the previous commenters and support the issues they have highlighted. Specifically, there needs to be more evidence to convince the reader that the wave-cut bench shorelines are indeed shorelines and that the rounded, imbricated cobbles are indeed the result of an outburst flood and not simply from subglacial meltwater flow, which could have removed any till cover on the bedrock (i.e., a vanished protector: Veillette and Roy, 1995; https://doi.org/10.4095/202923) which would affect the inheritance of the bedrock sample. Additionally, as previously mentioned, a more discussion on how the glacial lake was reconstructed and possible inheritance from the 10Be should be explored as other suggested hypotheses seem plausible.
I can appreciate the limited dataset available to the authors, which they fully acknowledge, calling for additional fieldwork in the region. However, until additional data is available, either other hypotheses should be explored, or additional discussions to address the issues presented by other commenters is needed. Again, I fully appreciate the difficulty of this type of investigation and I do hope this discussion sparks future work in this remote region of Canada where little fieldwork has been conducted.
Jessey Rice- Geological Survey of Canada, Ottawa
We thank the comment author for their positive comments and thoughtful feedback. As we stress earlier in our response letter, our study relies on legacy data from a 2003 field expedition, and we do not have data from additional shoreline sites. We nevertheless pushed forward with publishing these data to draw attention to this work, and to highlight the exciting potential for several facets of Quaternary work, including constraining the configuration of pre-LGM ice sheets, understanding glacial inception, as well as quantifying the timing and amount of freshwater release into the North Atlantic. We have stressed in our current study that attaining such information will be the focus of future field studies.
In response to these comments, and similar ones raised by reviewers and other community comments, we will add the following text:
“Till cover would cause the age assigned to the glacial lake shoreline to be older”
“Evidence for outburst flooding is based on the rounded, imbricated cobbles in inlet channels. We found these deposits in multiple inlet channels leading to the sample site, which we interpreted as a lake shoreline. We hypothesize that the lake level may have fallen rapidly because there is no evidence for bands of lake shorelines at progressively lower elevations. Nevertheless, future work is required to verify such evidence for glacial outburst flooding”
In the methods section we will add the following text:
The lowest elevation wave-cut bench was sampled to avoid possible inheritance of cosmogenic nuclides. We believe that substantial inheritance is unlikely because the sampled wave-cut bench is 3 meters below the highest observed platform elevation (Figure 4C). Because there is a relationship between cosmogenic nuclide concentration and depth, we can calculate the expected inheritance at 3 meters depth.
We use the following equation,
,
where N is cosmogenic nuclide concentration, x is depth below the surface, is rock density, and is attenuation length. We assume density is 2.9 g/cm3 and attenuation length is 165 cm.
If we consider the maximum inheritance expected at the surface in the Torngat Mountains region to be 5.7x106 atoms/g quartz (340 ky equivalent) (Staiger et al., 2005), the maximum concentration at 3 m depth below surface elevation should be near 2.8x104 atoms/g quartz (4.5 ky exposure). If we do not use the maximum concentration found but use a more realistic value found for similar altitudes 1x106 atoms/g (0.9 ky exposure), the possible inheritance is within our external age uncertainty of 3 ky.
Citation: https://doi.org/10.5194/cp-2021-132-AC5 - AC9: 'Reply on CC3', Tamara Pico, 13 Jan 2022
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AC5: 'Reply on CC3', Tamara Pico, 10 Jan 2022
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CC4: 'Comment on cp-2021-132', Jakob Heyman, 26 Nov 2021
Data comments:
* The longitude of the samples given in Table S1 (-65.79) appear to be located too far west. Based on Figure 3, the longitude should be something around -64.56. If you have more detailed coordinates for the samples it would be good to include them.
* The 10Be concentration uncertainties in Table S1 appear to be too small based on the internal exposure age uncertainties in Table S1. Should they perhaps be multiplied by 10?
Citation: https://doi.org/10.5194/cp-2021-132-CC4 -
AC6: 'Reply on CC4', Tamara Pico, 10 Jan 2022
Community Comment #4
Data comments:
* The longitude of the samples given in Table S1 (-65.79) appear to be located too far west. Based on Figure 3, the longitude should be something around -64.56. If you have more detailed coordinates for the samples it would be good to include them.
Thank you for catching this mistake. When we realized this error, we attempted to replace the supplementary text, however it can only be replaced in the revision. The revised manuscript will reflect this change.
* The 10Be concentration uncertainties in Table S1 appear to be too small based on the internal exposure age uncertainties in Table S1. Should they perhaps be multiplied by 10?
Thanks for spotting this. The correct uncertainties are 1.038E+04 and 1.046E+04. The uncertainties in the AMS ratio were correct. The correct nuclide concentration uncertainties were used in the CRONUS input. We will update the Table.
Citation: https://doi.org/10.5194/cp-2021-132-AC6
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AC6: 'Reply on CC4', Tamara Pico, 10 Jan 2022
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EC1: 'Responding to comments/reviews', Alberto Reyes, 29 Nov 2021
Dear Dr. Pico,
The public discussion period for your manuscript is still open for another ~two weeks. But all formal peer-reviews have been submitted now. While waiting for the discussion period to close, you and your co-authors can certainly respond to the reviews and comments in the interactive discussion.
Sincerely,
Alberto
Citation: https://doi.org/10.5194/cp-2021-132-EC1 -
EC2: 'Editor Comment', Alberto Reyes, 19 Dec 2021
Dear Dr Pico and co-authors,
The discussion period for your manuscript is now over. The two formal reviewers and commenters identifed several key points of contention, with (at least in my opinion) the most important being:
-impact of nuclide inheritance and exposure/erosion history on interpretation of the 10Be data
-low number of dated samples
-unclear or under-described geomorphic evidence for the ice-dammed lake geometry and drainage
I agree with the reviewers/commenters that these issues raise important questions about the interpretations in the manuscript. But I'd like to give you the opportunity to address the criticisms, by responding to the reviews and comments in the interactive discussion. This does not yet require preparation of a revised manuscript, but rather indicate how you would respond to the criticisms and suggestions.
Sincerely,
Alberto (handling editor)
Citation: https://doi.org/10.5194/cp-2021-132-EC2 -
AC7: 'Reply on EC2', Tamara Pico, 10 Jan 2022
We thank the editor, two reviewers, and four community comment authors for their positive and constructive comments in regard to the submitted manuscript.
In responding to these reviews, we have noted how we will revise the manuscript to improve the clarity of our arguments. In particular, we have performed additional calculations to explore the uncertainty on estimated lake volume, we have created a new figure contextualizing the sample location, and we have added text to provide additional geomorphic observations for glacial lake shoreline evidence. Furthermore, we have added text describing alternate interpretations of our dataset. We will incorporate all minor text edit suggestions made by the reviewers. We believe incorporating these comments in this revision will strengthen our manuscript.
Our study relies on legacy data from a 2003 field expedition, and we do not have data from additional shoreline sites or paired cosmogenic isotopes. We nevertheless pushed forward with publishing these data to draw attention to this work, and to highlight the exciting potential for several facets of Quaternary work, including constraining the configuration of pre-LGM ice sheets, understanding glacial inception, as well as quantifying the timing and amount of freshwater release into the North Atlantic. We have stressed in our current study that attaining such information will be the focus of future field studies.
In the following, we address each of the comments raised in the reviews and provide a detailed listing of the associated revisions to the text. We intersperse the reviewers’ comments (black font) with our responses (in italic black font). Line numbers correspond to the original submitted version of the manuscript.
Citation: https://doi.org/10.5194/cp-2021-132-AC7
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AC7: 'Reply on EC2', Tamara Pico, 10 Jan 2022
Status: closed
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RC1: 'Comment on cp-2021-132', Anonymous Referee #1, 29 Oct 2021
The manuscript by Pico et al. reports evidence of a pre-LGM (Last Glacial Maximum), ice-dammed proglacial lake in the Torngat Mountains in Labrador, with two cosmogenic exposure ages of a wave-cut platform at the lake shoreline. The record, albeit incomplete, provides important evidence of ice-free conditions in the Torngat Mountains during MIS 3, which is contrary to many previous reconstructions of pre-LGM ice sheets and assumptions of the location of ice-sheet inception prior to the LGM. The data are worth publishing as they represent an important limit on ice-sheet extent prior to the LGM. The manuscript is written well, will be of broad interest in the paleoclimate community, and is appropriate for Climate of the Past. The following comments are suggestions for providing more context and perhaps more geologic information regarding the existence of the pre-LGM lake, which would help to motivate future work on the geologic record of the area as the authors repeatedly state is necessary.
While the geomorphology of the lake shoreline and the cosmogenic ages of the wave-cut platforms are well supported, the evidence of one or more outburst floods from the lake is relatively unclear in the manuscript. Most ice-dammed lakes are unstable and are known to drain and refill, the authors do not report any direct geologic evidence of outburst flooding. This may be a result of the subsequent glaciation in the area and the removal of any geomorphic or sedimentary evidence of outburst floods through lake outlets, which are more abundant (and perhaps better preserved) for ice-dammed lakes that existed in Canada during deglaciation. The authors should provide more geologic evidence for outburst floods from the lake, or a more detailed explanation of the why they believe the lake drained via outburst floods.
Finally, the volume of the lake should include some additional considerations of uncertainty given the unclear shape of the damming ice margins during MIS 3. The reported lake volume is based on some detailed consideration of the effects of isostatic rebound, but does not report the uncertainty of the lake volume calculation based on the presumed ice margin positions. This should be taken into account in paragraphs from lines 301-318 to more appropriately limit the potentially contribution to Heinrich Events during MIS 3.
Line by line comments:
Line 67: reference for the marine oxygen isotope data?
Line 82 (and elsewhere): should the names of glacial lakes be capitalized, as in Glacial Lake Koroc? They appear to considered proper nouns in other literature and are capitalized in the paragraph on lines 180-191.
Lines 129 and 130: use of “constraints” here and elsewhere. Editors may suggest replacing this with “limits”.
Lines 133: change “previous unpublished” to “new”?
Line 134: this is the first appearance of “pre-LGM glacial lake Koroc” (unless my PDF reader is mistaken). Two considerations: (1) add the name of the lake to the abstract, (2) perhaps give the lake a unique name? It appears to be distinct from the younger Glacial Lake Koroc, with a much higher shoreline elevation and presumably much deeper, and perhaps should have a unique name.
Figure 3 - The caption indicates that the boxed area with a white dashed outline is the assumed area of the lake, but it is unclear what the label “assumed glacial lake boundaries” means.
Lines 196-200: Is it possible to provide more geologic information about the lake shorelines? If so, considering showing a map with the location of the landforms and sediments representing flow into the lake, or a map that traces out the distribution of shoreline deposits and/or landforms representing the lake. The existence of the lake would be strengthened by more details about its record.
Lines 203-206: the relative location of the two samples is unclear. This should be explained here. Any photos of the sample sites would be worth including in the paper or in the supplement. The photos in Figure 4 show examples of the wave-cut benches but do not specify the sample locations.
Line 205: is the channel in Figure 4A an inlet or outlet channel? If an inlet, the authors should elaborate on how this channel and any associated deposits are indicative of an outburst flood.
Line 220: should the reference be “Willenbring and Staiger, 2005”?
Lines 232-234: More information is needed to justify the assumed duration of ice cover. It would be good to include at least a few sentences about the basis of the “apparent exposure age” given on lines 232-234. This is warranted given the potential uncertainty of the duration of ice cover, as it is the only limit on it. Additionally, the preservation of wave-cut platforms suggests cold-based ice at the sample site, but the authors do not explicitly state this. Consider doing this in the Methods paragraphs, as there appears to be sufficient evidence for it and it alleviates at least one concern about the exposure history of the site.
Line 260: change “presently” to “at present” or “currently”
Lines 255-268: Provide some estimate of uncertainty in the lake volume calculation given the unknown ice margin positions.
Line 301: This would be an appropriate place to add some explanation of why the lake drained as outburst floods (see general comment above).
Line 308: would it be possible to add to Figure 3A or 3B the flow direction of water when the lake drained?
Line 407, 409: it would be more appropriate to state “the first direct on-land evidence for a proglacial lake”, because the evidence of an outburst flood not reported in the paper.
