the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Synchronizing ice-core and U/Th time scales in the Last Glacial Maximum using Hulu Cave 14C and new 10Be measurements from Greenland and Antarctica
Florian Adolphi
Marcus Christl
Kees C. Welten
Thomas Woodruff
Marc Caffee
Anders Svensson
Raimund Muscheler
Sune Olander Rasmussen
Abstract. Between 15 and 27 ka b2k (thousands of years before 2000 CE) during the last glacial, Greenland experienced a prolonged cold stadial phase, interrupted by two short-lived warm interstadials. Greenland ice-core calcium data show two periods, preceding the interstadials, of anomalously high atmospheric dust loading, the origin of which is not well understood. At approximately the same time as the Greenland dust peaks, the Chinese Hulu Cave speleothems exhibit a climatic signal suggested to be a response to Heinrich Event 2, a period of enhanced ice-rafted debris deposition in the North Atlantic. In the climatic signal of Antarctic ice cores, moreover, a relative warming occurs between 23 and 24.5 ka b2k that is generally interpreted as a counterpart to a cool climate phase in the Northern Hemisphere. Proposed centennial-scale offsets between the polar ice-core time scales and the speleothem time scale hamper the precise reconstruction of the global sequence of these climatic events. Here, we examine two new 10Be datasets from Greenland (NorthGRIP) and Antarctic (WDC) ice cores to test the agreement between different time scales, by taking advantage of the globally synchronous cosmogenic radionuclide production rates.
Evidence of an event similar to the Maunder Solar Minimum is found in the new 10Be datasets, supported by lower-resolution radionuclide data from Greenland and 14C in the Hulu Cave speleothem, representing a good synchronization candidate at around 22 ka b2k. By matching the respective 10Be data, we determine the offset between the Greenland ice-core time scale, GICC05, and the WDC Antarctic time scale, WD2014, to be 125±40 years. Furthermore, via radionuclide wiggle-matching, we determine the offset between the Hulu speleothem and ice core timescales to be 375 years for GICC05 (75–625 years at 68 % confidence), and 225 years for WD2014 (-25–425 years at 68 % confidence). The rather wide uncertainties are intrinsic to the wiggle-matching algorithm and the limitations set by data resolution. The undercounting of annual layers in GICC05 inferred from the offset is hypothesized to have been caused by a combination of underdetected annual layers, especially during periods with low winter precipitation, and misinterpreted unusual patterns in the annual signal, during the extremely cold period often referred to as Heinrich Stadial 1.
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Giulia Sinnl et al.
Status: closed
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CC1: 'Comment on cp-2022-62', Xiyu Dong, 09 Oct 2022
This paper (https://doi.org/10.1038/s41467-022-33583-4）also discuss the improvement of Greenland and Antarctic ice-core chronologies during the Last Glacial Maximum, which can be included in the Introduction and discussion section.
Citation: https://doi.org/10.5194/cp-2022-62-CC1 -
CC2: 'Reply on CC1', Giulia Sinnl, 21 Oct 2022
Thank you very much for your suggestion. We will certainly comment on the recent publication by Xiyu Dong et al. about Heinrich Event 2.
Kind regards,
Sinnl et al.
Citation: https://doi.org/10.5194/cp-2022-62-CC2
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CC2: 'Reply on CC1', Giulia Sinnl, 21 Oct 2022
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CC3: 'Comment on cp-2022-62', Minjie Zheng, 20 Nov 2022
Thanks for this interesting manuscript. I would like to mention that there are also Be10 data covering this period from the NEEM ice core (zheng et al 2021, https://doi.org/10.1016/j.quascirev.2021.106881). The average resolution of the NEEM record for this period 20ka-25ka is 150-year/sample.
It would be nice to also include this NEEM record or at least, mention why you excluded this NEEM record from your analysis.
Citation: https://doi.org/10.5194/cp-2022-62-CC3 -
AC3: 'Reply on CC3', Giulia Sinnl, 16 Jan 2023
Thank you for your comment. We are aware of the NEEM dataset, but the resolution of 150 years didn't allow for a clear identification of our most important 10Be tie point. We will refer to the NEEM dataset and motivate its exclusion from our study in the revised manuscript.
Kind regards,
Giulia Sinnl et al.
