the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Apr 2025
Deglaciation of the Sierra Nevada (USA) during Heinrich Event 1
Abstract. A polar jet stream (PJS) split by the Laurentide Ice Sheet (LIS) is a well-established feature of Ice-Age atmospheric circulation. California’s central Sierra Nevada Mountains (37–38° N) lie near the reconstructed position of the PJS’s southern branch. Previous studies concluded that rapid deglaciation began here at ca. 16–15 ka after millennia of relatively stability at ~60 % LGM length. However, this conclusion is largely based on the behavior of glaciers in a single valley, Bishop Creek Canyon. We report 31 new 10Be samples from two new locations – Lyell Canyon and Mono Creek Canyon – and 26 recalculated 36Cl dates from Bishop Creek Canyon (n = 57). These dates indicate rapid deglaciation began at 16.4 ± 0.8 ka and lasted for ca. 1.0 kyr. Placing two previously published paleoenvironmental reconstructions (Swamp Lake and McLean’s Cave) with centennial-or-better-scale resolution on new age-depth models that provide age-uncertainty estimates, we find evidence for warming in the central Sierra Nevada at 16.4 ± 0.4 ka and drying at 16.20 ± 0.13 ka. Collectively, we interpret that rapid deglaciation began at 16.20 ± 0.13 ka. This timing is indistinguishable from that of Heinrich Event 1 (HE1), which occurred between 16.22 ± 0.04 ka and 16.04 ± 0.04 ka. We hypothesize that the Sierra Nevada’s deglaciation was driven by a northward repositioning and focusing of the winter-storm track over western North America in response to PJS reunification, bringing warmer and drier weather to the central Sierra Nevada, and that PJS reunification occurred in response HE1 thinning the LIS.
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Status: closed (peer review stopped)
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RC1: 'Comment on egusphere-2025-1370', Matteo Spagnolo, 03 May 2025
Dear Editor and Authors,
I read with great interest this manuscript, which attempts to chronologically constraint the post-Tioga4 deglaciation in the Sierra Nevada, linking it to key palaeoclimatic events that likely drove a relatively rapid and widespread glacier retreat in this region.
The quality and quantity of the work undertaken and presented is truly impressive. The authors have really considered many different aspects which could have impacted the results and looked at these in great detail. A good number of other proxies are also revisited and considered to strengthen their interpretation. The implications from a paleoglaciology and palaeoclimatology point of view are important. And the topic is certainly of interest to a wide international audience. All in all, I therefore encourage a swift acceptance for publication of the submitted manuscript. However, I believe there are a number of potential improvements, the most important ones I list below, in no particular order. I consider these of a minor nature, and I believe it should take relatively little effort to address these. Notice that I will also attach a version of the manuscript with some 55 comments and additional edits, which might be useful to improve the manuscript too.
- The manuscript is too long. I appreciate there is a lot to talk about, but to make it as attractive as possible to a wide audience I recommend some trimming. Could some aspects be moved the supplementary material? For example, do we really need such long narrative around the age calculations (though I appreciate the destination journal somewhat justify it)? Could this be moved to the supplementary and referred to in the main text?
- A clearer justification of the main research focus, i.e. duration of deglaciation (rather than what most other cosmo glacio papers deal with, that is moraine ages). Why does it matter?
- Is there a wider lesson to be learned, which could be of relevance to other palaeoglaciology/climatology studies elsewhere, i.e. not specific to the Sierra or North America?
- Somewhat related to point 1, though I appreciate the effort, I think that the work undertaken to establish snow shielding is a bit of an overkill, given that ultimately we have very little knowledge of how this varied in the past. Can the past 50 or a 100 years or even a few centuries be truly representative of conditions over a timeframe 2 orders of magnitude longer? I suggest the text that relates to this is reduced considerably.
- Please try to steer away from the description of glacial dynamics in terms of percentage of LGM glacier length and use ELA instead, even if transient (not connected to a moraine), which is far more important and robust glacier mass balance wise
- Could you quantify a retreat rate? This will help putting your “rapid and abrupt” deglaciation into context.