Line 420: change “tease out” to “resolve”?
Citation: https://doi.org/10.5194/cp-2021-132-RC1 -
AC1: 'Reply on RC1', Tamara Pico, 10 Jan 2022
Reviewer 1
The manuscript by Pico et al. reports evidence of a pre-LGM (Last Glacial Maximum), ice-dammed proglacial lake in the Torngat Mountains in Labrador, with two cosmogenic exposure ages of a wave-cut platform at the lake shoreline. The record, albeit incomplete, provides important evidence of ice-free conditions in the Torngat Mountains during MIS 3, which is contrary to many previous reconstructions of pre-LGM ice sheets and assumptions of the location of ice-sheet inception prior to the LGM. The data are worth publishing as they represent an important limit on ice-sheet extent prior to the LGM. The manuscript is written well, will be of broad interest in the paleoclimate community, and is appropriate for Climate of the Past. The following comments are suggestions for providing more context and perhaps more geologic information regarding the existence of the pre-LGM lake, which would help to motivate future work on the geologic record of the area as the authors repeatedly state is necessary.
We thank the reviewer for these positive comments.
While the geomorphology of the lake shoreline and the cosmogenic ages of the wave-cut platforms are well supported, the evidence of one or more outburst floods from the lake is relatively unclear in the manuscript. Most ice-dammed lakes are unstable and are known to drain and refill, the authors do not report any direct geologic evidence of outburst flooding. This may be a result of the subsequent glaciation in the area and the removal of any geomorphic or sedimentary evidence of outburst floods through lake outlets, which are more abundant (and perhaps better preserved) for ice-dammed lakes that existed in Canada during deglaciation. The authors should provide more geologic evidence for outburst floods from the lake, or a more detailed explanation of the why they believe the lake drained via outburst floods.
We agree with the reviewer that the manuscript should be more specific about outburst flooding evidence to contextualize our findings for the readers.
In response to this comment and similar comments by Reviewer 2 we have revised the text to avoid overextending our data interpretation. We have replaced wording about “glacial outburst flooding” in the title and abstract with “glacial lake”, to be specific that our data supports evidence of a glacial lake more strongly than evidence for outburst flooding.
In addition, we have highlighted the evidence for outburst flooding by adding the following sentence:
“Evidence for outburst flooding is based on the rounded, imbricated cobbles in inlet channels. We found these deposits in multiple inlet channels leading to the sample site, which we interpreted as a lake shoreline. We hypothesize that the lake level may have fallen rapidly because there is no evidence for bands of lake shorelines at progressively lower elevations. Nevertheless, future work is required to verify such evidence for glacial outburst flooding”
Finally, the volume of the lake should include some additional considerations of uncertainty given the unclear shape of the damming ice margins during MIS 3. The reported lake volume is based on some detailed consideration of the effects of isostatic rebound, but does not report the uncertainty of the lake volume calculation based on the presumed ice margin positions. This should be taken into account in paragraphs from lines 301-318 to more appropriately limit the potentially contribution to Heinrich Events during MIS 3.
We agree with the reviewer that additional exploration of possible ice margins would contextualize the uncertainty on lake volume estimates for readers. In response to these comments, we performed additional calculations that vary ice margin location to explore the uncertainty on estimated lake volume.
The estimated total lake volume is highly sensitive to our choice of assumed ice margins. We shifted the assumed ice margin boundary northward from 58 N to 58.5 N (~56 km), and found that this reduced the estimated lake volume from 1.14x1012 m3 to 5.42x1011 m3 (reduced by ~50%). Shifting the assumed ice margin boundary northward from 58 N to 58.25 N (~28 km) reduces the estimated lake volume from 1.14x1012 m3 to 7.62x1011 m3 (reduced by ~30%). Shifting the assumed ice margin boundary southward from 58 N to 57.75 N increases the estimated lake volume 1.14x1012 m3 to 1.44x1012 m3.
To address the uncertainty on lake volume resulting from our ice margin assumptions, we will add the following text (line 318):
The total lake volume depends strongly on the assumed ice margin positions that bound the glacial lake. Given the uncertainty on the ice-dammed locations of this lake, we explored three possible scenarios varying the assumed southern position of the ice margin that bound pre-LGM Glacial Lake Koroc. For the intermediate scenario, the southern ice margin boundary is at 58 N, resulting in an estimated lake volume of 1.14x1012 m3. For the smaller lake scenario, we shift the assumed ice margin boundary northward from 58 N to 58.25 N (~28 km), which reduces the estimated lake volume from 1.14x1012 m3 to 7.62x1011 m3 (reduced by ~30%). For the larger lake scenario, shifting the assumed ice margin boundary southward from 58 N to 57.75 N increases the estimated lake volume 1.14x1012 m3 to 1.44x1012 m3 (increased by ~25%).
Line by line comments:
Line 67: reference for the marine oxygen isotope data?
Thank you for catching this. We now reference Channell et al., 2012 here.
Line 82 (and elsewhere): should the names of glacial lakes be capitalized, as in Glacial Lake Koroc? They appear to considered proper nouns in other literature and are capitalized in the paragraph on lines 180-191.
We have edited the text to read “Glacial Lake Koroc” throughout the manuscript.
Lines 129 and 130: use of “constraints” here and elsewhere. Editors may suggest replacing this with “limits”.
Done!
Lines 133: change “previous unpublished” to “new”?
We have left this wording as is since we are reporting legacy data (therefore not new).
Line 134: this is the first appearance of “pre-LGM glacial lake Koroc” (unless my PDF reader is mistaken). Two considerations: (1) add the name of the lake to the abstract, (2) perhaps give the lake a unique name? It appears to be distinct from the younger Glacial Lake Koroc, with a much higher shoreline elevation and presumably much deeper, and perhaps should have a unique name.
We appreciate this suggestion. We have edited the abstract to include the name “pre-LGM Glacial Lake Koroc”
Figure 3 - The caption indicates that the boxed area with a white dashed outline is the assumed area of the lake, but it is unclear what the label “assumed glacial lake boundaries” means.
We have edited this text to read “assumed location of ice margins bounding glacial lake” to clarify that these represent assumed ice margin limits.
Lines 196-200: Is it possible to provide more geologic information about the lake shorelines? If so, considering showing a map with the location of the landforms and sediments representing flow into the lake, or a map that traces out the distribution of shoreline deposits and/or landforms representing the lake. The existence of the lake would be strengthened by more details about its record.
Unfortunately, our study relies on legacy data from a 2003 field expedition, and we do not have data from additional shoreline sites. Attaining such information will be the focus of future field studies.
Lines 203-206: the relative location of the two samples is unclear. This should be explained here. Any photos of the sample sites would be worth including in the paper or in the supplement. The photos in Figure 4 show examples of the wave-cut benches but do not specify the sample locations.
The location of the two samples is at the site marked by the white circle in Figure 3. There is only one location, which is shown in Figure 4b.
We now include a schematic figure in Figure 4 that shows the sample location on this set of wave-cut benches.
We have added text to the manuscript to read “The two samples are at the same location shown in Figure 4C”
Line 205: is the channel in Figure 4A an inlet or outlet channel? If an inlet, the authors should elaborate on how this channel and any associated deposits are indicative of an outburst flood.
The channel in the forefront of Figure 4A on the interpreted wave-cut bench is an inlet channel leading into the glacial lake. The inlet channel would have served as a conduit for water to enter the glacial lake, which can be considered base level for the inlet. Therefore, the inlet channel itself is not indicative of an outburst flood.
We now note in the text:
Evidence for outburst flooding is based on the rounded, imbricated cobbles in inlet channels. We found these deposits in multiple inlet channels leading to the sample site, which we interpreted as a lake shoreline. We hypothesize that the lake level may have fallen rapidly because there is no evidence for bands of lake shorelines at progressively lower elevations. Nevertheless, future work is required to verify such evidence for glacial outburst flooding
Line 220: should the reference be “Willenbring and Staiger, 2005”?
No, the reference is Jane Willenbring’s PhD thesis. This is the same person (co-author of study).
Lines 232-234: More information is needed to justify the assumed duration of ice cover. It would be good to include at least a few sentences about the basis of the “apparent exposure age” given on lines 232-234. This is warranted given the potential uncertainty of the duration of ice cover, as it is the only limit on it. Additionally, the preservation of wave-cut platforms suggests cold-based ice at the sample site, but the authors do not explicitly state this. Consider doing this in the Methods paragraphs, as there appears to be sufficient evidence for it and it alleviates at least one concern about the exposure history of the site.
We will add the following text to the Methods section to note the likelihood of cold based ice and a discussion about the assumed duration of ice cover (line 235):
“The sample site was likely covered by cold-based ice for some duration (Staiger et al., 2005).”
Line 260: change “presently” to “at present” or “currently”
Done!
Lines 255-268: Provide some estimate of uncertainty in the lake volume calculation given the unknown ice margin positions.
We have now included a new calculation in response to these comments. We will add the following text:
The total lake volume depends strongly on the assumed ice margin positions that bound the glacial lake. Given the uncertainty on the ice-dammed locations of this lake, we explored three possible scenarios varying the assumed southern position of the ice margin that bound pre-LGM Glacial Lake Koroc. For the intermediate scenario, the southern ice margin boundary is at 58 N, resulting in an estimated lake volume of 1.14x1012 m3. For the smaller lake scenario, we shift the assumed ice margin boundary northward from 58 N to 58.25 N (~28 km), which reduces the estimated lake volume from 1.14x1012 m3 to 7.62x1011 m3 (reduced by ~30%). For the larger lake scenario, shifting the assumed ice margin boundary southward from 58 N to 57.75 N increases the estimated lake volume 1.14x1012 m3 to 1.44x1012 m3 (increased by ~25%).
Line 301: This would be an appropriate place to add some explanation of why the lake drained as outburst floods (see general comment above).
As discussed above, in response to this comment we have added the following text:
Evidence for outburst flooding is based on the rounded, imbricated cobbles in inlet channels. We found these deposits in multiple inlet channels leading to the sample site, which we interpreted as a lake shoreline. We hypothesize that the lake level may have fallen rapidly because there is no evidence for bands of lake shorelines at progressively lower elevations. Nevertheless, future work is required to verify such evidence for glacial outburst flooding
Line 308: would it be possible to add to Figure 3A or 3B the flow direction of water when the lake drained?
Thank you for this suggestion. We will add the potential flow route lines to this figure.
Line 407, 409: it would be more appropriate to state “the first direct on-land evidence for a proglacial lake”, because the evidence of an outburst flood not reported in the paper.
Yes, we agree with this suggestion. We will update this wording accordingly.
Line 420: change “tease out” to “resolve”?
Done!
Citation: https://doi.org/10.5194/cp-2021-132-AC1
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AC1: 'Reply on RC1', Tamara Pico, 10 Jan 2022
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CC1: 'Comment on cp-2021-132', Nicolas Young, 01 Nov 2021
Pico et al present an interesting manuscript that tackles two outstanding questions in Quaternary glacial geology and paleoclimatology: what were the dimensions of the Laurentide Ice Sheet during Marine Isotope Stage (MIS) 3 and what triggered Heinrich events? Pico et al suggest that the formation and sudden discharge of a proglacial ice-dammed lake in the Torngat Mountains (Labrador, Canada) sometime during MIS 3 could have served as a triggering mechanism for a MIS 3 Heinrich event(s). Indeed, the authors present an impressive reconstruction of the region’s paleotopography and potential paleolake dimensions and volume that hypothetically acted as a trigger for a MIS 3 Henrich event. This argument relies heavily on 1) field evidence for the existence of a paleo ice-dammed lake with wave action eroding bedrock platforms, and 2) the age of the lake dating to MIS 3 based on interpretation of two 10Be ages from a bedrock surface near the proposed shoreline. Based on our experience measuring 10Be in glaciated landscapes exhibiting weathering zones and spatial patterns of ice-sheet erosion, we think there are more likely, alternative interpretations of an ice-margin history that would result in the apparent 10Be ages reported by Pico et al.