Citation: https://doi.org/10.5194/cp-2022-62-AC3
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AC3: 'Reply on CC3', Giulia Sinnl, 16 Jan 2023
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RC1: 'Comment on cp-2022-62', Anonymous Referee #1, 06 Dec 2022
In this study, new 10Be NorthGRIP and WAIS Divide ice core measurements were compared to Hulu cave 14C measurements to constrain the age scales of these records through the Last Glacial Maximum (LGM). This exercise is important for improving the understanding centennial-scale climatic events in the LGM. Radionuclide particle production changes as a result of solar activity variability. Once changes in its transport to and deposition at ice core sites are accounted for, radionuclide particle variability in proxy records is therefore independent from changes in climate. It is beneficial to use radionuclide particle records to constrain proxy age scales due to this independence. In this study, a time period characteristically similar to the Maunder Minimum identified in the LGM 10Be records was used to synchronize the Greenland (GICC05) and Antarctic (WD2014) ice core ages scales. Using this analysis, an ~125-year difference between the age scales prior to synchronization was determined. A wiggle-matching algorithm was also used to synchronize the ice core age scales to the Hulu Cave age scale. The offsets between the Hulu Cave age scale and the GICC05 and WD2014 age scales were ~375 years and ~225 years, respectively.
This study is important to publish because highly temporally resolved paleoclimate datasets can only be compared to other archives if the age scales of the datasets are accurate. An improvement in the accuracy of proxy age scales therefore leads to better understanding of the timing and progression of climate events and therefore a better understanding of the climate system.
Major comments:
The introduction needs to be revised. The introduction is a long description of several well-known past climate events without giving the readers any context for why they are being presented with this information. Is there something about these climate events that is unresolved that is addressed in this study? It is a nice literature review, but why is it given?
Along the same lines, in the introduction, the authors state that the “objective” of the study is a “comparison of three timescales.” This isn’t really an objective. The comparison is really the approach used to address the objective, which I believe is to improve the accuracy of the timing of climate events in the LGM, which is necessary to understand (eventually) the mechanisms behind them.
The age scale development of the three proxies is then nicely summarized in the introduction, but again, the readers are not given any information about how the current study fits into any of it until ~line 155. It would be very helpful if the authors explained the flaws in the previous dating methods much earlier. Otherwise, the reader does not know why they are being given the summary. It needs to be made very clear that the benefit of using radionuclide records is that their variability is independent from (at least in the case of flux) climatic events. Therefore, the circular nature of dating proxies using climate events and then, in turn, interpreting the timing of those events, is avoided.
The uncertainties in the age scale offsets are rather large given the small magnitude of the offsets. In the conclusion, it would be helpful if the authors could suggest ways in which these uncertainties could be reduced in future studies.
The focus of this study is the LGM, but as the authors state that the age scales were not stretched in this study. How would the age scales before and after the LGM be affected by the offsets suggested here? Are offsets of a few hundred years too small to make much of a difference?
The conclusion made in this study is that age scale corrections need to be made to the ice core records, and that the problem is the result of annual-layer-undercounting. Why does the problem lie with the ice cores? Is the age scale of the Hulu Cave record that much more certain?
Minor comments:
Line 60: This sentence is confusing: “During this time, a phase of massive discharge of icebergs from the Laurentide ice sheet was inferred from the ice-rafted debris content of North Atlantic marine sediments, defining the occurrence of the Heinrich Event 2 (HE-2; Bard et al., 2000; Peck et al., 2006).” You mean that HS2 happened at the same time as the LGM, right? Simplify this sentence.
Lines 63-66: “The term Heinrich Stadial (HS) is often used to indicate the period affected by the HE. The duration of HS-1, for example, is limited to the 14.5-17.5 ka b2k interval within GS-2.1 (Broecker and Barker, 2007), while for HS-2, a correspondence with the late 65 GS-3 is often argued for, based on speleothem water isotope records (e.g. Li et al., 2021)”. It would be very useful if the timing of each Heinrich Stadial and each Greenland Stadial referenced was defined and easily referenced. Maybe a table could be added?
Even though it is commonly used, please add a sentence defining the IntCal20 curve.
Line 100: “and GICC05 was extended to these ice cores.” This is odd phrasing. I’m not sure what this means.
Why was before 2000 (b2k) used instead of the conventional, before 1950?
Lines 117-119: “The authors duly excluded the GI-2−AIM-2 pair from their lead-lag analysis, firstly because the GISP2 CH4 record did not support synchronicity with the GI-2 temperature increase, and, secondly, because the older HE-4 and HE-5 were similarly associated with higher CH4 levels.” What is meant by “because the older HE-4 and HE-5 were similarly associated with higher CH4 levels?” Does this mean that GI2 and the HE’s can’t be distinguished, and that the HE’s are associated with stadials?
Lines 128-130: This needs to be more prominent: “Resolving some time-scale issues, which we will delineate shortly, will clarify the distinctive timing factors of the global climate around HE-2, compared to the ‘conventional’ bipolar seesaw scenario.”