- Figure-wise, all great and useful. Dare I ask you to consider providing a 3D cartoon-like/schematic diagram showing the glacier retreating through the studied sites, exemplifying key geomorphological/geological evidence that justify your conclusion along with the ages of the various samples? These diagrams could perhaps be added to figs 3-5.
Looking forward to seeing this great work published as soon as possible,
Matteo Spagnolo
- AC1: 'Reply on RC1', Richard Becker, 25 Jul 2025
-
RC2: 'Comment on egusphere-2025-1370', Anonymous Referee #2, 17 May 2025
Dear Editor and Authors:
I appreciate the opportunity to read this Discussion paper that provides new cosmogenic nuclide ages on post-LGM moraines in the Sierra Nevada and attempts to place the glacial history of this area in a broader regional and global context. The new glacial chronologies will be of great interest to those studying the history of this region, and are an exciting addition to the multi-archive, multi-proxy body of work on western North American paleoclimate.
I find the new ages to be interesting and well-presented. My primary comments on this manuscript are centered on the treatment of regional proxy information and the interpretations about the broader expression of Heinrich Stadial 1, so this is what I will discuss for the remainder of this comment. I find that the discussion of the temporal evolution of events in the North Atlantic region and the western US is a bit muddled in the discussion. The previously published proxy information that the authors present here could be treated more carefully and clearly.
The authors emphasize a very narrow window for “Heinrich Event 1” which they define from a Spanish stalagmite record from Ostolo Cave. This is shown in the graphical abstract and in Figure 8 with yellow shading and labeled “HE1”. The Ostolo Cave record suggests a change in the d18O of the moisture source as well as temperature in an excursion contemporaneous with IRD in the Bay of Biscay (Eynaud et al., 2012) and the change in seawater isotopic composition (Voelker et al., 2009). The Ostolo Cave paper by Bernall-Wormull which you cite, interprets this negative excursion in d18O as follows:
“An exceptional light δ18Ospeleo excursion centered at 16.2–16.0 kyr B.P. is interpreted to reflect the major phase of HE1 iceberg melting reaching the Iberian Peninsula, which drastically changed the δ18O composition of regional precipitation.”
While this is a precisely dated record, it is a regional expression of the Heinrich Event in Spain and should be taken in context with other North Atlantic records of the event – which Bernall-Wormhull also show in their figure 4. I suggest you also compare to the records of IRD and foraminifera d18O which suggest a broader peak for meltwater release between ~16.5 and 16 ka – rather than shading this sharp speleothem δ18O excursion as the full expression of the Heinrich Event in your Figure 8 and associated discussion in the text. The record of 231Pa/230Th in North Atlantic sediments suggests that AMOC slowdown began closer to 18 ka, again reaching a minimum close to 16 ka (McManus et al., 2004). Similar timing of freshwater release to the North Atlantic is used in the TRACE and iTRACE transient climate model simulations (He et al., 2011; He et al., 2021). A summary of Heinrich Stadial 1 model results and proxy records for the western US is provided in Oster et al., 2023. This supports that the window that you have chosen for Heinrich Event 1 is too narrow.
Additionally, there is a long-standing discussion of Heinrich Stadial 1 in the western US and whether it contained 2 phases - one that was overall drier- particularly in the southwest, and one that was much wetter centered on 16 ka. This discussion is ongoing, but much evidence has come from the timing of lake high stands and how they have varied across the region (Broecker and Putnam, 2012; McGee et al., 2018; Hudson et al., 2019; Oster et al., 2020). At any rate, the development of Heinrich Stadial 1 in this region has been well-explored in the literature, and that discussion should be reflected here.
Following this idea, the discussion on Great Basin Lakes (Section 5.5.3) is oversimplified and under-cited. There have been very nice compilations of Great Basin Lake high stands that include analysis of the timing and geographic patterns and include modeling and other regional syntheses (McGee et al., 2018; Hudson et al., 2019). The presentation of the lake data in Figure 8d is also oversimplified which carries through to the graphical abstract. Only some of the lakes are labeled. It is unclear if only one age is provided per lake or if there are more. This needs to be clarified in the figure and caption.