10Be measurements in bedrock
The authors report two statistically indistinguishable 10Be ages from bedrock surfaces of 35.9 ± 1.1 ka and 36.0 ± 1.1 ka, although it is not clear whether the samples are from the arrows in Fig. 4 or elsewhere. The authors recognize that this landscape was likely covered by ice during the Last Glacial Maximum/MIS 2 and assume that the most recent episode of deglaciation occurred ca. 10 ka. The authors suggest that the apparent 10Be ages represent ~10 ka of recent surface exposure leading up to the year of sample collection, and ~25 ka of exposure (or exposure equivalent) occurred prior to the last deglaciation but fail to evaluate the range of possibilities for the exposure interval(s) represented by the residual 10Be inventory.
To make sense of the excess 10Be in these samples and to consider the sample locations’ full exposure/burial history, the authors assume that ice advanced over the sample sites at 30 ka and remained over these sites until 10 ka (20 kyr of ice cover). Therefore, there is an additional 20 ka of ice-cover history that is not reflected in the apparent 10Be ages (i.e., no nuclide production under ice). The authors use this 20-kyr interval of burial to suggest that the likely exposure age is 56 ka (36 ka measured + 20 ka assumed burial), which also implies the excess 10Be accumulated between 56 ka and 30 ka. In turn this “likely” age of 56 ka is used to date the drainage of an ice-dammed lake and further suggest that this region underwent an episode of deglaciation at this time.
We believe that alternative explanations are equally, if not more plausible.
The excess 10Be in the surfaces (the ~25 kyr worth of 10Be) could have accumulated in the samples just about any time in the last million or more years. Without 26Al data, we are essentially “blind” to when it may have accumulated. With 26Al data demonstrating no isotopic disequilibrium, one could then only reduce the time during which the 10Be could have accumulated to the last ~100-200 kyr. We also cannot know if the excess 10Be accumulated during a single episode of pre-LGM surface exposure, multiple episodes of pre-LGM surface exposure, single episode of subsurface isotope accumulation, or multiple episodes of subsurface isotope accumulation. Among all of these options, none of which are uniquely constrainable with available data, what support is there for the interpretation of a single period of MIS 3 exposure that the authors put forward? For the chosen interpretation to be supported, a fuller discussion and ruling out of alternative interpretations is required.
We offer two other possibilities that are more likely:
1) At 890 m asl in a landscape classified as “intermediate weathering,” studies show that bedrock surfaces almost always contain inheritance, usually 10s of kyr of inheritance in age equivalent terms. We did not see any text confronting – and ruling out – a likely scenario that wave action from a lake, or meltwater flow, during the last deglaciation stripped surficial debris off of the sampled outcrops leaving planar bedrock surfaces with inherited 10Be that lacks a unique temporal explanation. We think this is a likely explanation for the measured inventories.
2) We are unaware of studies where an apparent 10Be age that is known to be influenced by isotopic inheritance (as Pico et al show here) is interpreted literally because, again, there is no way to determine when exactly inherited 10Be accumulates in a rock sample (see above). Here, the authors are prescribing that the inherited 10Be accumulated between 56 ka and 30 ka. In fact, if the excess 10Be accumulated at any other time(s), then the paleo ice-dammed lake likely cannot be MIS 3 in age. There are, however, some reasonable exposure/burial histories in this region that would result in measurable amounts of isotopic inheritance in bedrock. An alternative scenario, in addition to the one we mention above, is that the currently unglaciated terrain where these bedrock samples were collected was also ice free during the Last Interglacial (MIS 5e) when temperatures likely exceeded those of the Holocene. In this scenario, bedrock exposure during MIS 5e would result in the accumulation of 10Be and during post-MIS 5e glaciation across the region, the Laurentide Ice Sheet did not erode through the required 2-3 m of bedrock needed to reset the cosmogenic clock. We think this is particularly likely in a region that was susceptible to non- or minimally erosive cold-based to polythermal ice; 20+ years of 10Be measurements in Canadian Arctic and sub-Arctic have revealed that bedrock is often influenced by isotopic inheritance. By Pico et al’s own conclusion, this region likely experienced times of cold-based ice and they invoke this phenomenon to explain the preservation of paleoshorelines. Thus, the measured 10Be inventory presented by the authors could simply reflect 10 ka of recent exposure combined with 10Be accumulated during MIS 5e or perhaps even leftover 10Be from even earlier interglacials (e.g., MIS 11), minus the amount of 10Be that was stripped from the bedrock as a result of the depth of subglacial erosion that occurred during post MIS 5e ice cover. Slightly more subglacial erosion (but not exceeding 2-3 m) during the interval of ice cover would result in younger apparent 10Be ages, while slightly less erosion would result in older apparent 10Be ages, but both end members would yield apparent 10Be ages influenced by isotopic inheritance.
It may be tempting to consider the statistically indistinguishable apparent 10Be ages presented here as evidence of a true exposure signal that are not influenced by erosion. To fully evaluate this possibility, however, full descriptions of the sample locations are needed; it is presently unclear where these bedrock samples are from (e.g., listed sample coordinates plot closer to Ungava Bay than the site shown in the accompanying figure, and only at 140 m asl). For example, if these samples are close to one another then the two bedrock surfaces almost certainly experienced the same long-term exposure-burial histories and therefore the similar apparent 10Be ages could be a function of residing under the same amount of till cover that was later stripped away (shielding; 10Be production at depth). Additionally, it is not unreasonable to suspect that similar 10Be ages are a function of the same magnitude of subglacial erosion through the 10Be production-depth profile. In this latter scenario, the full exposure-burial history of the sample locations would be greater than ~36 ka, but the excess 10Be that could potentially explain this full history has been eroded away.
The claim that this region, a suspected LIS inception point that therefore spends most of its history under ice, deglaciated during MIS 3 is a fairly extraordinary suggestion. We suggest this claim requires firm chronological constraints. Because it is highly likely this region, like today, was ice free during MIS 5e, we think the authors should provide compelling evidence for why the inherited 10Be is not simply a relic from MIS 5e (or earlier interglacials) that has been preserved under a minimally erosive Laurentide Ice Sheet.
Geomorphic evidence for a paleo ice-dammed lake
Considering the importance of this suspected paleo ice-dammed lake, the geomorphic evidence pointing to this lake’s existence needs to be better described. Pico et al provide one figure (Fig. 4) with two pictures meant to document paleo-shorelines attributed to this ice-dammed lake. We do not see anything definitive in these pictures that suggests the presence of a large ice-dammed lake nor clear evidence of wave-cut shorelines. In particular, in Figure 4b, there appears to be extensive till and/or regolith cover with a few bedrock surfaces nearby. Without further information, this appears to be till and/or regolith cover that was washed aside as meltwater flowed across the landscape during a period of deglaciation, not necessarily associated with an ice-dammed lake. To be clear, it is possible that a large ice-dammed lake at ~890 m asl existed, but the evidence for this needs to be more thoroughly documented.
Lastly, if there was a large ice-dammed lake and the 10Be samples are from wave-cut benches, the interpretation of the apparent 10Be ages and lake chronology presented by Pico et al would require that wave action on the edge of the lake remove 2-3 m of bedrock to reset the cosmogenic clock. This magnitude of wave-based erosion might be possible at the outlet channel hosting a catastrophic outburst flood, but 2-3 m of wave-cut erosion by an ephemeral ice-dammed lake into crystalline granitic terrain is unlikely. Moreover, if wave action only removed a few 10s of cm or ~1 meter, instead of 2-3 m, then the measured 10Be inventories reflect a component of 10Be produced at depth and the full exposure history of the bedrock surface is more, but this evidence has been eroded away. We note that the text speculates that there was outburst flooding, but the evidence provided for this is also consistent with a variety of smaller scale glaciofluvial processes. Rather than a large ice-dammed lake that suddenly drained, we wonder why this cannot be a case of terrain that was ice-covered during the LGM and during landscape deglaciation, the likely significant amounts of meltwater runoff simply washed away till and/or regolith exposing bare bedrock surfaces that contain 10Be from a previous period of exposure. Note this would be consistent with the alternative interpretation of the 10Be inventories that we mention above.
We accept that it is possible, although unlikely, that these bedrock surfaces were exposed during MIS 3. To support their interpretation, the authors would need to provide strong evidence as to why they prefer that inherited 10Be in their samples was from MIS 3 exposure instead of any number of alternative scenarios, including but not limited to some of those we discuss above.
Nicolás E. Young (Lamont-Doherty Earth Observatory, Columbia University)
Jason P. Briner (Dept. of Geology, University at Buffalo)
Gifford H. Miller (INSTAAR & Dept. of Geological Sciences, University of Colorado Boulder)
Citation: https://doi.org/10.5194/cp-2021-132-CC1 -
AC3: 'Reply on CC1', Tamara Pico, 10 Jan 2022
Community Comment # 1
Pico et al present an interesting manuscript that tackles two outstanding questions in Quaternary glacial geology and paleoclimatology: what were the dimensions of the Laurentide Ice Sheet during Marine Isotope Stage (MIS) 3 and what triggered Heinrich events? Pico et al suggest that the formation and sudden discharge of a proglacial ice-dammed lake in the Torngat Mountains (Labrador, Canada) sometime during MIS 3 could have served as a triggering mechanism for a MIS 3 Heinrich event(s). Indeed, the authors present an impressive reconstruction of the region’s paleotopography and potential paleolake dimensions and volume that hypothetically acted as a trigger for a MIS 3 Henrich event. This argument relies heavily on 1) field evidence for the existence of a paleo ice-dammed lake with wave action eroding bedrock platforms, and 2) the age of the lake dating to MIS 3 based on interpretation of two 10Be ages from a bedrock surface near the proposed shoreline. Based on our experience measuring 10Be in glaciated landscapes exhibiting weathering zones and spatial patterns of ice-sheet erosion, we think there are more likely, alternative interpretations of an ice-margin history that would result in the apparent 10Be ages reported by Pico et al.
We thank the authors of this comment for their positive comments on our manuscript and for their constructive feedback.
10Be measurements in bedrock
The authors report two statistically indistinguishable 10Be ages from bedrock surfaces of 35.9 ± 1.1 ka and 36.0 ± 1.1 ka, although it is not clear whether the samples are from the arrows in Fig. 4 or elsewhere. The authors recognize that this landscape was likely covered by ice during the Last Glacial Maximum/MIS 2 and assume that the most recent episode of deglaciation occurred ca. 10 ka. The authors suggest that the apparent 10Be ages represent ~10 ka of recent surface exposure leading up to the year of sample collection, and ~25 ka of exposure (or exposure equivalent) occurred prior to the last deglaciation but fail to evaluate the range of possibilities for the exposure interval(s) represented by the residual 10Be inventory.
To make sense of the excess 10Be in these samples and to consider the sample locations’ full exposure/burial history, the authors assume that ice advanced over the sample sites at 30 ka and remained over these sites until 10 ka (20 kyr of ice cover). Therefore, there is an additional 20 ka of ice-cover history that is not reflected in the apparent 10Be ages (i.e., no nuclide production under ice). The authors use this 20-kyr interval of burial to suggest that the likely exposure age is 56 ka (36 ka measured + 20 ka assumed burial), which also implies the excess 10Be accumulated between 56 ka and 30 ka. In turn this “likely” age of 56 ka is used to date the drainage of an ice-dammed lake and further suggest that this region underwent an episode of deglaciation at this time.
We believe that alternative explanations are equally, if not more plausible.
The excess 10Be in the surfaces (the ~25 kyr worth of 10Be) could have accumulated in the samples just about any time in the last million or more years. Without 26Al data, we are essentially “blind” to when it may have accumulated. With 26Al data demonstrating no isotopic disequilibrium, one could then only reduce the time during which the 10Be could have accumulated to the last ~100-200 kyr. We also cannot know if the excess 10Be accumulated during a single episode of pre-LGM surface exposure, multiple episodes of pre-LGM surface exposure, single episode of subsurface isotope accumulation, or multiple episodes of subsurface isotope accumulation. Among all of these options, none of which are uniquely constrainable with available data, what support is there for the interpretation of a single period of MIS 3 exposure that the authors put forward? For the chosen interpretation to be supported, a fuller discussion and ruling out of alternative interpretations is required.