Lines 31-33: This point should also be more prominent: “Traces of volcanic eruptions and cosmogenic radionuclides provide synchronization tools that do not rely on the precise identification of climatic match-points and on the assumption of their synchronicity.”
It is hard to see the effect of the 10Be flux calculation when the concentration and flux aren’t plotted together.
Lines 241-243: “A carbon-cycle model (here the box-diffusion model by Siegenthaler, 1983) is necessary to derive the atmospheric Δ14C signal, i.e. the decay and fractionation-corrected ratio of 14C/12C relative to a standard (Stuiver & Pollach, 1977), from the measured ice-core 10Be.” Please clarify what is meant by “from the measured ice core 10Be.” How was the 10Be used in the model?
Lines 258-260: “The strength of the geomagnetic field directly affects both the 10Be and 14C production rates. Although each radionuclide may be affected differently (Masarik & Beer, 2009), most studies do not find any significant difference in production rates (e.g. Kovaltsov et al., 2012; Herbst et al. 2017).”… I thought that the geomagnetic field did affect production rates? Please clarify this sentence.
Lines 256-258: “To compare the measured and the modelled Δ14C, in this study we will make use of linear detrending, as this largely removes the systematic offsets associated with the unknown carbon cycle history and inventories.” Were both datasets detrended? Please clarify what was done to detrend the data and which datasets were used.
What is the orange in Fig. 5?
Line 505-506: “The stack is shown in fig. S1, with uncertainty bands derived from the standard deviation of the 1000 simulated fluxes.” Is there a reason the stack isn’t shown in the main manuscript? Is it not particularly relevant?
Lines 516-518: “The G2B event: a relatively abrupt increase of 30 ‰ in the modelled Δ14C from 10Be, reaching its maximum at 21,725 years b2k (GICC05 ages), about 100 years after the maximum is reached in 10Be fluxes.” This is a bit confusing because this event is called the “G2B” event, but then it is stated that happens 100 years after the 10Be. Please explain. Also, if the timing were the same as the 10Be event, wouldn’t you expect this, considering that the 10Be data are an input that is used to produce the modelled Δ14C data?
Table 3 is very helpful!
Lines 654-656: “We cannot provide an Antarctic comparison in this context, as the WD2014 chronology does not currently apply to other ice cores, hence an updated Antarctic synchronization across AIM-2 would be required.” I don’t quite understand this. Isn’t WD2014 applied to the South Pole Ice core (SPICE) and to Skytrain (paper recently submitted to CP)?
Technical changes:
Line 50: change “being debated” to “under debate”
Line 52: add comma after “cold period”
Line 54: “was established to have lasted until”…change to something like “suggest that the LGM lasted until…”
Line 56: “since it coincides with the age limits of our new Greenland 10Be dataset”…what does this mean?
Line 108: Define δ18Oice
Figure 1 caption: The second “d” should be “e”
Sections 2.1 and 2.2: references to the 10Be methods???
Line 323: “quantify the impact of 10Be measurements uncertainty” change to “quantify the impact of 10Be measurement uncertainty”
Line 275 vs. line 390: 21.7 ka event in one line and 22.7 ka event in another. Are these different events?
Line 451: “some eruptions are better visible in the”…change to “more visible”
Line 461-462: “and we obtained a timescale correction, which we apply in the following to the GRIP data.” This is confusing. Please simplify this sentence.
Line 560: “Measured Hulu Cave used for synchronization” You should add that this is Δ14C.
Line 672: change “As much as the GICC05 layers are concerned,” to “As far as the GICC05 layers are concerned”
Line 677: change “Acknowledging the 125-years offset” to “Acknowledging the 125-year offset”
Line 677-678: change “the 375 years offset between GICC05” to “the 375-year offset between GICC05”
Lines 746-747: “However, the onset of the Greenlandic dust peak moved to be roughly synchronous with the signal in the Hulu speleothem that has been linked to the HS-2 onset.” Turn this into a sentence.
Citation: https://doi.org/10.5194/cp-2022-62-RC1 - AC1: 'Reply on RC1', Giulia Sinnl, 16 Jan 2023
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RC2: 'Comment on cp-2022-62', Pete D. Akers, 03 Jan 2023
General comments
In this paper, the authors examine the chronologies in three sets of paleo records over the Last Glacial Maximum: Greenland ice cores, a West Antarctic ice core, and Hulu Cave speleothems. To examine the synchroneity of the existing chronologies for each record set, the authors use cosmogenic nuclides as the comparative proxy under the assumption that changes in these nuclide fluxes are globally synchronous and avoid the impacts of local climate that can cause issues when matching other environmental proxies like stable isotopes of oxygen and carbon. They use these comparison to estimate that over a century offset exists between the Greenland and Antarctic chronologies, and both ice core records are offset from the speleothem record by 200 additional years.