The 6-degree jump (lines 949-950 and other places) appears from Figure 8 to be defined by the high stands of Lake Cochise and Lake Russell or Surprise. However, there are numerous other high stands on your figure prior to 16.4 ka that are further north than Cochise. Cochise and Estancia are also outside of the Great Basin. It is not consistent with the uncertainty on the ages of lake high stands to pinpoint 6 degree jump within a 200 year interval.
Provide a citation for the hypothesis on the delay of Owens and Russell high stands due to meltwater (Lines 956-958). It is stated as a fact here but with a question mark in your figure – is this a hypothesis put forward by this paper or elsewhere?
Regarding your McLean’s Cave age model, there is a paper currently in review that includes new U-series dates for McLean’s Cave and a new age model run using the COPRA algorithm (Breitenbach et al., 2012), that I believe shifts the minimum in d13C of this record slightly older than what your Bchron age model put it at and closer to 16.25 ka. I realize this information is not yet accessible, but caution against putting too much stock in the exact timing of the shift in McLean’s Cave δ13C computed from the Bchron model here. This is consistent with the comment to broaden the constraints of the timing on Heinrich Stadial 1 used in this paper, which can be done with the records available in the literature.
I am also curious about the emphasis on the ice sheet thinning rather than changes in freshwater flux and AMOC that are frequently modeled and evaluated in discussions of Heinrich Event impacts. This comes through in the discussion of the TrACE results (Example Lines 1005 to 1017) which misses the influx of meltwater to the North Atlantic and the subsequent impacts to AMOC. Other modeled scenarios of Heinrich 1 (McGee et al., 2018; Oster et al., 2023) investigate the cascading influences of this freshwater flux to the North Atlantic. It was surprising to me to equate the changes noted around 16 ka in the proxy records from western North America with the shift in the ice sheet parameterized with ICE-5G between 15 ka and 14 ka in the model simulations. There are proxy records from western North America that cover this interval, including the records in your Figure 8. It is more appropriate to compare the atmospheric changes and drivers in TrACE at 16 ka to the proxy records from 16 ka. I think the authors should consider the influences of meltwater flux, SST changes and AMOC shifts and those teleconnections which do align in timing with the shifts seen in western North American proxy records (McGee et al., 2018; Oster al., 2023).
Below are more targeted comments by line:
Abstract:
Line 19 – Does the “60% LGM length” refer to the length of glaciers? Not clear.
Overall abstract is too specific with place names and is difficult to follow.
Graphical Abstract – For the great basin highstands – which lakes are you talking about here – what are the ages from? The citations you have are for papers that include multiple lakes, and the high stands vary across the region. Are the ages for individual lake highstands or are they curves for different lakes (multiple points per lake). (Similar comments on Figure 8).
Introduction:
The discussion of timing in the introduction needs to be more concrete and include the available age information of the time periods discussed leading into this study. In part this is presented in the setting section, but the information should be included in the introduction to set up your central questions/objectives. Specifically:
-Line 63 – provide a time range to go with “broadly in-phase with the global Last Glacial Maximum”.
Lines 66-71 – please also provide timing estimates for these advances and retreats based on the references included here. It is important to situate your discussion of Heinrich 1 in time.
-Line 82 – first mention of the Tioga 4 glaciation – need to define what the timing of this glaciation is understood to be for non-specialists.
-Line224 – do not think you need to include the phrase “is especially relevant to this manuscript”. You can explain the proximity to your sites and make the relevance apparent.
Line 617 – it is more typical to report 2 sigma uncertainties for U-series ages.
Lines 668-674 – This reads a bit too colloquially – the discussion prior to this centers on why the different calculators may be returning different ages. But then these sentences dismiss this question and say that it is not the central task of this paper to figure this out. Suggest rewriting to this to emphasize what is important, rather than what isn’t and to tone down colloquial language.
Overall, I suggest fewer parenthetical observations like the example in Lines 682-683. These are distracting and are used unevenly. Just include ideas in the sentences proper.
Line 734 – missing word “Accepting the bulk organic radiocarbon dates as accurate provides minimum limits…” or something like this.
Lines 741-749 – suggest adding the ages of the outliers into the paragraph in the appropriate places to aid comparison with the mean.
Line 759 – “probably” should be “probable”
Lines 783-788 – add the numbers into this narrative so that it is easier to follow and see what your interpreted chronology is.
