the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Mar 2024
Late Quaternary glacial maxima in Southern Patagonia: insights from the Lago Argentino glacier lobe
Abstract. Determining the timing and extent of Quaternary glaciations around the globe is critical to understanding the drivers behind climate change and glacier fluctuations. Despite synchronous ice-volume and extent change across hemispheres, evidence from the southern mid-latitudes indicates that local glacial maxima occurred earlier in the glacial cycle, preceding the global Last Glacial Maximum (LGM), implying that feedbacks in the climate system or ice dynamics played a role beyond the underlying orbital parameters. To shed light on these processes, we investigated the glacial landforms shaped by the Lago Argentino glacier (50° S), an outlet lobe of the former Patagonian Ice Sheet in southern Argentina, during the last two glacial cycles. We mapped geomorphological features on the landscape and dated moraine boulders and outwash sediments using 10Be cosmogenic nuclides and feldspar infrared stimulated luminescence (IRSL) to constrain the chronology of glacial advance and retreat. We, therefore, provide the first published age constraints on the middle-to-late Pleistocene glaciations at Lago Argentino, and report that this outlet lobe expanded during Marine Isotope Stage 6, at 153.0 ± 14.7 ka, and during Marine Isotope Stage 3, culminating at 44.5 ± 8.0 ka and at 36.6 ± 1.0 ka. Our results indicate that the most recent was its most extensive advance during the last glacial period, and hypothesize that this was a result of longer and colder winters, as well as increased precipitation delivered by a northward migration of the Southern Westerly Winds belt, highlighting the role of local and regional climate feedbacks in driving ice mass changes in the southern mid-latitudes.
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Status: open (until 21 May 2024)
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RC1: 'Comment on cp-2024-24', Andrew Hein, 10 Apr 2024
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This is an interesting and valuable manuscript that introduces new geomorphological mapping and geochronology (cosmogenic 10Be and IRSL dating) that firmly establishes the timing of the local Last Glacial Maximum (LGM) of the Lago Argentino glacier lobe. This glacier lobe reached its maximum during Marine Isotope Stage (MIS) 3, at about 45 ka and again at about 37 ka, rather than during the global LGM, which occurred more recently during MIS 2. It also shows that there are older glacial moraines preserved in the valley that could indicate a more extensive advance dating to the penultimate MIS 6 glaciation (~160 ka), but that is less certain as I will discuss later. The authors find no evidence for glacial advances in the valley that correspond in timing to the global LGM and suggest such deposits could be less extensive and therefore remain preserved beneath the present lake. The overall timing of the local maximum is quite similar to other valleys in southern Patagonia, such as the Torres del Paine and Ultima Esperanza valleys just to the south. The authors suggest ideas as to why the local LGM occurred earlier than the global LGM, inferring more intense winters and a northward expansion of the southern westerly winds (SWW) causing an increase in precipitation during MIS 3.
The manuscript makes a valuable contribution to knowledge on glacier behaviour during the last glacial cycle in a part of Patagonia where this data is currently limited. The paper's thrust tackles the local LGM, and the data presented supports the interpretations made. However, the interpretations made regarding the age of the older moraine system are, in my view, not supported by the data. I would suggest some changes to this part of the manuscript before publication. When those changes are made, I would recommend publication of the manuscript.
Main Revisions
The cosmogenic 10Be ages from moraine boulders on the Arroyo Verde II moraines are few and widely scattered. Four exposure ages range from 132-243 ka. While two of the exposure ages are similar (~160 ka), the data are insufficient to conclude that this moraine was formed during MIS 6. It may well prove to be a correct age assignment, but the current data are not sufficient evidence. I recommend the manuscript is changed to indicate that this age assignment is no more than tentative, given the paucity of data to support the age. This will require changes in the abstract, conclusions and throughout the body of the text (e.g., section 5.3) and figures (e.g., Figure 6) where the MIS 6 age is currently indicated as the definitive age of the advance.