I found this paper to be very well written and organized. Their approach of linking 10Be with 14C data is well-thought out and impressive in its level of detail dedicated to modelling and cross-examining between records. The paper overall is fairly heavy on methods and technical speak, but such a focus is likely necessary due to the nature of this study, and I applaud the authors for providing very detailed information on all parts of this work. In particular, there is very good supportive data in the captions (e.g., supporting citations, clear descriptions of figure color/symbols), which I often find lacking in papers under review. That said, some captions are very lengthy, and while I think the supportive information is good, some considerations could be made for making the information in the captions more succinct. Similarly, I’d suggest examining if any of the deep technical details could be streamlined without sacrificing important context, which I think would help the paper prove more approachable to a broader audience. However, I think this manuscript still manages to have a clear narrative structure and is one of the better manuscripts that I’ve reviewed in terms of presenting clear results and conclusions while still providing deep technical detail.
My specific comments are minor. Some of the modelling aspects of the paper are outside my scientific expertise, but the described approaches and discussion of the results are all logical to me and supported by ample evidence. Some additional focus could possibly be made toward considering in more detail the impact these new chronologies should have on paleoclimate reconstructions, or how the ice core community should respond to the findings of century-scale offsets in chronologies. Such additional information are not required edits from me, but offered here as consideration to the authors if they wish to elaborate more.
There are many acronyms/abbreviations in the paper. To improve readability, I suggest eliminating the ones that are only used once or twice, which I list in the technical corrections.
Throughout the paper, uncertainties in age corrections and estimates are usually given as 1 SD, but a 95% confidence interval would likely be more informative and representative. As of now, the offsets between the different chronology sets are sometimes within the 95% confidence window. I think the magnitude of difference and consistency of difference is large enough to believe you are observing true offsets, but you might want to target this point more with a dedicated response to avoid the appearance that you used 1 SD values to make the results look more differentiated from each other.
Some restructuring of the introduction would probably make the paper more effective. It takes quite a while before you point out what your paper is doing, and what problem currently exists that you are attempting to solve. So we read about LGM events like stadials and HEs, but without the context for why we need to know this. Likewise, there is a lot of discussion about the specifics of the three different chronologies without the reader’s context for why this information will be important later. Being more explicit about the currently known or expected issues in record synchroneity early on and repeatedly linking the background information to these issues and your study approaches will make for a tighter, more effective introduction.
Specific comments
65: I’m slightly unclear in what this sentence is meaning by “correspondence”. The first part makes it clear that HS-1 is only part of GS 2.1, but “a correspondence with the late GS-3” could be taken either as 1) HS-2 is similar to HS-1 in that it only covers part of GS-3 (the late part) or 2) unlike HS-1, HS-2 coincides with the entirety of the late GS-3 unit. A rewording will help the point you are intending to make be clear.
83: The objective is determining if the accepted chronologies at three sites actually align when compared with a globally synchronous marker, right? “Comparison of three time scales in the LGM” is vague and doesn’t really capture the point of your work.
589: This is probably my biggest comment here, in that all the age offset “fault” is taken to lie with the ice cores. I think that this is reasonable, since the layer counting of ice cores has known issues, but it would still be good to elaborate a little more on what the U-Th uncertainties might represent. How much older would the “too old” estimates be, as cited by Corrick? Do any estimates exist for this time period? This might get more into the mechanics of U-Th dating, thorium corrections, U-Th half life estimates, etc, than warranted by your study, but any additional clarification and constraint you can give on this point will make your argument stronger toward the ice core re-dating conclusion.
626: MCE was defined back at line 98 but hasn’t been used until now. I had forgotten what this acronym stood for and had to look it up again. It would probably be good to redefine it again here (or around 700 when it is used a lot again) for the reader.
Technical corrections
60: Probably worth spelling out Heinrich events here, since it starts the sentences and HEs is only used in this way once here.
70: “it has been suggested that during HS-1, the empirical” – removing a comma will make the sentence read better
76: Check this sentence for phrasing. I think the first comma needs to go after GS-3, but the sentence does not make full sense at the moment. It is also rather long and run-on.
80: Similarly to HEs, I would spell out Heinrich stadials here to aid the reader already seeing lots of abbreviations, and also HSs is only used once here.
88: Just a consideration that you could term this 14Ccalcite to be consistent with the other stalagmite proxies.
168: The only “previously reported” radionuclide tie point? Or is this the only excursion that can function as a tie point according to some parameter?
179: Table should be capitalized. Also Fig. and Table throughout manuscript.
187: Period after “Table 1”. Also in all other captions for figures and tables as well. May be caught in proofing.