Section 5.4 – it is not clear where the constraints on temperature and precipitation changes given in this section are coming from. It does not appear that independent glacier mass-balance modeling was undertaken for this study. Where do the estimates of 2 degrees and 35% precipitation change come from? The papers cited in this section are not unique to changes in the Sierra Nevada. The Wolfe 1992 paper is also not included in the reference list. The authors should carry out the glacier mass-balance work for their location specifically or at the very least be clearer about how they arrive at these numbers for temperature and precipitation change. They can also find independent estimates of temperature and precipitation changes during this interval from climate modeling work, such as the TRACE simulations mentioned later, to compare with their estimates.
Lines 888-889 – The Great Basin lakes that are included in this comparison need to be listed by name and latitude as the timing of lake high stands varies with latitude during the deglaciation (see for example McGee et al., 2018).
Lines 892-893 – Again suggest that you expand to other North Atlantic records to document the timing of the Heinrich Event and not the regional Ostolo Cave record on its own.
Line 903 – change the partial derivative symbol being used in the carbon isotope notation to a lower-case delta symbol
930-931 – As stated above, 16.2 is not a defined beginning of Heinrich Event 1, rather there are records from the North Atlantic indicating that IRD and AMOC slowdown began well before this.
Line 938 – It is not clear where the estimate of a 40% reduction in North Pacific moisture is coming from. Please provide citations or analysis to support this number.
Lines 941-942 – The increase in speleothem d13C beginning at ~16.25 ka does imply that conditions may start to become drier, but overall the McLean’s Cave record indicates relatively wet conditions between ~16.4 and 16 ka.
Lines 986-989 – Again, these dates are not an appropriate choice for the start of the Heinrich event based on evidence form the North Atlantic.
Lines 1018-1028: This section linking the WAIS divide record to the tropical Pacific is missing the subsequent link to western North American climate. If this is important, include a description of this mechanism with citations. The last two lines “implying a substantial thinning of the LIS” require citations.
Figure 9 – illustrates the proposed ice sheet thinning –While the majority of the text suggests a big change at 16.2 – this figure shows the resultant ice sheet much later at 15.6 ka. This is also based on another reconstruction than ICE-5G, while ICE-5G is emphasized earlier. I’m not sure how much this figure adds given the inconsistencies with the timing and other aspects of the hypothesis.
References: McGee, E. Moreno-Chamarro, J. Marshall, E. D. Galbraith, Western U.S. lake expansions during Heinrich stadials linked to Pacific Hadley circulation. Sci. Adv. 4, eaav0118 (2018)
M. Hudson, B. J. Hatchett, J. Quade, D. P. Boyle, S. D. Bassett, G. Ali, M. G. De los Santos, North-south dipole in winter hydroclimate in the western United States during the last deglaciation. Sci. Rep. 9, 4826 (2019).
He, Z. Liu, B. L. Otto-Bliesner, E. C. Brady, C. Zhu, R. Tomas, P. U. Clark, J. Zhu, A. Jahn, S. Gu, J. Zhang, J. Nusbaumer, D. Noone, H. Cheng, Y. Wang, M. Yan, Y. Bao, Hydroclimate footprint of pan-Asian monsoon water isotope during the last deglaciation. Sci. Adv. 7, eabe2611 (2021)
F. McManus, R. Francois, J.-M. Gherardi, L. D. Keigwin, S. Brown-Leger, Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes. Nature 428, 834–837 (2004)
J.L. Oster, S. Macarewich, M. Lofverstrom, C. de Wet, I, Montanez, J. Lora, C. Skinner, C. Tabor, North Atlantic Meltwater during Heinrich Stadial 1 drives wetter climate with more atmospheric rivers in western North America. Sci Adv, eadj2225 (2023)Broecker and A. Putnam, How did the hydrologic cycle respond to the two-phase Mystery Interval. Quat. Sci. Rev. 57, 17-25 (2012)
J.L. Bernal-Wormull, A. Moreno, C. Pérez-Mejías, M. Bartolomé, A. Aranburu, M. Arriolabengoa, E. Iriarte, I. Cacho, C. Spötl, R.L. Edwards, H. Cheng, Immediate temperature response in northern Iberia to last deglacial changes in the North Atlantic, Geology, 49, 999-1003 (2021)
Voelker, A.H.L. de Abreu L., Schonfeld J. Erlenkeuser H. and Abrantes F. 2009 Hydrographic conditions along the western Iberian margin during marine isotope stage2 Geochemistry Geophysics Geosystems v. 10 Q12U08Eynaud, F., et al., New constraints on European glacial freshwater releases to the North Atlantic Ocean, Geophysical Research Letters, 39, L15601 (2012)
J. L. Oster, I. E. Weisman, W. D. Sharp, Multi-proxy stalagmite records from northern California reveal dynamic patterns of regional hydroclimate over the last glacial cycle. Quat. Sci. Rev. 241, 106411 (2020).