We appreciate the perspectives raised in this comment and respond below to how we will incorporate such alternate interpretations in the revision of the manuscript.
We offer two other possibilities that are more likely:
1) At 890 m asl in a landscape classified as “intermediate weathering,” studies show that bedrock surfaces almost always contain inheritance, usually 10s of kyr of inheritance in age equivalent terms. We did not see any text confronting – and ruling out – a likely scenario that wave action from a lake, or meltwater flow, during the last deglaciation stripped surficial debris off of the sampled outcrops leaving planar bedrock surfaces with inherited 10Be that lacks a unique temporal explanation. We think this is a likely explanation for the measured inventories.
The authors of this comment are correct to note that we cannot rule out the possibility of till cover. This cover would make the ages older since the till would shield the bedrock surface.
We will add the following the text to the manuscript to note the possibility of till cover:
“Till cover would cause the age assigned to the glacial lake shoreline to be older”
2) We are unaware of studies where an apparent 10Be age that is known to be influenced by isotopic inheritance (as Pico et al show here) is interpreted literally because, again, there is no way to determine when exactly inherited 10Be accumulates in a rock sample (see above). Here, the authors are prescribing that the inherited 10Be accumulated between 56 ka and 30 ka. In fact, if the excess 10Be accumulated at any other time(s), then the paleo ice-dammed lake likely cannot be MIS 3 in age. There are, however, some reasonable exposure/burial histories in this region that would result in measurable amounts of isotopic inheritance in bedrock. An alternative scenario, in addition to the one we mention above, is that the currently unglaciated terrain where these bedrock samples were collected was also ice free during the Last Interglacial (MIS 5e) when temperatures likely exceeded those of the Holocene. In this scenario, bedrock exposure during MIS 5e would result in the accumulation of 10Be and during post-MIS 5e glaciation across the region, the Laurentide Ice Sheet did not erode through the required 2-3 m of bedrock needed to reset the cosmogenic clock. We think this is particularly likely in a region that was susceptible to non- or minimally erosive cold-based to polythermal ice; 20+ years of 10Be measurements in Canadian Arctic and sub-Arctic have revealed that bedrock is often influenced by isotopic inheritance. By Pico et al’s own conclusion, this region likely experienced times of cold-based ice and they invoke this phenomenon to explain the preservation of paleoshorelines. Thus, the measured 10Be inventory presented by the authors could simply reflect 10 ka of recent exposure combined with 10Be accumulated during MIS 5e or perhaps even leftover 10Be from even earlier interglacials (e.g., MIS 11), minus the amount of 10Be that was stripped from the bedrock as a result of the depth of subglacial erosion that occurred during post MIS 5e ice cover. Slightly more subglacial erosion (but not exceeding 2-3 m) during the interval of ice cover would result in younger apparent 10Be ages, while slightly less erosion would result in older apparent 10Be ages, but both end members would yield apparent 10Be ages influenced by isotopic inheritance.
It may be tempting to consider the statistically indistinguishable apparent 10Be ages presented here as evidence of a true exposure signal that are not influenced by erosion. To fully evaluate this possibility, however, full descriptions of the sample locations are needed; it is presently unclear where these bedrock samples are from (e.g., listed sample coordinates plot closer to Ungava Bay than the site shown in the accompanying figure, and only at 140 m asl). For example, if these samples are close to one another then the two bedrock surfaces almost certainly experienced the same long-term exposure-burial histories and therefore the similar apparent 10Be ages could be a function of residing under the same amount of till cover that was later stripped away (shielding; 10Be production at depth). Additionally, it is not unreasonable to suspect that similar 10Be ages are a function of the same magnitude of subglacial erosion through the 10Be production-depth profile. In this latter scenario, the full exposure-burial history of the sample locations would be greater than ~36 ka, but the excess 10Be that could potentially explain this full history has been eroded away.
The claim that this region, a suspected LIS inception point that therefore spends most of its history under ice, deglaciated during MIS 3 is a fairly extraordinary suggestion. We suggest this claim requires firm chronological constraints. Because it is highly likely this region, like today, was ice free during MIS 5e, we think the authors should provide compelling evidence for why the inherited 10Be is not simply a relic from MIS 5e (or earlier interglacials) that has been preserved under a minimally erosive Laurentide Ice Sheet.
We appreciate the comment author’s careful consideration of our manuscript and ideas for alternate hypotheses. We agree that there is considerable age uncertainty on the data presented in our study, and we hope that we have been clear that we view this dataset as preliminary evidence for future detailed fieldwork. Alternate hypotheses about the age of samples are certainly plausible. Such findings will be compelling in their own right since there is little data on pre-LGM glacial lakes in this region.
To note the possibility of alternate ages, we will include the following text on line 291:
Should an improved understanding of the ice burial history suggest a longer period of ice sheet cover, this glacial lake shoreline may be assigned older ages than those presented here. For example, depending on the history of ice cover and erosion, the lake shoreline may correspond to earlier periods of retracted ice such as MIS 5a or MIS 5e. Nevertheless, given the preservation of this lake shoreline feature, increasingly older ages are less likely.
Geomorphic evidence for a paleo ice-dammed lake
Considering the importance of this suspected paleo ice-dammed lake, the geomorphic evidence pointing to this lake’s existence needs to be better described. Pico et al provide one figure (Fig. 4) with two pictures meant to document paleo-shorelines attributed to this ice-dammed lake. We do not see anything definitive in these pictures that suggests the presence of a large ice-dammed lake nor clear evidence of wave-cut shorelines. In particular, in Figure 4b, there appears to be extensive till and/or regolith cover with a few bedrock surfaces nearby. Without further information, this appears to be till and/or regolith cover that was washed aside as meltwater flowed across the landscape during a period of deglaciation, not necessarily associated with an ice-dammed lake. To be clear, it is possible that a large ice-dammed lake at ~890 m asl existed, but the evidence for this needs to be more thoroughly documented.
The noted shoreline is not in line with any fracture plane, and because it is eroded into bedrock it cannot be a solifluction or gelifluction lobe. For other glacial erosion processes we would not expect to see such a planar surface. Therefore, we interpret this geomorphic feature as a shoreline platform.
Lastly, if there was a large ice-dammed lake and the 10Be samples are from wave-cut benches, the interpretation of the apparent 10Be ages and lake chronology presented by Pico et al would require that wave action on the edge of the lake remove 2-3 m of bedrock to reset the cosmogenic clock. This magnitude of wave-based erosion might be possible at the outlet channel hosting a catastrophic outburst flood, but 2-3 m of wave-cut erosion by an ephemeral ice-dammed lake into crystalline granitic terrain is unlikely. Moreover, if wave action only removed a few 10s of cm or ~1 meter, instead of 2-3 m, then the measured 10Be inventories reflect a component of 10Be produced at depth and the full exposure history of the bedrock surface is more, but this evidence has been eroded away. We note that the text speculates that there was outburst flooding, but the evidence provided for this is also consistent with a variety of smaller scale glaciofluvial processes. Rather than a large ice-dammed lake that suddenly drained, we wonder why this cannot be a case of terrain that was ice-covered during the LGM and during landscape deglaciation, the likely significant amounts of meltwater runoff simply washed away till and/or regolith exposing bare bedrock surfaces that contain 10Be from a previous period of exposure. Note this would be consistent with the alternative interpretation of the 10Be inventories that we mention above.
Wave-cut benches in granite do exist, for example, on Sharp Island, Hong Kong (Berry & Ruxton, 1957).
Although the bedrock in our study region is gneiss, it is highly fractured rock. Frost action and ice wedging acts on preexisting pervasive fractures, resulting in a more easily weathered rock. Studies based on glaciated gneiss terrains suggest that such fractured rock results in plucking-dominated erosion, which is likely for our study area (Krabbendam & Bradwell, 2014)
- Berry, B. P. Ruxton, Structure and Form of Cheung Chau Island, Hong Kong. Geol. Mag. XCIV (1957).
- Krabbendam, T. Bradwell, Quaternary evolution of glaciated gneiss terrains : pre-glacial weathering vs . glacial erosion. Quat. Sci. Rev. 95, 20–42 (2014).
We accept that it is possible, although unlikely, that these bedrock surfaces were exposed during MIS 3. To support their interpretation, the authors would need to provide strong evidence as to why they prefer that inherited 10Be in their samples was from MIS 3 exposure instead of any number of alternative scenarios, including but not limited to some of those we discuss above.
Our study relies on legacy data from a 2003 field expedition, and we do not have data from additional shoreline sites. We nevertheless pushed forward with publishing these data to draw attention to this work, and to highlight the exciting potential for several facets of Quaternary work, including constraining the configuration of pre-LGM ice sheets, understanding glacial inception, as well as quantifying the timing and amount of freshwater release into the North Atlantic. We have stressed in our current study that attaining such information will be the focus of future field studies.
We appreciate the ideas raised in this comment. In response, we will edit the manuscript to include discussion of some of the alternate hypotheses raised by the authors of this comment.
Citation: https://doi.org/10.5194/cp-2021-132-AC3
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AC3: 'Reply on CC1', Tamara Pico, 10 Jan 2022
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RC2: 'Comment on cp-2021-132', Anonymous Referee #2, 05 Nov 2021
Manuscript Number: cp-2021-132
Manuscript Type: Research Article
Title: Was there a glacial outburst flood in the Torngat Mountains during Marine Isotope Stage 3?
Authors: Tamara Pico, Jane Willenbring, April S. Dalton and Sidney HemmingComments to Authors:
Pico et al’s manuscript provides evidence of an outburst flood from Glacial Lake Koroc in the Torngat Mountain of northern Quebec and Labrador, Canada. Using two 10Be cosmogenic exposure ages they suggest this outburst occurred during MIS 3 and the flood volume could have contributed to surface ocean freshening and measurable meltwater signal in ð¿18O records. The manuscript is, for the most part, clearly written, and is well supported by figures. However, I have a number of concerns that need to be addressed before this manuscript is ready for publication.
1). The authors constrain the age of Glacial Lake Koroc using two 10Be cosmogenic exposure ages. My main concern is the small sample size and the possibility of inheritance in these samples. The presence of a cold-based ice sheet covering the study area is highlighted in the text (lines 293-296). It, therefore, seems equally plausible that the high elevation shorelines dated in this study could be one of the earliest stages of a post-LGM lake in the region and samples simply appear older due to inherited 10Be concentrations. The authors explore the possibility of erosion and burial of the sample site but no mention of inheritance in the samples is made.
2). More detail is needed regarding the geomorphic/sedimentological evidence for pre-LGM Glacial Lake Koroc. No details regarding the mechanism of failure and evidence for a spillway that drained the lake are discussed, despite ‘outburst flood’ being in the title. During LGM ice recession, multiple glacial lakes occupied the Torngat Mountains. Could this not have been the case for pre-LGM and if so, what is the likelihood that this lake didn’t drain into another lower elevation lake nearby? Is it likely that the whole lake drained during the outburst event? Without the elevation of the spillway, this cannot be assessed.
3). The main conclusion drawn in this study is the contribution an outburst flood from this lake could have had to surface ocean freshening and possible implications this may have had for Heinrich events.
‘A freshwater volume of 1.14x1012 m3, associated with the glacial lake outburst described in this
study could contribute to the large ð¿18O recorded for MIS 3 Heinrich events (minimum volume
required = 1.4x1013 -2.3 x1014 m3 ; Hemming, 2004).’
The minimum volume stated is from Hemming, 2004 is ‘a value assuming a volume the area of the Heinrich layers, and the thickness of the mixed layer is mixed one time with enough ice and water to make the d18O excursion’. However, my understanding of the paper is that 0.6 -1.9 Sv of water over 1 yr to 500 yrs is needed to explain the observed ð¿18O excursion. The estimate presented in this manuscript is 0.004x 106 over 3 days. This seems to be significantly less water than is needed to contribute to the ð¿18O excursion observed during a ~500 yrs of a typical Heinrich Event.