Table 1: Geographic coordinates need degrees symbols. All sites should have same level of significant decimal digits (see WDC longitude).
192: Timescale here is one word whereas earlier it is written as two words (time scale). Consistency needed (or use “chronology” term instead).
Figure 1: Be aware that overlapping symbols in red and green (Fig 1e) may cause accessibility issues for colorblind readers. Check with a colorblind filter to make sure figure color schemes are accessible.
Figure 2: The image is fuzzy compared to Figure 1, so just make sure there is a high quality version for final submission.
241: “here, the box-diffusion…”
264: GCRs not GCR’s. Also, consider just spelling this out since it is the only use of this abbreviation in the manuscript.
246: The comma after 14C seems oddly placed for phrasing in this sentence.
Figure 6: No y axis marks for d, but some superfluous yellow ones at right. Check y-axes labels and numbers for overlap (e.g., a+b, c+d).
Figure 8: I appreciate the dedication to information in these captions. This caption, however, is very wordy and could use some of the methods text in it to be greatly summarized or moved into the main text or supplement.
Citation: https://doi.org/10.5194/cp-2022-62-RC2 - AC2: 'Reply on RC2', Giulia Sinnl, 16 Jan 2023
Status: closed
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CC1: 'Comment on cp-2022-62', Xiyu Dong, 09 Oct 2022
This paper (https://doi.org/10.1038/s41467-022-33583-4）also discuss the improvement of Greenland and Antarctic ice-core chronologies during the Last Glacial Maximum, which can be included in the Introduction and discussion section.
Citation: https://doi.org/10.5194/cp-2022-62-CC1 -
CC2: 'Reply on CC1', Giulia Sinnl, 21 Oct 2022
Thank you very much for your suggestion. We will certainly comment on the recent publication by Xiyu Dong et al. about Heinrich Event 2.
Kind regards,
Sinnl et al.
Citation: https://doi.org/10.5194/cp-2022-62-CC2
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CC2: 'Reply on CC1', Giulia Sinnl, 21 Oct 2022
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CC3: 'Comment on cp-2022-62', Minjie Zheng, 20 Nov 2022
Thanks for this interesting manuscript. I would like to mention that there are also Be10 data covering this period from the NEEM ice core (zheng et al 2021, https://doi.org/10.1016/j.quascirev.2021.106881). The average resolution of the NEEM record for this period 20ka-25ka is 150-year/sample.
It would be nice to also include this NEEM record or at least, mention why you excluded this NEEM record from your analysis.
Citation: https://doi.org/10.5194/cp-2022-62-CC3 -
AC3: 'Reply on CC3', Giulia Sinnl, 16 Jan 2023
Thank you for your comment. We are aware of the NEEM dataset, but the resolution of 150 years didn't allow for a clear identification of our most important 10Be tie point. We will refer to the NEEM dataset and motivate its exclusion from our study in the revised manuscript.
Kind regards,
Giulia Sinnl et al.
Citation: https://doi.org/10.5194/cp-2022-62-AC3
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AC3: 'Reply on CC3', Giulia Sinnl, 16 Jan 2023
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RC1: 'Comment on cp-2022-62', Anonymous Referee #1, 06 Dec 2022
In this study, new 10Be NorthGRIP and WAIS Divide ice core measurements were compared to Hulu cave 14C measurements to constrain the age scales of these records through the Last Glacial Maximum (LGM). This exercise is important for improving the understanding centennial-scale climatic events in the LGM. Radionuclide particle production changes as a result of solar activity variability. Once changes in its transport to and deposition at ice core sites are accounted for, radionuclide particle variability in proxy records is therefore independent from changes in climate. It is beneficial to use radionuclide particle records to constrain proxy age scales due to this independence. In this study, a time period characteristically similar to the Maunder Minimum identified in the LGM 10Be records was used to synchronize the Greenland (GICC05) and Antarctic (WD2014) ice core ages scales. Using this analysis, an ~125-year difference between the age scales prior to synchronization was determined. A wiggle-matching algorithm was also used to synchronize the ice core age scales to the Hulu Cave age scale. The offsets between the Hulu Cave age scale and the GICC05 and WD2014 age scales were ~375 years and ~225 years, respectively.
This study is important to publish because highly temporally resolved paleoclimate datasets can only be compared to other archives if the age scales of the datasets are accurate. An improvement in the accuracy of proxy age scales therefore leads to better understanding of the timing and progression of climate events and therefore a better understanding of the climate system.
Major comments:
The introduction needs to be revised. The introduction is a long description of several well-known past climate events without giving the readers any context for why they are being presented with this information. Is there something about these climate events that is unresolved that is addressed in this study? It is a nice literature review, but why is it given?