Citation: https://doi.org/10.5194/egusphere-2025-1370-RC2 - AC2: 'Reply on RC2', Richard Becker, 25 Jul 2025
Status: closed (peer review stopped)
-
RC1: 'Comment on egusphere-2025-1370', Matteo Spagnolo, 03 May 2025
Dear Editor and Authors,
I read with great interest this manuscript, which attempts to chronologically constraint the post-Tioga4 deglaciation in the Sierra Nevada, linking it to key palaeoclimatic events that likely drove a relatively rapid and widespread glacier retreat in this region.
The quality and quantity of the work undertaken and presented is truly impressive. The authors have really considered many different aspects which could have impacted the results and looked at these in great detail. A good number of other proxies are also revisited and considered to strengthen their interpretation. The implications from a paleoglaciology and palaeoclimatology point of view are important. And the topic is certainly of interest to a wide international audience. All in all, I therefore encourage a swift acceptance for publication of the submitted manuscript. However, I believe there are a number of potential improvements, the most important ones I list below, in no particular order. I consider these of a minor nature, and I believe it should take relatively little effort to address these. Notice that I will also attach a version of the manuscript with some 55 comments and additional edits, which might be useful to improve the manuscript too.
- The manuscript is too long. I appreciate there is a lot to talk about, but to make it as attractive as possible to a wide audience I recommend some trimming. Could some aspects be moved the supplementary material? For example, do we really need such long narrative around the age calculations (though I appreciate the destination journal somewhat justify it)? Could this be moved to the supplementary and referred to in the main text?
- A clearer justification of the main research focus, i.e. duration of deglaciation (rather than what most other cosmo glacio papers deal with, that is moraine ages). Why does it matter?
- Is there a wider lesson to be learned, which could be of relevance to other palaeoglaciology/climatology studies elsewhere, i.e. not specific to the Sierra or North America?
- Somewhat related to point 1, though I appreciate the effort, I think that the work undertaken to establish snow shielding is a bit of an overkill, given that ultimately we have very little knowledge of how this varied in the past. Can the past 50 or a 100 years or even a few centuries be truly representative of conditions over a timeframe 2 orders of magnitude longer? I suggest the text that relates to this is reduced considerably.
- Please try to steer away from the description of glacial dynamics in terms of percentage of LGM glacier length and use ELA instead, even if transient (not connected to a moraine), which is far more important and robust glacier mass balance wise
- Could you quantify a retreat rate? This will help putting your “rapid and abrupt” deglaciation into context.
- Figure-wise, all great and useful. Dare I ask you to consider providing a 3D cartoon-like/schematic diagram showing the glacier retreating through the studied sites, exemplifying key geomorphological/geological evidence that justify your conclusion along with the ages of the various samples? These diagrams could perhaps be added to figs 3-5.
Looking forward to seeing this great work published as soon as possible,
Matteo Spagnolo
- AC1: 'Reply on RC1', Richard Becker, 25 Jul 2025
-
RC2: 'Comment on egusphere-2025-1370', Anonymous Referee #2, 17 May 2025
Dear Editor and Authors:
I appreciate the opportunity to read this Discussion paper that provides new cosmogenic nuclide ages on post-LGM moraines in the Sierra Nevada and attempts to place the glacial history of this area in a broader regional and global context. The new glacial chronologies will be of great interest to those studying the history of this region, and are an exciting addition to the multi-archive, multi-proxy body of work on western North American paleoclimate.