Line edits:
Line 24: Provide a value for the magnitude of freshwater flux in the abstract. Consider also adding the lake name to the abstract
Line 27: Present freshwater flood volume in km3 rather than m3
Line 123: Delete space
Figure 3: Please add the drainage route to panel B. It would also be useful to add the location of mapped shorelines to this figure
Line 206: Evidence for outlet needs to be clearly stated
Line 232/233: More information is needed regarding the duration of ice cover during the LGM. The uncertainty surrounding the 20 kyr ice cover is very briefly mentioned and needs to be more clearly stated
Line 232: Add space ‘concentration( Gosse and Phillips, 2001)’
Line 235: Add space ‘calculation(Jones et al., 2019)’
Line 230: Add a brief statement to highlight that this is a minimum age
Line 234: You describe the impact of GIA on your ages however no age GIA correct age is available to the reader. Consider adding these ages to the text and Table S2.
Line 206: ‘The shoreline is an erosional feature, and there is a small inlet channel at this elevation with rounded imbricated cobbles, suggestive of outburst flooding.’ Imbricated sediment can be produced by outburst events. However, is these are within a small inlet channel how do they suggest outburst flooding?
Line 263: Should glacial not be capitalized in ‘Pre-LGM glacial Lake Koroc”?
Supplementary Material Line 18: Table S2 seems to be identical to the table above
Citation: https://doi.org/10.5194/cp-2021-132-RC2 -
AC2: 'Reply on RC2', Tamara Pico, 10 Jan 2022
Reviewer 2
Pico et al’s manuscript provides evidence of an outburst flood from Glacial Lake Koroc in the Torngat Mountain of northern Quebec and Labrador, Canada. Using two 10Be cosmogenic exposure ages they suggest this outburst occurred during MIS 3 and the flood volume could have contributed to surface ocean freshening and measurable meltwater signal in ð¿18O records. The manuscript is, for the most part, clearly written, and is well supported by figures. However, I have a number of concerns that need to be addressed before this manuscript is ready for publication.
We thank the reviewer for this positive appraisal of our manuscript. We have addressed the concerns of Reviewer 2 below.
1). The authors constrain the age of Glacial Lake Koroc using two 10Be cosmogenic exposure ages. My main concern is the small sample size and the possibility of inheritance in these samples. The presence of a cold-based ice sheet covering the study area is highlighted in the text (lines 293-296). It, therefore, seems equally plausible that the high elevation shorelines dated in this study could be one of the earliest stages of a post-LGM lake in the region and samples simply appear older due to inherited 10Be concentrations. The authors explore the possibility of erosion and burial of the sample site but no mention of inheritance in the samples is made.
We agree that the possibility of inheritance is an important question to address in our manuscript for the reader. We will add the following text to the methods section of the manuscript:
The lowest elevation wave-cut bench was sampled to avoid possible inheritance of cosmogenic nuclides. We believe that substantial inheritance is unlikely because the sampled wave-cut bench is 3 meters below the highest observed platform elevation (Figure 4C). Because there is a relationship between cosmogenic nuclide concentration and depth, we can calculate the expected inheritance at 3 meters depth.
We use the following equation,
,
where N is cosmogenic nuclide concentration, x is depth below the surface, is rock density, and is attenuation length. We assume density is 2.9 g/cm3 and attenuation length is 165 cm.
If we consider the maximum inheritance expected at the surface in the Torngat Mountains region to be 5.7x106 atoms/g quartz (340 ky equivalent) (Staiger et al., 2005), the maximum concentration at 3 m depth below surface elevation should be near 2.8x104 atoms/g quartz (4.5 ky exposure). If we do not use the maximum concentration found but use a more realistic value found for similar altitudes 1x106 atoms/g (0.9 ky exposure), the possible inheritance is within our external age uncertainty of 3 ky.
We have added an additional figure to show the sampled location on the wave-cut platforms and benches, which will now be included in Figure 4.
2). More detail is needed regarding the geomorphic/sedimentological evidence for pre-LGM Glacial Lake Koroc. No details regarding the mechanism of failure and evidence for a spillway that drained the lake are discussed, despite ‘outburst flood’ being in the title. During LGM ice recession, multiple glacial lakes occupied the Torngat Mountains. Could this not have been the case for pre-LGM and if so, what is the likelihood that this lake didn’t drain into another lower elevation lake nearby? Is it likely that the whole lake drained during the outburst event? Without the elevation of the spillway, this cannot be assessed.
We agree with the Reviewer that more detail is needed regarding the geomorphic sedimentological evidence for pre-LGM Glacial Lake Koroc. However, our study relies on legacy data from a 2003 field expedition, and we do not have data from additional shoreline sites. Attaining such information will be the focus of future field studies.
We agree that this point should be clarified in the text. We have added the following text:
There is no evidence that the lakes drained to lower elevation regions, as no lower elevation shorelines were identified. However partial or complete draining of the lake to lower elevation locations is possible. Future work focused on obtaining sediment cores in the valley could allow us to learn whether multiple lake levels occurred.
In response to this comment, and similar comments by reviewer 1, we have changed the wording in the title to “glacial lake” instead of “outburst flood” to be more cautious in our interpretation of evidence.
3). The main conclusion drawn in this study is the contribution an outburst flood from this lake could have had to surface ocean freshening and possible implications this may have had for Heinrich events.
‘A freshwater volume of 1.14x1012 m3, associated with the glacial lake outburst described in this
study could contribute to the large ð¿18O recorded for MIS 3 Heinrich events (minimum volume
required = 1.4x1013 -2.3 x1014 m3 ; Hemming, 2004).’
The minimum volume stated is from Hemming, 2004 is ‘a value assuming a volume the area of the Heinrich layers, and the thickness of the mixed layer is mixed one time with enough ice and water to make the d18O excursion’. However, my understanding of the paper is that 0.6 -1.9 Sv of water over 1 yr to 500 yrs is needed to explain the observed ð¿18O excursion. The estimate presented in this manuscript is 0.004x 106 over 3 days. This seems to be significantly less water than is needed to contribute to the ð¿18O excursion observed during a ~500 yrs of a typical Heinrich Event.
In response to these comments, we will edit the text to include the following explanation: “Pre-LGM Glacial Lake Koroc represents one freshwater source, which, in addition to other freshwater sources from other glacial lakes that may have coexisted in the region, could constitute a substantial freshwater input to the ocean.”
Line edits:
Line 24: Provide a value for the magnitude of freshwater flux in the abstract. Consider also adding the lake name to the abstract
According to this suggestion and that of reviewer 1, we have edited the abstract to include the name “pre-LGM glacial lake Koroc”. We will also add the estimate of freshwater flux to the abstract.
Line 27: Present freshwater flood volume in km3 rather than m3
Done!
Line 123: Delete space
Done!
Figure 3: Please add the drainage route to panel B. It would also be useful to add the location of mapped shorelines to this figure
Thank you for this suggestion. We have added the potential drainage route to this figure. We also note that the mapped shoreline site is shown by the white circle in Figure 3.
Line 206: Evidence for outlet needs to be clearly stated
To clarify evidence for the glacial lake inlet we will add the following text:
Evidence for outburst flooding is based on the rounded, imbricated cobbles in inlet channels. We found these deposits in multiple inlet channels leading to the sample site, which we interpreted as a lake shoreline. We hypothesize that the lake level may have fallen rapidly because there is no evidence for bands of lake shorelines at progressively lower elevations. Nevertheless, future work is required to verify such evidence for glacial outburst flooding.
Line 232/233: More information is needed regarding the duration of ice cover during the LGM. The uncertainty surrounding the 20 kyr ice cover is very briefly mentioned and needs to be more clearly stated
The issue of ice cover duration is addressed on lines 281 to 284:
Although the duration of ice cover at this site is unknown, there is evidence for ice cover at the LGM (Staiger et al., 2005), and this region likely deglaciated between 11 and 8 ka (Dalton et al., 2020). The age correction for ice burial depends on the timing of glaciation, which occurred after the existence of pre-LGM glacial lake Koroc. For example, if ice advanced over our site at 30 ka and the entire area was glaciated until 10 ka, then there would have been 20 ky of ice cover, which would shift the age to 56 ka. We therefore consider 36 3 ka to represent a minimum age, and 56 3 ka a likely age for the identified pre-LGM glacial lake Koroc.
We have added the following sentence in include a recent publication reviewing possible ice margin configurations during MIS 3:
“Although poorly constrained, Dalton et al., 2022 suggest the Laurentide Ice Sheet margin may have been to the east of Ungava Bay in the Torngat Mountains during MIS 3 (Dalton et al., 2022).”
- S. Dalton, C. R. Stokes, C. L. Batchelor, Evolution of the Laurentide and Innuitian ice sheets prior to the Last Glacial Maximum ( 115 ka to 25 ka ). Earth-Science Rev. 224, 103875 (2022).
To flag the question of ice cover duration in the methods section, we will add the following text:
“Uncertainty on ice cover duration will impact sample age constraints (see Discussion)”
Line 232: Add space ‘concentration( Gosse and Phillips, 2001)’
Done!
Line 235: Add space ‘calculation(Jones et al., 2019)’
Done!
Line 230: Add a brief statement to highlight that this is a minimum age
We have incorporated this suggestion now by adding the following text to line 230: “and thus represent a minimum age”.
Line 234: You describe the impact of GIA on your ages however no age GIA correct age is available to the reader. Consider adding these ages to the text and Table S2.
In the text we have explained that we do not account for GIA in the age correction since it is smaller than the uncertainty (line 235).
Line 206: ‘The shoreline is an erosional feature, and there is a small inlet channel at this elevation with rounded imbricated cobbles, suggestive of outburst flooding.’ Imbricated sediment can be produced by outburst events. However, is these are within a small inlet channel how do they suggest outburst flooding?
The reviewer is correct that the rounded, imbricated cobbles are not necessarily indicative of outburst flooding. We found these deposits in multiple inlet channels leading to the sample site and at other parts of the discontinuous eroded notch, which we interpreted in the field as a lake shoreline. We hypothesize that the lake level fell quite rapidly because we don’t see bands of lake shorelines at progressively lower elevations. Indeed, it is somewhat hard to imagine a lake margin at 890 m elevation dammed from the ocean by ice ending in any way other than a geologically near-instantaneous event. It is possible that these were simply not preserved. However, in the field, we believed that we saw spillway features similar in morphology to those at lower elevations in the region (those at lower elevation date to the ~8-9 ka) lake draining events that have been documented by (Dube-Loubert et al., 2018).
We hope that this report inspires further investigation of the features in this important field area. Unfortunately, we ran out of time for further investigations in 2003 and don’t want the fact that these features exist and have curious ages to be lost to the glacial geologic history. This discussion paper serves the purpose of making these features known so that the community can investigate in the future.
Line 263: Should glacial not be capitalized in ‘Pre-LGM glacial Lake Koroc”?
We will now capitalize this term throughout the manuscript.
Supplementary Material Line 18: Table S2 seems to be identical to the table above
The top table in Table S2 is repeated from Table S1c to include this information alongside the location information.
Citation: https://doi.org/10.5194/cp-2021-132-AC2 - AC8: 'Reply on RC2', Tamara Pico, 13 Jan 2022
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AC2: 'Reply on RC2', Tamara Pico, 10 Jan 2022
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CC2: 'Comment on cp-2021-132', Martin Roy, 10 Nov 2021
The manuscript of Pico et al. presents evidence for a glacial lake that developed before the LGM in the Koroc River valley and adjoining basins, to the west of the Torngat Mountains (NE Quebec-Labrador). It is then hypothesized that the drainage of this lake into the Labrador Sea may have contributed to climate perturbations (Heinrich events) during Marine Isotope Stage 3 – an interval of the last glacial cycle for which the authors claim to be characterized by a significantly reduced ice cover in the region formerly covered by the Laurentide ice sheet (LIS).
As glacial geologists working since several years on the deglaciation of the greater Ungava Bay region, we were extremely enthusiastic about this new piece of information. Indeed, the Labrador-Ungava region is a key area for the study of the evolution of the LIS during the last glacial cycle. Previous work in this region has showed the occurrence of several ice-dammed lakes that developed during the last deglaciation and their location near a critical sector of the North Atlantic makes freshwater forcings from this region highly relevant to paleoclimate studies. It is also a remote area difficult to access and as such, many aspects of its geological record remain to be studied.