Along the same lines, in the introduction, the authors state that the “objective” of the study is a “comparison of three timescales.” This isn’t really an objective. The comparison is really the approach used to address the objective, which I believe is to improve the accuracy of the timing of climate events in the LGM, which is necessary to understand (eventually) the mechanisms behind them.
The age scale development of the three proxies is then nicely summarized in the introduction, but again, the readers are not given any information about how the current study fits into any of it until ~line 155. It would be very helpful if the authors explained the flaws in the previous dating methods much earlier. Otherwise, the reader does not know why they are being given the summary. It needs to be made very clear that the benefit of using radionuclide records is that their variability is independent from (at least in the case of flux) climatic events. Therefore, the circular nature of dating proxies using climate events and then, in turn, interpreting the timing of those events, is avoided.
The uncertainties in the age scale offsets are rather large given the small magnitude of the offsets. In the conclusion, it would be helpful if the authors could suggest ways in which these uncertainties could be reduced in future studies.
The focus of this study is the LGM, but as the authors state that the age scales were not stretched in this study. How would the age scales before and after the LGM be affected by the offsets suggested here? Are offsets of a few hundred years too small to make much of a difference?
The conclusion made in this study is that age scale corrections need to be made to the ice core records, and that the problem is the result of annual-layer-undercounting. Why does the problem lie with the ice cores? Is the age scale of the Hulu Cave record that much more certain?
Minor comments:
Line 60: This sentence is confusing: “During this time, a phase of massive discharge of icebergs from the Laurentide ice sheet was inferred from the ice-rafted debris content of North Atlantic marine sediments, defining the occurrence of the Heinrich Event 2 (HE-2; Bard et al., 2000; Peck et al., 2006).” You mean that HS2 happened at the same time as the LGM, right? Simplify this sentence.
Lines 63-66: “The term Heinrich Stadial (HS) is often used to indicate the period affected by the HE. The duration of HS-1, for example, is limited to the 14.5-17.5 ka b2k interval within GS-2.1 (Broecker and Barker, 2007), while for HS-2, a correspondence with the late 65 GS-3 is often argued for, based on speleothem water isotope records (e.g. Li et al., 2021)”. It would be very useful if the timing of each Heinrich Stadial and each Greenland Stadial referenced was defined and easily referenced. Maybe a table could be added?
Even though it is commonly used, please add a sentence defining the IntCal20 curve.
Line 100: “and GICC05 was extended to these ice cores.” This is odd phrasing. I’m not sure what this means.
Why was before 2000 (b2k) used instead of the conventional, before 1950?
Lines 117-119: “The authors duly excluded the GI-2−AIM-2 pair from their lead-lag analysis, firstly because the GISP2 CH4 record did not support synchronicity with the GI-2 temperature increase, and, secondly, because the older HE-4 and HE-5 were similarly associated with higher CH4 levels.” What is meant by “because the older HE-4 and HE-5 were similarly associated with higher CH4 levels?” Does this mean that GI2 and the HE’s can’t be distinguished, and that the HE’s are associated with stadials?
Lines 128-130: This needs to be more prominent: “Resolving some time-scale issues, which we will delineate shortly, will clarify the distinctive timing factors of the global climate around HE-2, compared to the ‘conventional’ bipolar seesaw scenario.”
Lines 31-33: This point should also be more prominent: “Traces of volcanic eruptions and cosmogenic radionuclides provide synchronization tools that do not rely on the precise identification of climatic match-points and on the assumption of their synchronicity.”
It is hard to see the effect of the 10Be flux calculation when the concentration and flux aren’t plotted together.
Lines 241-243: “A carbon-cycle model (here the box-diffusion model by Siegenthaler, 1983) is necessary to derive the atmospheric Δ14C signal, i.e. the decay and fractionation-corrected ratio of 14C/12C relative to a standard (Stuiver & Pollach, 1977), from the measured ice-core 10Be.” Please clarify what is meant by “from the measured ice core 10Be.” How was the 10Be used in the model?
Lines 258-260: “The strength of the geomagnetic field directly affects both the 10Be and 14C production rates. Although each radionuclide may be affected differently (Masarik & Beer, 2009), most studies do not find any significant difference in production rates (e.g. Kovaltsov et al., 2012; Herbst et al. 2017).”… I thought that the geomagnetic field did affect production rates? Please clarify this sentence.
Lines 256-258: “To compare the measured and the modelled Δ14C, in this study we will make use of linear detrending, as this largely removes the systematic offsets associated with the unknown carbon cycle history and inventories.” Were both datasets detrended? Please clarify what was done to detrend the data and which datasets were used.
What is the orange in Fig. 5?