I find the new ages to be interesting and well-presented. My primary comments on this manuscript are centered on the treatment of regional proxy information and the interpretations about the broader expression of Heinrich Stadial 1, so this is what I will discuss for the remainder of this comment. I find that the discussion of the temporal evolution of events in the North Atlantic region and the western US is a bit muddled in the discussion. The previously published proxy information that the authors present here could be treated more carefully and clearly.
The authors emphasize a very narrow window for “Heinrich Event 1” which they define from a Spanish stalagmite record from Ostolo Cave. This is shown in the graphical abstract and in Figure 8 with yellow shading and labeled “HE1”. The Ostolo Cave record suggests a change in the d18O of the moisture source as well as temperature in an excursion contemporaneous with IRD in the Bay of Biscay (Eynaud et al., 2012) and the change in seawater isotopic composition (Voelker et al., 2009). The Ostolo Cave paper by Bernall-Wormull which you cite, interprets this negative excursion in d18O as follows:
“An exceptional light δ18Ospeleo excursion centered at 16.2–16.0 kyr B.P. is interpreted to reflect the major phase of HE1 iceberg melting reaching the Iberian Peninsula, which drastically changed the δ18O composition of regional precipitation.”
While this is a precisely dated record, it is a regional expression of the Heinrich Event in Spain and should be taken in context with other North Atlantic records of the event – which Bernall-Wormhull also show in their figure 4. I suggest you also compare to the records of IRD and foraminifera d18O which suggest a broader peak for meltwater release between ~16.5 and 16 ka – rather than shading this sharp speleothem δ18O excursion as the full expression of the Heinrich Event in your Figure 8 and associated discussion in the text. The record of 231Pa/230Th in North Atlantic sediments suggests that AMOC slowdown began closer to 18 ka, again reaching a minimum close to 16 ka (McManus et al., 2004). Similar timing of freshwater release to the North Atlantic is used in the TRACE and iTRACE transient climate model simulations (He et al., 2011; He et al., 2021). A summary of Heinrich Stadial 1 model results and proxy records for the western US is provided in Oster et al., 2023. This supports that the window that you have chosen for Heinrich Event 1 is too narrow.
Additionally, there is a long-standing discussion of Heinrich Stadial 1 in the western US and whether it contained 2 phases - one that was overall drier- particularly in the southwest, and one that was much wetter centered on 16 ka. This discussion is ongoing, but much evidence has come from the timing of lake high stands and how they have varied across the region (Broecker and Putnam, 2012; McGee et al., 2018; Hudson et al., 2019; Oster et al., 2020). At any rate, the development of Heinrich Stadial 1 in this region has been well-explored in the literature, and that discussion should be reflected here.
Following this idea, the discussion on Great Basin Lakes (Section 5.5.3) is oversimplified and under-cited. There have been very nice compilations of Great Basin Lake high stands that include analysis of the timing and geographic patterns and include modeling and other regional syntheses (McGee et al., 2018; Hudson et al., 2019). The presentation of the lake data in Figure 8d is also oversimplified which carries through to the graphical abstract. Only some of the lakes are labeled. It is unclear if only one age is provided per lake or if there are more. This needs to be clarified in the figure and caption.
The 6-degree jump (lines 949-950 and other places) appears from Figure 8 to be defined by the high stands of Lake Cochise and Lake Russell or Surprise. However, there are numerous other high stands on your figure prior to 16.4 ka that are further north than Cochise. Cochise and Estancia are also outside of the Great Basin. It is not consistent with the uncertainty on the ages of lake high stands to pinpoint 6 degree jump within a 200 year interval.
Provide a citation for the hypothesis on the delay of Owens and Russell high stands due to meltwater (Lines 956-958). It is stated as a fact here but with a question mark in your figure – is this a hypothesis put forward by this paper or elsewhere?
Regarding your McLean’s Cave age model, there is a paper currently in review that includes new U-series dates for McLean’s Cave and a new age model run using the COPRA algorithm (Breitenbach et al., 2012), that I believe shifts the minimum in d13C of this record slightly older than what your Bchron age model put it at and closer to 16.25 ka. I realize this information is not yet accessible, but caution against putting too much stock in the exact timing of the shift in McLean’s Cave δ13C computed from the Bchron model here. This is consistent with the comment to broaden the constraints of the timing on Heinrich Stadial 1 used in this paper, which can be done with the records available in the literature.