There has been a significant amount of published work in similarly dry environments in neighbouring valleys within Argentine Patagonia demonstrating that outwash terrace surfaces are better targets for exposure dating of “old” glacial advances (i.e., pre-last glacial cycle) than corresponding moraine limits, which tend have scattered and too-young exposure ages (e.g., Hein et al., 2009; 2011; 2017; Darvill et al., 2015; Mendelova et al., 2020; Leger et al., 2023). In the Lago Pueyrredón and Lago Buenos Aires valleys’, the outwash terraces linked directly to (and in between) moraines with MIS 6 boulder ages, instead had MIS 8 outwash cobble ages (i.e., 100 ka older), suggesting moraine degradation had led to erroneously young boulder exposure ages (Hein et al., 2009; 2017). In a more extreme case, moraine boulders in the Lago Pueyrredón valley with exposure ages mostly between 190 – 230 ka, but with a single 600 ka outlier, had corresponding outwash ages of ~600 ka, consistent with the age of the oldest boulder outlier (suggesting all of the other moraine boulders were too young). Given this regional context from neighbouring valleys, it could be the case that the 243 ka boulder age in this study is closer to the age of the Arroyo Verde II moraine. In other words, it may very well be that the Arroyo Verde II moraine was deposited during MIS 8 instead of MIS 6, much like the “Hatcher” and “Moreno” moraine systems in the Lago Pueyrredón and Lago Buenos Aires valleys a little further north (47S) in central Patagonia. The revisions to the present manuscript should leave open the possibility that the sampled moraines could indeed be older. Moving forward, this could be tested by increasing the dataset and targeting the outwash cobbles. If both outwash and moraine boulders give MIS 6 ages, then the uncertainty in age assignment would be greatly reduced (e.g., Leger et al., 2023).
Suggested minor changes
Abstract:L0-5: “Despite synchronous ice-volume and extent change across hemispheres, evidence from the southern mid-latitudes indicates that local glacial maxima occurred earlier in the glacial cycle” – this sentence appears to contradict itself. Suggest re-writing.
L35: It’s not necessarily that they are poorly preserved to the west, but the LGM ice sheet probably terminated in the sea, so fewer moraines exist to the west.
L40: Capitalise “Late” Glacial
L115: I have sampled outwash cobbles in the Lago Argentino valley, and looked at several boulders. Many of these were fluted and ventifacted as a consequence of being downwind of the dust source (outwash plains). I would imagine this type of aeolian erosion would be most severe in the centre of the valley. Was this observed? Did the samples collected have evidence for aeolian erosion?
- Please indicate whether such aeolian erosion is present (and its severity) at an appropriate point in the manuscript.
- Please also note any evidence for moraine degradation at the sample sites (I note from Figure 3I that boulder AV-01 appears to have ~50 cm of recent exhumation). Is this common across the sampling sites?
L131: I would encourage aborting use the 1.4 mm ka-1 erosion rate, but instead use the 0.2 mm ka-1 rate from Douglass et al., 2007. Kaplan always meant for the 1.4 rate to indicate a “maximum” erosion rate, but it seems to have been adopted as “the” erosion rate in a lot of literature – check discussion on this topic in Hein et al. (2017). If it is used, please explicitly indicate this is a maximum erosion rate.
L175: state how far west of Arroyo Verde I in km (for consistency with subsequent description)
L175: It is not clear from where the elevations of the moraines are taken from, the terminal moraine elevation? The maximum height in the mapping area east of the lake? Or is it just the maximum elevation before they are no longer traceable?
L194: remove “in” before “Strelin and Malagnino”
Figure 2: hard to see palaeo channels
Section 5.1: Earlier the young age of 132 ka was rejected as recently exposed through exhumation, so I’m wondering why it is included in the calculation of the mean age of 153 ka. Is a mean age sensible with so few data, and such large geological scatter? Some take the oldest boulder age as a strategy (e.g., Kaplan et al., 2005). Please explain interpretation here.
L243: “Isotope” rather than “Isotopic”
L255: “Isotope” rather than “Isotopic”
Section 5.2: a potential added issue for the samples from the centre of the valley is subsequent fluvial erosion causing undercutting/slumping of moraines and therefore degradation/exhumation of boulders if the sampled surfaces were in contact with former drainage routes.
Figure 6: Depending on how the MIS 6/8 discussion evolves, it may be sensible to increase the age scale to include MIS 8 and the chronologies that align with that age.
- Please indicate how the “Patagonian moraine age distribution” sites were chosen for this figure. For example, other major valleys not mentioned include Lago Buenos Aires, Lago Pueyrredón, and others.
Section 5.3: A relevant reference may be a new marine core off the Chilean coast that indicates glacier activity during MIS 6. Hagemann et al., 2024: https://doi.org/10.1073/pnas.230298312
L300: I don’t think there is evidence that these are separate re-advances? If so, make this clear (i.e., cross-cutting relationships, etc).
L306: “and were more extensive than during the global LGM”
L315: The reference to çiner et al. 2022 should be under “c” in the reference list
L350 – Currently the core of westerlies is at about 50 S (i.e., Lago Argentino), so it seems cooling would be more important than a northward migration of the westerlies?
L361 – suggest adding Mendelova 2020 to that reference list
Citation: https://doi.org/10.5194/cp-2024-24-RC1
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