The conclusions drawn in this study are mainly based on the assumption that a glacial lake existed and that it drained during MIS 3. The geological evidence presented in support to this MIS 3 ice-dammed lake and flood are:
1) a wave-cut bench (c.f., shoreline) at an elevation of 890 m used to reconstruct a high-elevation ice-dammed lake in the Koroc River valley;
2) an inlet channel filled with cobbles recording the abrupt drainage of the lake;
3) two cosmogenic ages from bedrock surfaces exposed by lakeshore erosion indicating an age of ~36 ka (MIS 3).
It appears that these landforms and 10Be ages come from a single site (as suggested by the geographic coordinates in the Table). Also, no geological evidence is presented for the damming mechanism of the lake, which is based on the presumed occurrence of the LIS margin in the lower reach of the Koroc River and valley glaciers in its upper reach.
Careful examination of the manuscript and data therein, and consideration of available geological maps (see below), leads us to conclude that, at this stage, the geomorphological evidence and geochronological data reported are unfortunately too fragmentary to support the existence of this glacial lake and outburst flood during MIS 3. More importantly, we believe that the interpretation of the geomorphological features reported may be incorrect, while the proposed ice margin configuration for the LIS and valley glaciers appears inconsistent with the reduced ice cover scenario presented for MIS 3 in the study.
We further expand these points in the paragraphs below, in addition to comment on geochronological approach used.
Geomorphological evidence for a pre-LGM glacial Lake Koroc and outburst flood
The evidence supporting the existence of this glacial lake is essentially enclosed in the two photographs presented in Figure 4. In Fig. 4A, it is not clear to what geomorphological feature(s) the authors refer to when “interpolating” a former lake level running up high across the valley. Despite the low resolution of the photo, an apparent trim line can be seen and from this distance, it seems to separate permafrost terrains (frost-shattered bedrock and felsenmeer) present on the high mountain top from the lower freshly glacially-eroded valley walls. In the foreground of this photo, it is also difficult to identify the geomorphological feature supposed to represent a former shoreline, but the action of freeze-thaw processes on the bedrock and deposits is clearly present with slopes partly covered by solifluction lobes dissected by meltwater channels.
The photo in 4B is supposed to show a wave-cut bench and bedrock surfaces exposed by lakeshore erosion. However, the entire feature lacks the continuity that would normally characterize a shoreline. Based on this photo, this setting could be interpreted as a frost-shattered erosional bedrock surface (felsenmeer), while the presence of the smoother rock surfaces on the gently sloping terrain may reflect erosion (block removal) by meltwater. Erosion of Archean gneissic rocks by lakeshore wave-action would require a lot of time, which is generally not the case with the short-lived ice-dammed lakes of Ungava-Labrador.
Given the importance of this glacial lake in the study, and considering the associated implications for paleogeographic reconstructions, the manuscript should better document the characteristics of the shorelines reported. An unanswered question concerns the number of sites with shorelines that have been documented? The manuscript reports currently one, although it is mentioned that “this glacial lake was identified based on geomorphological mapping of shorelines, spillways, and drainage channels” (Lines199-200). Indeed, these features form key indicators for identifying a glacial lake and the results from this mapping should be clearly presented to constrain the areal extent of the lake and build a proper case for its existence. A glacial lake of this importance should have left several geomorphological evidence of its presence in the study area, even in the context of preservation by a non-erosive cold-based ice.
Similarly, the claim of the “first direct evidence on land for an MIS 3 jokulhaup” (Lines 363 and 407) can be questioned. This outburst flood was identified on the basis of “a small inlet channel at this elevation with rounded, imbricated cobbles” (c.f. Fig. 4). Again, the geological support for this outburst flood should be better documented and the feature reported here should be better detailed, especially regarding its spatial relationship with respect to the lake configuration (location in the study area?).
As mentioned in the manuscript, the drainage of glacial lakes through outburst floods occurred during the last deglaciation in the Ungava-Labrador region. The nearby Lake Naskaupi indeed drained through outburst flooding events that left behind a wealth of large-scale geomorphological deposits and landforms, such as km-size ripples and other features like multi-tens of meters thick alluvial bars (Dubé-Loubert and Roy, 2017; Dubé-Loubert et al., 2018). Even if we assume that part of these features was eroded by the ice advance of the last glaciation (MIS 2), evidence for the outburst drainage of a lake the size of the hypothetical pre-LGM Lake Koroc should still be present in the geomorphological and sedimentary records of this region.
We note that insights on whether a pre-LGM glacial lake existed in the Koroc River valley may gained in part from two detailed Quaternary geology maps produced by the Geological Survey of Canada that cover the whole Koroc River drainage basin (Paradis and Parent, 2002a, 2002b). These maps do show evidence for glacial lake(s) at lower elevations in the Koroc River valley, but these glacial lake deposits and landforms (deltas and shorelines) are part of a morpho-sedimentary assemblage typical of the early Holocene deglaciation. Small and scarce occurrences of glacial lake deposits are found in places at higher elevations, but they likely relate to ephemeral lakes that were dammed by small retreating glacier tongues during the last deglaciation, as indicated by belts of minor moraine crests in these minor valleys. The maps also show that the high-elevation terrains of the study area consist essentially of bedrock exposures that are commonly blanketed and flanked by block fields (felsenmeer) produced by frost action, with abundant solifluction lobes – a setting reminiscent of the features presented in the photos of Figure 4. Given the ease of access to these maps (references below), this critical information should have been taken into consideration when attempting to reconstruct such a lake.
Configuration of the ice masses damming the lake
Accepting the premise that a pre-LGM glacial lake occupied the Koroc River valley also raises other important questions regarding the configuration and origin of the ice masses that dammed the lake at that time. Given the lack of geological data constraining ice-margin positions during MIS 3 in Labrador-Ungava, the authors use a series of assumptions about the ice dam locations.
Some of these are coherent with paleogeographic reconstructions of the last deglaciation, when the lower reach of the rivers flowing into southeastern Ungava Bay was dammed by the margin of the retreating Labrador Ice Dome – a damming mechanism that resulted in the accumulation of large amount of meltwater in river valleys such as the Koroc River. Although this scenario is consistent with the last deglaciation, it nonetheless involves a substantial ice mass located in north-central Quebec in order to have an ice margin extending down into the Ungava Bay lowlands. This ice margin would also have to be very thick and voluminous to hold the large meltwater body (high hydrostatic pressure) associated with a pre-LGM Lake Koroc with a surface-elevation at ~900 m. These considerations appear counterintuitive in the context of a drastically reduced ice cover for MIS 3 that is presented in the manuscript.
Damming a high-elevation glacial lake (~900 m) in this region also requires the presence of ice masses in the upper reaches of the Koroc River drainage watershed, without which, the lake would drain via different low-elevation outlets. To circumvent this problem, it is stipulated that several valley glaciers occupied low-lying topographic depressions to prevent excess meltwater to flow eastward across the continental divide separating the Ungava Bay and Labrador Sea watersheds. Unfortunately, no geological evidence is presented to support the existence of these valley glaciers. The manuscript refers to the work of Ives to support the existence of these valley glaciers in the Torngats, although it does not relate to old valley glaciations. Accordingly, the manuscript should build a better case for this aspect of the reconstruction.
Again, drawing comparisons based on the geological maps reported above, the deglacial (Holocene) geomorphology of the numerous secondary valleys of this region shows a record of small-scale moraines and ice-contact deposits that tend to suggest that these valley glaciers were perhaps not large enough to hold such a large lake. For these valley glaciers to be efficient for the damming of the pre-LGM Lake Koroc would imply larger valley glaciers and ice caps, which in turn calls for a large amount of ice above the Torngats at that time – a scenario inconsistent with the hypothesis of a significantly reduced ice cover presented for this time interval.
Geochronological constraints
An important limitation regarding the conclusions reached by the study relates to the fact the MIS 3 age assigned to the old glacial Lake Koroc is based on only two cosmogenic ages. This represents an extremely low number of geochronological constraints considering the method employed here and given the complexity and variety of terrains present in formerly glaciated environments.
For instance, several studies have documented complex arrangements of subglacial thermal conditions in the Labrador-Ungava region, which show evidence that high-elevation regions were likely characterized by at least a partial cold-based ice cover (Marquette et al., 2004; Staiger et al., 2005; Rice et al., JQS 2019; Dubé-Loubert et al., Geomorph. 2021). Although this provides a setting for a non-erosive ice cover and thus the preservation of old geomorphological features, it does warrant caution in the interpretation of cosmogenic ages, which may be affected by inheritance signal from previous exposures and result with age overestimates.
However, the manuscript presents a different interpretation for the two pre-deglaciation 10Be ages obtained. The favored age of ~56 ± 3 ka derives from a correction applied to the two samples to take into account a period of burial by ice; a plausible choice of calculation and consistent with the hypothesis presented in the study. Still, considering the inheritance problem inherent to high elevation terrains of this region, it would have been essential to carry out paired isotope (Al/Be) measurements to characterize the history of ice cover of this site – an approach that would greatly facilitate the interpretation of the results. In addition, as for the geomorphological evidence for the lake, it would have been important to present 10Be ages from more than one site in order to support the deductions made on the basis of the two geochronological constraints.
Summary
We believe the manuscript presents an interesting hypothesis regarding the possibility of a pre-LGM glacial lake to the east of the Torngat Mountains and the potential contribution of its drainage to the forcing mechanisms responsible for the climate variability (Heinrich events) of MIS 3. Unfortunately, the geomorphological evidence and geochronological data supporting the existence of a glacial Lake Koroc during MIS 3 are too fragmentary and unconvincing at the moment, while the proposed configuration of the damming LIS margins and valley glaciers appears inconsistent with the scenario of significantly reduced ice cover presented. As stated at numerous places in the manuscript, we concur with the authors that further fieldwork is required to convincingly show that old (relict) geomorphological features are present on high-elevation terrains of Labrador-Ungava. At this point we would simply urge caution in using such a small amount of geological evidence to claim the existence of a glacial lake and outburst flood, especially when using the reconstruction to constrain the ice margin of the Laurentide ice sheet during MIS 3 in one of its core sectors.
Martin Roy – Department of Earth and atmospheric sciences / Geotop Research Center, University of Quebec at Montreal, Montréal (QC), Canada.
Hugo Dubé-Loubert – Quebec Ministry of Energy and Natural Resources, Val d’Or (QC), Canada.
References for the maps by the Geological Survey of Canada.
Quaternary geology of the Koroc River in Quebec-Newfoundland and Labrador (eastern and western map sheets 2013A and 2013A, scale 1:125,000).
Paradis, SJ and Parent, M, 2002. Géologie des formations en surface, Rivière Koroc (moitié est), Québec-Terre-Neuve-et-Labrador, Commission Géologique du Canada, Carte 2013A, échelle 1 :125,000. (available on https://geoscan.nrcan.gc.ca)
Paradis, SJ and Parent, M, 2002. Géologie des formations en surface, Rivière Koroc (moitié ouest), Québec-Terre-Neuve-et-Labrador, Commission Géologique du Canada, Carte 2014A, échelle 1 :125,000). (available on https://geoscan.nrcan.gc.ca)
Citation: https://doi.org/10.5194/cp-2021-132-CC2 -
AC4: 'Reply on CC2', Tamara Pico, 10 Jan 2022
Community Comment # 2
The manuscript of Pico et al. presents evidence for a glacial lake that developed before the LGM in the Koroc River valley and adjoining basins, to the west of the Torngat Mountains (NE Quebec-Labrador). It is then hypothesized that the drainage of this lake into the Labrador Sea may have contributed to climate perturbations (Heinrich events) during Marine Isotope Stage 3 – an interval of the last glacial cycle for which the authors claim to be characterized by a significantly reduced ice cover in the region formerly covered by the Laurentide ice sheet (LIS).
As glacial geologists working since several years on the deglaciation of the greater Ungava Bay region, we were extremely enthusiastic about this new piece of information. Indeed, the Labrador-Ungava region is a key area for the study of the evolution of the LIS during the last glacial cycle. Previous work in this region has showed the occurrence of several ice-dammed lakes that developed during the last deglaciation and their location near a critical sector of the North Atlantic makes freshwater forcings from this region highly relevant to paleoclimate studies. It is also a remote area difficult to access and as such, many aspects of its geological record remain to be studied.