I am also curious about the emphasis on the ice sheet thinning rather than changes in freshwater flux and AMOC that are frequently modeled and evaluated in discussions of Heinrich Event impacts. This comes through in the discussion of the TrACE results (Example Lines 1005 to 1017) which misses the influx of meltwater to the North Atlantic and the subsequent impacts to AMOC. Other modeled scenarios of Heinrich 1 (McGee et al., 2018; Oster et al., 2023) investigate the cascading influences of this freshwater flux to the North Atlantic. It was surprising to me to equate the changes noted around 16 ka in the proxy records from western North America with the shift in the ice sheet parameterized with ICE-5G between 15 ka and 14 ka in the model simulations. There are proxy records from western North America that cover this interval, including the records in your Figure 8. It is more appropriate to compare the atmospheric changes and drivers in TrACE at 16 ka to the proxy records from 16 ka. I think the authors should consider the influences of meltwater flux, SST changes and AMOC shifts and those teleconnections which do align in timing with the shifts seen in western North American proxy records (McGee et al., 2018; Oster al., 2023).
Below are more targeted comments by line:
Abstract:
Line 19 – Does the “60% LGM length” refer to the length of glaciers? Not clear.
Overall abstract is too specific with place names and is difficult to follow.
Graphical Abstract – For the great basin highstands – which lakes are you talking about here – what are the ages from? The citations you have are for papers that include multiple lakes, and the high stands vary across the region. Are the ages for individual lake highstands or are they curves for different lakes (multiple points per lake). (Similar comments on Figure 8).
Introduction:
The discussion of timing in the introduction needs to be more concrete and include the available age information of the time periods discussed leading into this study. In part this is presented in the setting section, but the information should be included in the introduction to set up your central questions/objectives. Specifically:
-Line 63 – provide a time range to go with “broadly in-phase with the global Last Glacial Maximum”.
Lines 66-71 – please also provide timing estimates for these advances and retreats based on the references included here. It is important to situate your discussion of Heinrich 1 in time.
-Line 82 – first mention of the Tioga 4 glaciation – need to define what the timing of this glaciation is understood to be for non-specialists.
-Line224 – do not think you need to include the phrase “is especially relevant to this manuscript”. You can explain the proximity to your sites and make the relevance apparent.
Line 617 – it is more typical to report 2 sigma uncertainties for U-series ages.
Lines 668-674 – This reads a bit too colloquially – the discussion prior to this centers on why the different calculators may be returning different ages. But then these sentences dismiss this question and say that it is not the central task of this paper to figure this out. Suggest rewriting to this to emphasize what is important, rather than what isn’t and to tone down colloquial language.
Overall, I suggest fewer parenthetical observations like the example in Lines 682-683. These are distracting and are used unevenly. Just include ideas in the sentences proper.
Line 734 – missing word “Accepting the bulk organic radiocarbon dates as accurate provides minimum limits…” or something like this.
Lines 741-749 – suggest adding the ages of the outliers into the paragraph in the appropriate places to aid comparison with the mean.
Line 759 – “probably” should be “probable”
Lines 783-788 – add the numbers into this narrative so that it is easier to follow and see what your interpreted chronology is.
Section 5.4 – it is not clear where the constraints on temperature and precipitation changes given in this section are coming from. It does not appear that independent glacier mass-balance modeling was undertaken for this study. Where do the estimates of 2 degrees and 35% precipitation change come from? The papers cited in this section are not unique to changes in the Sierra Nevada. The Wolfe 1992 paper is also not included in the reference list. The authors should carry out the glacier mass-balance work for their location specifically or at the very least be clearer about how they arrive at these numbers for temperature and precipitation change. They can also find independent estimates of temperature and precipitation changes during this interval from climate modeling work, such as the TRACE simulations mentioned later, to compare with their estimates.
Lines 888-889 – The Great Basin lakes that are included in this comparison need to be listed by name and latitude as the timing of lake high stands varies with latitude during the deglaciation (see for example McGee et al., 2018).