Line 505-506: “The stack is shown in fig. S1, with uncertainty bands derived from the standard deviation of the 1000 simulated fluxes.” Is there a reason the stack isn’t shown in the main manuscript? Is it not particularly relevant?
Lines 516-518: “The G2B event: a relatively abrupt increase of 30 ‰ in the modelled Δ14C from 10Be, reaching its maximum at 21,725 years b2k (GICC05 ages), about 100 years after the maximum is reached in 10Be fluxes.” This is a bit confusing because this event is called the “G2B” event, but then it is stated that happens 100 years after the 10Be. Please explain. Also, if the timing were the same as the 10Be event, wouldn’t you expect this, considering that the 10Be data are an input that is used to produce the modelled Δ14C data?
Table 3 is very helpful!
Lines 654-656: “We cannot provide an Antarctic comparison in this context, as the WD2014 chronology does not currently apply to other ice cores, hence an updated Antarctic synchronization across AIM-2 would be required.” I don’t quite understand this. Isn’t WD2014 applied to the South Pole Ice core (SPICE) and to Skytrain (paper recently submitted to CP)?
Technical changes:
Line 50: change “being debated” to “under debate”
Line 52: add comma after “cold period”
Line 54: “was established to have lasted until”…change to something like “suggest that the LGM lasted until…”
Line 56: “since it coincides with the age limits of our new Greenland 10Be dataset”…what does this mean?
Line 108: Define δ18Oice
Figure 1 caption: The second “d” should be “e”
Sections 2.1 and 2.2: references to the 10Be methods???
Line 323: “quantify the impact of 10Be measurements uncertainty” change to “quantify the impact of 10Be measurement uncertainty”
Line 275 vs. line 390: 21.7 ka event in one line and 22.7 ka event in another. Are these different events?
Line 451: “some eruptions are better visible in the”…change to “more visible”
Line 461-462: “and we obtained a timescale correction, which we apply in the following to the GRIP data.” This is confusing. Please simplify this sentence.
Line 560: “Measured Hulu Cave used for synchronization” You should add that this is Δ14C.
Line 672: change “As much as the GICC05 layers are concerned,” to “As far as the GICC05 layers are concerned”
Line 677: change “Acknowledging the 125-years offset” to “Acknowledging the 125-year offset”
Line 677-678: change “the 375 years offset between GICC05” to “the 375-year offset between GICC05”
Lines 746-747: “However, the onset of the Greenlandic dust peak moved to be roughly synchronous with the signal in the Hulu speleothem that has been linked to the HS-2 onset.” Turn this into a sentence.
Citation: https://doi.org/10.5194/cp-2022-62-RC1 - AC1: 'Reply on RC1', Giulia Sinnl, 16 Jan 2023
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RC2: 'Comment on cp-2022-62', Pete D. Akers, 03 Jan 2023
General comments
In this paper, the authors examine the chronologies in three sets of paleo records over the Last Glacial Maximum: Greenland ice cores, a West Antarctic ice core, and Hulu Cave speleothems. To examine the synchroneity of the existing chronologies for each record set, the authors use cosmogenic nuclides as the comparative proxy under the assumption that changes in these nuclide fluxes are globally synchronous and avoid the impacts of local climate that can cause issues when matching other environmental proxies like stable isotopes of oxygen and carbon. They use these comparison to estimate that over a century offset exists between the Greenland and Antarctic chronologies, and both ice core records are offset from the speleothem record by 200 additional years.
I found this paper to be very well written and organized. Their approach of linking 10Be with 14C data is well-thought out and impressive in its level of detail dedicated to modelling and cross-examining between records. The paper overall is fairly heavy on methods and technical speak, but such a focus is likely necessary due to the nature of this study, and I applaud the authors for providing very detailed information on all parts of this work. In particular, there is very good supportive data in the captions (e.g., supporting citations, clear descriptions of figure color/symbols), which I often find lacking in papers under review. That said, some captions are very lengthy, and while I think the supportive information is good, some considerations could be made for making the information in the captions more succinct. Similarly, I’d suggest examining if any of the deep technical details could be streamlined without sacrificing important context, which I think would help the paper prove more approachable to a broader audience. However, I think this manuscript still manages to have a clear narrative structure and is one of the better manuscripts that I’ve reviewed in terms of presenting clear results and conclusions while still providing deep technical detail.
My specific comments are minor. Some of the modelling aspects of the paper are outside my scientific expertise, but the described approaches and discussion of the results are all logical to me and supported by ample evidence. Some additional focus could possibly be made toward considering in more detail the impact these new chronologies should have on paleoclimate reconstructions, or how the ice core community should respond to the findings of century-scale offsets in chronologies. Such additional information are not required edits from me, but offered here as consideration to the authors if they wish to elaborate more.