The conclusions drawn in this study are mainly based on the assumption that a glacial lake existed and that it drained during MIS 3. The geological evidence presented in support to this MIS 3 ice-dammed lake and flood are:
1) a wave-cut bench (c.f., shoreline) at an elevation of 890 m used to reconstruct a high-elevation ice-dammed lake in the Koroc River valley;
2) an inlet channel filled with cobbles recording the abrupt drainage of the lake;
3) two cosmogenic ages from bedrock surfaces exposed by lakeshore erosion indicating an age of ~36 ka (MIS 3).
It appears that these landforms and 10Be ages come from a single site (as suggested by the geographic coordinates in the Table). Also, no geological evidence is presented for the damming mechanism of the lake, which is based on the presumed occurrence of the LIS margin in the lower reach of the Koroc River and valley glaciers in its upper reach.
Careful examination of the manuscript and data therein, and consideration of available geological maps (see below), leads us to conclude that, at this stage, the geomorphological evidence and geochronological data reported are unfortunately too fragmentary to support the existence of this glacial lake and outburst flood during MIS 3. More importantly, we believe that the interpretation of the geomorphological features reported may be incorrect, while the proposed ice margin configuration for the LIS and valley glaciers appears inconsistent with the reduced ice cover scenario presented for MIS 3 in the study.
We further expand these points in the paragraphs below, in addition to comment on geochronological approach used.
We thank the comment’s authors for their constructive feedback on our manuscript.
Geomorphological evidence for a pre-LGM glacial Lake Koroc and outburst flood
The evidence supporting the existence of this glacial lake is essentially enclosed in the two photographs presented in Figure 4. In Fig. 4A, it is not clear to what geomorphological feature(s) the authors refer to when “interpolating” a former lake level running up high across the valley. Despite the low resolution of the photo, an apparent trim line can be seen and from this distance, it seems to separate permafrost terrains (frost-shattered bedrock and felsenmeer) present on the high mountain top from the lower freshly glacially-eroded valley walls. In the foreground of this photo, it is also difficult to identify the geomorphological feature supposed to represent a former shoreline, but the action of freeze-thaw processes on the bedrock and deposits is clearly present with slopes partly covered by solifluction lobes dissected by meltwater channels.
There are solifluction lobes in this region, however these tend to be present in the valleys where there is abundant unconsolidated material. These are not present in the forefront of the photo of the wave-cut shoreline. The noted shoreline is not in line with any fracture plane, and because it is eroded into bedrock it cannot be a solifluction or gelifluction lobe. For other glacial erosion processes we would not expect to see such a planar surface. Therefore, we interpret this geomorphic feature as a shoreline platform.
The photo in 4B is supposed to show a wave-cut bench and bedrock surfaces exposed by lakeshore erosion. However, the entire feature lacks the continuity that would normally characterize a shoreline. Based on this photo, this setting could be interpreted as a frost-shattered erosional bedrock surface (felsenmeer), while the presence of the smoother rock surfaces on the gently sloping terrain may reflect erosion (block removal) by meltwater. Erosion of Archean gneissic rocks by lakeshore wave-action would require a lot of time, which is generally not the case with the short-lived ice-dammed lakes of Ungava-Labrador.
Although the bedrock in our study region is gneiss, it is highly fractured rock. Frost action and ice wedging acts on preexisting pervasive fractures, resulting in a more easily weathered rock. Studies based on glaciated gneiss terrains suggest that such fractured rock results in plucking-dominated erosion, which is likely for our study area (Krabbendam & Bradwell, 2014)
- Krabbendam, T. Bradwell, Quaternary evolution of glaciated gneiss terrains : pre-glacial weathering vs . glacial erosion. Quat. Sci. Rev. 95, 20–42 (2014).
Given the importance of this glacial lake in the study, and considering the associated implications for paleogeographic reconstructions, the manuscript should better document the characteristics of the shorelines reported. An unanswered question concerns the number of sites with shorelines that have been documented? The manuscript reports currently one, although it is mentioned that “this glacial lake was identified based on geomorphological mapping of shorelines, spillways, and drainage channels” (Lines199-200). Indeed, these features form key indicators for identifying a glacial lake and the results from this mapping should be clearly presented to constrain the areal extent of the lake and build a proper case for its existence. A glacial lake of this importance should have left several geomorphological evidence of its presence in the study area, even in the context of preservation by a non-erosive cold-based ice.
Future fieldwork should be aimed at assessing the areal extent of this lake. We agree that there are likely other localities where this glacial lake shoreline can be documented, and such investigation will be the subject of future inquiry.
Similarly, the claim of the “first direct evidence on land for an MIS 3 jokulhaup” (Lines 363 and 407) can be questioned. This outburst flood was identified on the basis of “a small inlet channel at this elevation with rounded, imbricated cobbles” (c.f. Fig. 4). Again, the geological support for this outburst flood should be better documented and the feature reported here should be better detailed, especially regarding its spatial relationship with respect to the lake configuration (location in the study area?).
In response to this comment, and similar comments by reviewers, we will edit the text to emphasize evidence for a glacial lake shoreline, rather than an outburst flood.
As mentioned in the manuscript, the drainage of glacial lakes through outburst floods occurred during the last deglaciation in the Ungava-Labrador region. The nearby Lake Naskaupi indeed drained through outburst flooding events that left behind a wealth of large-scale geomorphological deposits and landforms, such as km-size ripples and other features like multi-tens of meters thick alluvial bars (Dubé-Loubert and Roy, 2017; Dubé-Loubert et al., 2018). Even if we assume that part of these features was eroded by the ice advance of the last glaciation (MIS 2), evidence for the outburst drainage of a lake the size of the hypothetical pre-LGM Lake Koroc should still be present in the geomorphological and sedimentary records of this region.
Future work could focus on assessing the nature of pre-LGM glacial lakes and potential outburst floods in this region. For example, sediment cores sampling the Koroc valley may provide clues for the history of past lake levels.
In response to this comment and that of Reviewer 1, we have added the following text:
There is no evidence that the lakes drained to lower elevation regions, as no lower elevation shorelines were identified. However partial or complete draining of the lake to lower elevation locations is possible. Future work focused on obtaining sediment cores in the valley could allow us to learn whether multiple lake levels occurred.
We note that insights on whether a pre-LGM glacial lake existed in the Koroc River valley may gained in part from two detailed Quaternary geology maps produced by the Geological Survey of Canada that cover the whole Koroc River drainage basin (Paradis and Parent, 2002a, 2002b). These maps do show evidence for glacial lake(s) at lower elevations in the Koroc River valley, but these glacial lake deposits and landforms (deltas and shorelines) are part of a morpho-sedimentary assemblage typical of the early Holocene deglaciation. Small and scarce occurrences of glacial lake deposits are found in places at higher elevations, but they likely relate to ephemeral lakes that were dammed by small retreating glacier tongues during the last deglaciation, as indicated by belts of minor moraine crests in these minor valleys. The maps also show that the high-elevation terrains of the study area consist essentially of bedrock exposures that are commonly blanketed and flanked by block fields (felsenmeer) produced by frost action, with abundant solifluction lobes – a setting reminiscent of the features presented in the photos of Figure 4. Given the ease of access to these maps (references below), this critical information should have been taken into consideration when attempting to reconstruct such a lake.
We are aware of these geologic maps, and these maps were available for earlier studies in this region conducted by one of the coauthors. For these higher elevation regions, where the study site is located, these geologic maps are largely based on analyzing airphotos. The features are hard to see in airphotos. Ground truthing through field mapping could help to better document these features in published geologic maps.
Configuration of the ice masses damming the lake
Accepting the premise that a pre-LGM glacial lake occupied the Koroc River valley also raises other important questions regarding the configuration and origin of the ice masses that dammed the lake at that time. Given the lack of geological data constraining ice-margin positions during MIS 3 in Labrador-Ungava, the authors use a series of assumptions about the ice dam locations.
Some of these are coherent with paleogeographic reconstructions of the last deglaciation, when the lower reach of the rivers flowing into southeastern Ungava Bay was dammed by the margin of the retreating Labrador Ice Dome – a damming mechanism that resulted in the accumulation of large amount of meltwater in river valleys such as the Koroc River. Although this scenario is consistent with the last deglaciation, it nonetheless involves a substantial ice mass located in north-central Quebec in order to have an ice margin extending down into the Ungava Bay lowlands. This ice margin would also have to be very thick and voluminous to hold the large meltwater body (high hydrostatic pressure) associated with a pre-LGM Lake Koroc with a surface-elevation at ~900 m. These considerations appear counterintuitive in the context of a drastically reduced ice cover for MIS 3 that is presented in the manuscript.
Damming a high-elevation glacial lake (~900 m) in this region also requires the presence of ice masses in the upper reaches of the Koroc River drainage watershed, without which, the lake would drain via different low-elevation outlets. To circumvent this problem, it is stipulated that several valley glaciers occupied low-lying topographic depressions to prevent excess meltwater to flow eastward across the continental divide separating the Ungava Bay and Labrador Sea watersheds. Unfortunately, no geological evidence is presented to support the existence of these valley glaciers. The manuscript refers to the work of Ives to support the existence of these valley glaciers in the Torngats, although it does not relate to old valley glaciations. Accordingly, the manuscript should build a better case for this aspect of the reconstruction.
Again, drawing comparisons based on the geological maps reported above, the deglacial (Holocene) geomorphology of the numerous secondary valleys of this region shows a record of small-scale moraines and ice-contact deposits that tend to suggest that these valley glaciers were perhaps not large enough to hold such a large lake. For these valley glaciers to be efficient for the damming of the pre-LGM Lake Koroc would imply larger valley glaciers and ice caps, which in turn calls for a large amount of ice above the Torngats at that time – a scenario inconsistent with the hypothesis of a significantly reduced ice cover presented for this time interval.
We agree with the comment authors that MIS 3 ice margin locations in this region are poorly constrained. It is plausible that evidence from pre-LGM glacial lakes in this region could appear quite differently in the geological record compared with deglacial lake records. Unlike pre-LGM features, deglacial geomorphic features are more likely to be preserved as they would not have been eroded by advancing and retreating ice cover.
While we do not report evidence for ice margin locations damming pre-LGM Glacial Lake Koroc, future fieldwork could be tasked with mapping out the extent of pre-LGM glacial lake shorelines in this region, which would help bound the ice-dammed locations of this lake.
Since we are unable to make a stronger case for the existence of valley glaciations (or other ice margin locations) barring additional field data collection, we make assumptions for the locations of ice-dammed glacial lake boundaries.
In response to these comments, and those by Reviewer 1, we performed additional calculations that vary ice margin location to explore the uncertainty on estimated lake volume, and we will add the following text:
The total lake volume depends strongly on the assumed ice margin positions that bound the glacial lake. Given the uncertainty on the ice-dammed locations of this lake, we explored three possible scenarios varying the assumed southern position of the ice margin that bound pre-LGM Glacial Lake Koroc. For the intermediate scenario, the southern ice margin boundary is at 58 N, resulting in an estimated lake volume of 1.14x1012 m3. For the smaller lake scenario, we shift the assumed ice margin boundary northward from 58 N to 58.25 N (~28 km), which reduces the estimated lake volume from 1.14x1012 m3 to 7.62x1011 m3 (reduced by ~30%). For the larger lake scenario, shifting the assumed ice margin boundary southward from 58 N to 57.75 N increases the estimated lake volume 1.14x1012 m3 to 1.44x1012 m3 (increased by ~25%).
Geochronological constraints
An important limitation regarding the conclusions reached by the study relates to the fact the MIS 3 age assigned to the old glacial Lake Koroc is based on only two cosmogenic ages. This represents an extremely low number of geochronological constraints considering the method employed here and given the complexity and variety of terrains present in formerly glaciated environments.
For instance, several studies have documented complex arrangements of subglacial thermal conditions in the Labrador-Ungava region, which show evidence that high-elevation regions were likely characterized by at least a partial cold-based ice cover (Marquette et al., 2004; Staiger et al., 2005; Rice et al., JQS 2019; Dubé-Loubert et al., Geomorph. 2021). Although this provides a setting for a non-erosive ice cover and thus the preservation of old geomorphological features, it does warrant caution in the interpretation of cosmogenic ages, which may be affected by inheritance signal from previous exposures and result with age overestimates.