Lines 892-893 – Again suggest that you expand to other North Atlantic records to document the timing of the Heinrich Event and not the regional Ostolo Cave record on its own.
Line 903 – change the partial derivative symbol being used in the carbon isotope notation to a lower-case delta symbol
930-931 – As stated above, 16.2 is not a defined beginning of Heinrich Event 1, rather there are records from the North Atlantic indicating that IRD and AMOC slowdown began well before this.
Line 938 – It is not clear where the estimate of a 40% reduction in North Pacific moisture is coming from. Please provide citations or analysis to support this number.
Lines 941-942 – The increase in speleothem d13C beginning at ~16.25 ka does imply that conditions may start to become drier, but overall the McLean’s Cave record indicates relatively wet conditions between ~16.4 and 16 ka.
Lines 986-989 – Again, these dates are not an appropriate choice for the start of the Heinrich event based on evidence form the North Atlantic.
Lines 1018-1028: This section linking the WAIS divide record to the tropical Pacific is missing the subsequent link to western North American climate. If this is important, include a description of this mechanism with citations. The last two lines “implying a substantial thinning of the LIS” require citations.
Figure 9 – illustrates the proposed ice sheet thinning –While the majority of the text suggests a big change at 16.2 – this figure shows the resultant ice sheet much later at 15.6 ka. This is also based on another reconstruction than ICE-5G, while ICE-5G is emphasized earlier. I’m not sure how much this figure adds given the inconsistencies with the timing and other aspects of the hypothesis.
References: McGee, E. Moreno-Chamarro, J. Marshall, E. D. Galbraith, Western U.S. lake expansions during Heinrich stadials linked to Pacific Hadley circulation. Sci. Adv. 4, eaav0118 (2018)
M. Hudson, B. J. Hatchett, J. Quade, D. P. Boyle, S. D. Bassett, G. Ali, M. G. De los Santos, North-south dipole in winter hydroclimate in the western United States during the last deglaciation. Sci. Rep. 9, 4826 (2019).
He, Z. Liu, B. L. Otto-Bliesner, E. C. Brady, C. Zhu, R. Tomas, P. U. Clark, J. Zhu, A. Jahn, S. Gu, J. Zhang, J. Nusbaumer, D. Noone, H. Cheng, Y. Wang, M. Yan, Y. Bao, Hydroclimate footprint of pan-Asian monsoon water isotope during the last deglaciation. Sci. Adv. 7, eabe2611 (2021)
F. McManus, R. Francois, J.-M. Gherardi, L. D. Keigwin, S. Brown-Leger, Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes. Nature 428, 834–837 (2004)
J.L. Oster, S. Macarewich, M. Lofverstrom, C. de Wet, I, Montanez, J. Lora, C. Skinner, C. Tabor, North Atlantic Meltwater during Heinrich Stadial 1 drives wetter climate with more atmospheric rivers in western North America. Sci Adv, eadj2225 (2023)Broecker and A. Putnam, How did the hydrologic cycle respond to the two-phase Mystery Interval. Quat. Sci. Rev. 57, 17-25 (2012)
J.L. Bernal-Wormull, A. Moreno, C. Pérez-Mejías, M. Bartolomé, A. Aranburu, M. Arriolabengoa, E. Iriarte, I. Cacho, C. Spötl, R.L. Edwards, H. Cheng, Immediate temperature response in northern Iberia to last deglacial changes in the North Atlantic, Geology, 49, 999-1003 (2021)
Voelker, A.H.L. de Abreu L., Schonfeld J. Erlenkeuser H. and Abrantes F. 2009 Hydrographic conditions along the western Iberian margin during marine isotope stage2 Geochemistry Geophysics Geosystems v. 10 Q12U08Eynaud, F., et al., New constraints on European glacial freshwater releases to the North Atlantic Ocean, Geophysical Research Letters, 39, L15601 (2012)
J. L. Oster, I. E. Weisman, W. D. Sharp, Multi-proxy stalagmite records from northern California reveal dynamic patterns of regional hydroclimate over the last glacial cycle. Quat. Sci. Rev. 241, 106411 (2020).
Citation: https://doi.org/10.5194/egusphere-2025-1370-RC2 - AC2: 'Reply on RC2', Richard Becker, 25 Jul 2025
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