There are many acronyms/abbreviations in the paper. To improve readability, I suggest eliminating the ones that are only used once or twice, which I list in the technical corrections.
Throughout the paper, uncertainties in age corrections and estimates are usually given as 1 SD, but a 95% confidence interval would likely be more informative and representative. As of now, the offsets between the different chronology sets are sometimes within the 95% confidence window. I think the magnitude of difference and consistency of difference is large enough to believe you are observing true offsets, but you might want to target this point more with a dedicated response to avoid the appearance that you used 1 SD values to make the results look more differentiated from each other.
Some restructuring of the introduction would probably make the paper more effective. It takes quite a while before you point out what your paper is doing, and what problem currently exists that you are attempting to solve. So we read about LGM events like stadials and HEs, but without the context for why we need to know this. Likewise, there is a lot of discussion about the specifics of the three different chronologies without the reader’s context for why this information will be important later. Being more explicit about the currently known or expected issues in record synchroneity early on and repeatedly linking the background information to these issues and your study approaches will make for a tighter, more effective introduction.
Specific comments
65: I’m slightly unclear in what this sentence is meaning by “correspondence”. The first part makes it clear that HS-1 is only part of GS 2.1, but “a correspondence with the late GS-3” could be taken either as 1) HS-2 is similar to HS-1 in that it only covers part of GS-3 (the late part) or 2) unlike HS-1, HS-2 coincides with the entirety of the late GS-3 unit. A rewording will help the point you are intending to make be clear.
83: The objective is determining if the accepted chronologies at three sites actually align when compared with a globally synchronous marker, right? “Comparison of three time scales in the LGM” is vague and doesn’t really capture the point of your work.
589: This is probably my biggest comment here, in that all the age offset “fault” is taken to lie with the ice cores. I think that this is reasonable, since the layer counting of ice cores has known issues, but it would still be good to elaborate a little more on what the U-Th uncertainties might represent. How much older would the “too old” estimates be, as cited by Corrick? Do any estimates exist for this time period? This might get more into the mechanics of U-Th dating, thorium corrections, U-Th half life estimates, etc, than warranted by your study, but any additional clarification and constraint you can give on this point will make your argument stronger toward the ice core re-dating conclusion.
626: MCE was defined back at line 98 but hasn’t been used until now. I had forgotten what this acronym stood for and had to look it up again. It would probably be good to redefine it again here (or around 700 when it is used a lot again) for the reader.
Technical corrections
60: Probably worth spelling out Heinrich events here, since it starts the sentences and HEs is only used in this way once here.
70: “it has been suggested that during HS-1, the empirical” – removing a comma will make the sentence read better
76: Check this sentence for phrasing. I think the first comma needs to go after GS-3, but the sentence does not make full sense at the moment. It is also rather long and run-on.
80: Similarly to HEs, I would spell out Heinrich stadials here to aid the reader already seeing lots of abbreviations, and also HSs is only used once here.
88: Just a consideration that you could term this 14Ccalcite to be consistent with the other stalagmite proxies.
168: The only “previously reported” radionuclide tie point? Or is this the only excursion that can function as a tie point according to some parameter?
179: Table should be capitalized. Also Fig. and Table throughout manuscript.
187: Period after “Table 1”. Also in all other captions for figures and tables as well. May be caught in proofing.
Table 1: Geographic coordinates need degrees symbols. All sites should have same level of significant decimal digits (see WDC longitude).
192: Timescale here is one word whereas earlier it is written as two words (time scale). Consistency needed (or use “chronology” term instead).
Figure 1: Be aware that overlapping symbols in red and green (Fig 1e) may cause accessibility issues for colorblind readers. Check with a colorblind filter to make sure figure color schemes are accessible.
Figure 2: The image is fuzzy compared to Figure 1, so just make sure there is a high quality version for final submission.
241: “here, the box-diffusion…”
264: GCRs not GCR’s. Also, consider just spelling this out since it is the only use of this abbreviation in the manuscript.
246: The comma after 14C seems oddly placed for phrasing in this sentence.
Figure 6: No y axis marks for d, but some superfluous yellow ones at right. Check y-axes labels and numbers for overlap (e.g., a+b, c+d).
Figure 8: I appreciate the dedication to information in these captions. This caption, however, is very wordy and could use some of the methods text in it to be greatly summarized or moved into the main text or supplement.
Citation: https://doi.org/10.5194/cp-2022-62-RC2 - AC2: 'Reply on RC2', Giulia Sinnl, 16 Jan 2023
Giulia Sinnl et al.
Giulia Sinnl et al.
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