However, the manuscript presents a different interpretation for the two pre-deglaciation 10Be ages obtained. The favored age of ~56 ± 3 ka derives from a correction applied to the two samples to take into account a period of burial by ice; a plausible choice of calculation and consistent with the hypothesis presented in the study. Still, considering the inheritance problem inherent to high elevation terrains of this region, it would have been essential to carry out paired isotope (Al/Be) measurements to characterize the history of ice cover of this site – an approach that would greatly facilitate the interpretation of the results. In addition, as for the geomorphological evidence for the lake, it would have been important to present 10Be ages from more than one site in order to support the deductions made on the basis of the two geochronological constraints.
We are in full agreement with the authors of this comment that an accurate age for this glacial lake shoreline feature will require future analysis that incorporates paired cosmogenic isotope systems to derive a burial history for this site.
Summary
We believe the manuscript presents an interesting hypothesis regarding the possibility of a pre-LGM glacial lake to the east of the Torngat Mountains and the potential contribution of its drainage to the forcing mechanisms responsible for the climate variability (Heinrich events) of MIS 3. Unfortunately, the geomorphological evidence and geochronological data supporting the existence of a glacial Lake Koroc during MIS 3 are too fragmentary and unconvincing at the moment, while the proposed configuration of the damming LIS margins and valley glaciers appears inconsistent with the scenario of significantly reduced ice cover presented. As stated at numerous places in the manuscript, we concur with the authors that further fieldwork is required to convincingly show that old (relict) geomorphological features are present on high-elevation terrains of Labrador-Ungava. At this point we would simply urge caution in using such a small amount of geological evidence to claim the existence of a glacial lake and outburst flood, especially when using the reconstruction to constrain the ice margin of the Laurentide ice sheet during MIS 3 in one of its core sectors.
We thank the authors of this comment for their positive remarks and constructive comments. We hope that this report inspires further investigation of the features in this important field area. Unfortunately, we ran out of time for further investigations in 2003 and don’t want the fact that these features exist to be lost to the glacial geologic history. This discussion paper serves the purpose of making these features known so that the community can investigate in the future.
Citation: https://doi.org/10.5194/cp-2021-132-AC4
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AC4: 'Reply on CC2', Tamara Pico, 10 Jan 2022
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CC3: 'Comment on cp-2021-132', Jessey Rice, 24 Nov 2021
Pico et al. present an interesting discussion regarding a glacial lake reconstruction and outburst flood they attribute to a pre-LGM (MIS3) period. This reconstruction is supported by 10Be age calcualations from a bedrock upland and glacial lake volume calculations. As previous commenters have mentioned this region is of critical importance for freshwater forcing and represents a region where many outstanding questions regarding ice sheet dynamics and deglaciation persist.
As other commenters have presented detailed disucssions, I will keep my comment brief, however, I do share the concerns of the previous commenters and support the issues they have highlighted. Specifically, there needs to be more evidence to convince the reader that the wave-cut bench shorelines are indeed shorelines and that the rounded, imbricated cobbles are indeed the result of an outburst flood and not simply from subglacial meltwater flow, which could have removed any till cover on the bedrock (i.e., a vanished protector: Veillette and Roy, 1995; https://doi.org/10.4095/202923) which would affect the inheritance of the bedrock sample. Additionally, as previously mentioned, a more discussion on how the glacial lake was reconstructed and possible inheritance from the 10Be should be explored as other suggested hypotheses seem plausible.
I can appreciate the limited dataset available to the authors, which they fully acknowledge, calling for additional fieldwork in the region. However, until additional data is available, either other hypotheses should be explored, or additional discussions to address the issues presented by other commenters is needed. Again, I fully appreciate the difficulty of this type of investigation and I do hope this discussion sparks future work in this remote region of Canada where little fieldwork has been conducted.
Jessey Rice- Geological Survey of Canada, Ottawa
Citation: https://doi.org/10.5194/cp-2021-132-CC3 -
AC5: 'Reply on CC3', Tamara Pico, 10 Jan 2022
Community Comment # 3
Pico et al. present an interesting discussion regarding a glacial lake reconstruction and outburst flood they attribute to a pre-LGM (MIS3) period. This reconstruction is supported by 10Be age calculations from a bedrock upland and glacial lake volume calculations. As previous commenters have mentioned this region is of critical importance for freshwater forcing and represents a region where many outstanding questions regarding ice sheet dynamics and deglaciation persist.
As other commenters have presented detailed disucssions, I will keep my comment brief, however, I do share the concerns of the previous commenters and support the issues they have highlighted. Specifically, there needs to be more evidence to convince the reader that the wave-cut bench shorelines are indeed shorelines and that the rounded, imbricated cobbles are indeed the result of an outburst flood and not simply from subglacial meltwater flow, which could have removed any till cover on the bedrock (i.e., a vanished protector: Veillette and Roy, 1995; https://doi.org/10.4095/202923) which would affect the inheritance of the bedrock sample. Additionally, as previously mentioned, a more discussion on how the glacial lake was reconstructed and possible inheritance from the 10Be should be explored as other suggested hypotheses seem plausible.
I can appreciate the limited dataset available to the authors, which they fully acknowledge, calling for additional fieldwork in the region. However, until additional data is available, either other hypotheses should be explored, or additional discussions to address the issues presented by other commenters is needed. Again, I fully appreciate the difficulty of this type of investigation and I do hope this discussion sparks future work in this remote region of Canada where little fieldwork has been conducted.
Jessey Rice- Geological Survey of Canada, Ottawa
We thank the comment author for their positive comments and thoughtful feedback. As we stress earlier in our response letter, our study relies on legacy data from a 2003 field expedition, and we do not have data from additional shoreline sites. We nevertheless pushed forward with publishing these data to draw attention to this work, and to highlight the exciting potential for several facets of Quaternary work, including constraining the configuration of pre-LGM ice sheets, understanding glacial inception, as well as quantifying the timing and amount of freshwater release into the North Atlantic. We have stressed in our current study that attaining such information will be the focus of future field studies.
In response to these comments, and similar ones raised by reviewers and other community comments, we will add the following text:
“Till cover would cause the age assigned to the glacial lake shoreline to be older”
“Evidence for outburst flooding is based on the rounded, imbricated cobbles in inlet channels. We found these deposits in multiple inlet channels leading to the sample site, which we interpreted as a lake shoreline. We hypothesize that the lake level may have fallen rapidly because there is no evidence for bands of lake shorelines at progressively lower elevations. Nevertheless, future work is required to verify such evidence for glacial outburst flooding”
In the methods section we will add the following text:
The lowest elevation wave-cut bench was sampled to avoid possible inheritance of cosmogenic nuclides. We believe that substantial inheritance is unlikely because the sampled wave-cut bench is 3 meters below the highest observed platform elevation (Figure 4C). Because there is a relationship between cosmogenic nuclide concentration and depth, we can calculate the expected inheritance at 3 meters depth.
We use the following equation,
,
where N is cosmogenic nuclide concentration, x is depth below the surface, is rock density, and is attenuation length. We assume density is 2.9 g/cm3 and attenuation length is 165 cm.
If we consider the maximum inheritance expected at the surface in the Torngat Mountains region to be 5.7x106 atoms/g quartz (340 ky equivalent) (Staiger et al., 2005), the maximum concentration at 3 m depth below surface elevation should be near 2.8x104 atoms/g quartz (4.5 ky exposure). If we do not use the maximum concentration found but use a more realistic value found for similar altitudes 1x106 atoms/g (0.9 ky exposure), the possible inheritance is within our external age uncertainty of 3 ky.
Citation: https://doi.org/10.5194/cp-2021-132-AC5 - AC9: 'Reply on CC3', Tamara Pico, 13 Jan 2022
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AC5: 'Reply on CC3', Tamara Pico, 10 Jan 2022
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CC4: 'Comment on cp-2021-132', Jakob Heyman, 26 Nov 2021
Data comments:
* The longitude of the samples given in Table S1 (-65.79) appear to be located too far west. Based on Figure 3, the longitude should be something around -64.56. If you have more detailed coordinates for the samples it would be good to include them.
* The 10Be concentration uncertainties in Table S1 appear to be too small based on the internal exposure age uncertainties in Table S1. Should they perhaps be multiplied by 10?
Citation: https://doi.org/10.5194/cp-2021-132-CC4 -
AC6: 'Reply on CC4', Tamara Pico, 10 Jan 2022
Community Comment #4
Data comments:
* The longitude of the samples given in Table S1 (-65.79) appear to be located too far west. Based on Figure 3, the longitude should be something around -64.56. If you have more detailed coordinates for the samples it would be good to include them.
Thank you for catching this mistake. When we realized this error, we attempted to replace the supplementary text, however it can only be replaced in the revision. The revised manuscript will reflect this change.
* The 10Be concentration uncertainties in Table S1 appear to be too small based on the internal exposure age uncertainties in Table S1. Should they perhaps be multiplied by 10?
Thanks for spotting this. The correct uncertainties are 1.038E+04 and 1.046E+04. The uncertainties in the AMS ratio were correct. The correct nuclide concentration uncertainties were used in the CRONUS input. We will update the Table.
Citation: https://doi.org/10.5194/cp-2021-132-AC6
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AC6: 'Reply on CC4', Tamara Pico, 10 Jan 2022
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EC1: 'Responding to comments/reviews', Alberto Reyes, 29 Nov 2021
Dear Dr. Pico,
The public discussion period for your manuscript is still open for another ~two weeks. But all formal peer-reviews have been submitted now. While waiting for the discussion period to close, you and your co-authors can certainly respond to the reviews and comments in the interactive discussion.
Sincerely,
Alberto
Citation: https://doi.org/10.5194/cp-2021-132-EC1 -
EC2: 'Editor Comment', Alberto Reyes, 19 Dec 2021
Dear Dr Pico and co-authors,
The discussion period for your manuscript is now over. The two formal reviewers and commenters identifed several key points of contention, with (at least in my opinion) the most important being:
-impact of nuclide inheritance and exposure/erosion history on interpretation of the 10Be data
-low number of dated samples
-unclear or under-described geomorphic evidence for the ice-dammed lake geometry and drainage
I agree with the reviewers/commenters that these issues raise important questions about the interpretations in the manuscript. But I'd like to give you the opportunity to address the criticisms, by responding to the reviews and comments in the interactive discussion. This does not yet require preparation of a revised manuscript, but rather indicate how you would respond to the criticisms and suggestions.
Sincerely,
Alberto (handling editor)
Citation: https://doi.org/10.5194/cp-2021-132-EC2 -
AC7: 'Reply on EC2', Tamara Pico, 10 Jan 2022
We thank the editor, two reviewers, and four community comment authors for their positive and constructive comments in regard to the submitted manuscript.
In responding to these reviews, we have noted how we will revise the manuscript to improve the clarity of our arguments. In particular, we have performed additional calculations to explore the uncertainty on estimated lake volume, we have created a new figure contextualizing the sample location, and we have added text to provide additional geomorphic observations for glacial lake shoreline evidence. Furthermore, we have added text describing alternate interpretations of our dataset. We will incorporate all minor text edit suggestions made by the reviewers. We believe incorporating these comments in this revision will strengthen our manuscript.
Our study relies on legacy data from a 2003 field expedition, and we do not have data from additional shoreline sites or paired cosmogenic isotopes. We nevertheless pushed forward with publishing these data to draw attention to this work, and to highlight the exciting potential for several facets of Quaternary work, including constraining the configuration of pre-LGM ice sheets, understanding glacial inception, as well as quantifying the timing and amount of freshwater release into the North Atlantic. We have stressed in our current study that attaining such information will be the focus of future field studies.
In the following, we address each of the comments raised in the reviews and provide a detailed listing of the associated revisions to the text. We intersperse the reviewers’ comments (black font) with our responses (in italic black font). Line numbers correspond to the original submitted version of the manuscript.
Citation: https://doi.org/10.5194/cp-2021-132-AC7
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AC7: 'Reply on EC2', Tamara Pico, 10 Jan 2022
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