Interplay of North Atlantic Freshening and Deep Convection During the Last Deglaciation Constrained by Iberian Speleothems

Endres, Laura; Pérez-Mejías, Carlos; Ivanovic, Ruza; Gregoire, Lauren; Hughes, Anna L. C.; Cheng, Hai; Stoll, Heather

doi:https://doi.org/10.5194/egusphere-2025-3911

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https://doi.org/10.5194/egusphere-2025-3911

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26 Aug 2025

| 26 Aug 2025

Status: this preprint is open for discussion and under review for Climate of the Past (CP).

Interplay of North Atlantic Freshening and Deep Convection During the Last Deglaciation Constrained by Iberian Speleothems

Laura Endres, Carlos Pérez-Mejías, Ruza Ivanovic, Lauren Gregoire, Anna L. C. Hughes, Hai Cheng, and Heather Stoll

Abstract. The last deglaciation featured abrupt climate shifts driven by interactions among Earth system components, notably retreating ice sheets and meltwater input. While globally detected, the magnitude, timing, and sequence of North Atlantic source events remain uncertain. We present a Uranium-Thorium-dated stalagmite from northwestern Iberia spanning 24–12 ka BP, capturing both the impact of North Atlantic meltwater on surface ocean chemistry and regional air temperature changes. Our record reveals primarily gradual meltwater inflow during the Last Glacial Maximum and early deglaciation (about 20.8–18.2 ka BP), followed by abrupt increases during Heinrich Stadial 1. An abrupt cooling lags the first meltwater pulse by ca. 850 years, unlike later pulses. This evolving relationship between meltwater and cooling provides new constraints on the changing sensitivity of deep Atlantic convection to meltwater input throughout the deglaciation.

Received: 11 Aug 2025 – Discussion started: 26 Aug 2025

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Laura Endres, Carlos Pérez-Mejías, Ruza Ivanovic, Lauren Gregoire, Anna L. C. Hughes, Hai Cheng, and Heather Stoll

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RC1:
'Comment on egusphere-2025-3911', Anonymous Referee #1, 23 Sep 2025 reply
The manuscript by Endres et al. presents a high-resolution, Th/U-dated speleothem record from northwestern Iberia spanning 24-12 ka BP. The analyzed proxies are used to reconstruct North Atlantic surface ocean freshening (δ¹⁸O values) and regional temperature changes (δ¹³C) during the last deglaciation. The authors identify major freshening events during Heinrich Stadial 1, and claim that the initial cooling response lagged the first meltwater pulse by approximately 850 years, suggesting evolving AMOC sensitivity to freshwater forcing. Overall, the study provides interesting new constraints on the temporal relationship between ice sheet meltwater discharge and Atlantic Meridional Overturning Circulation strength. The manuscript is overall well-written and the analytical methods are sound. Also the interpretation of the proxies is based on an intensive work of the research group in that area. However, I have some comments on the description of statistical aspects including propagation of uncertainties as well as the discussion of the regional relevance of the results. Overall, the line of arguments could benefit from restructuring the discussion by re-integrating parts of the extended appendices back into the main text. It is a bit exhausting to repeatedly having to switch back and forth between main text and appendix, also given that the main text is not so long.
Other comments:
Age model. The authors state that they have optimized the age model by using the Sr/Mg ratio in the stalagmite. Please discuss a little but further why this approach is valid at this coastal location, where one could expect a significant and varying influence of sea spray, that could affect the Sr/Mg ratio as well. Also provide more details what has been exactly done and how the age model has been optimized. Have you smoothed the Sr/Mg ratio prior correlation analysis with growth rate? What was the highest correlation coefficient? How does this approach influence the finally given uncertainties of the age model (and then, the uncertainties of the break/changepoints).

Breakpoints/Changepoints. The outcomes of the study rely strongly on statistical time series analysis and the correct propagation of uncertainties. It is however unclear to me, how the age model uncertainties have been propagated into the uncertainties of the break/change point analysis. The resulting change/breakpoints are also only given in a table. It would be beneficial to visualize the timing of the events in the stalagmite also in Figure 3. Please also improve the quality of Figure 3, the discussed “drops” or other events in the δ¹³C record are not visible in the Figure, possibly because the y axis is too small. Please improve and make it easier for the reader to follow your arguments…

Meltwater model results. I am not a modeler and after reading the manuscript the relevance of the meltwater/tracer modeling is still unclear to me. I feel like the main conclusions could also be drawn without all this effort..? If I could follow the manuscript correctly, the model results are mainly part of the methods section, and there is only a limited discussion of model implications in the results/discussion. Could the results be helpful to explain a mechanism why there is a 850y lag between meltwater and temperature? Overall, the integration of the model results into the results/discussion section should be improved and the relevance clarified for non-experts in that field.

“Regional/global discussion”. The comparison with other records is very limited and could be strengthened. On the regional scope, only 2 other speleothem records from Iberia and the Mediterranean are mentioned and only in the appendix. It is unclear to me why the discussion then only focuses on comparing to records from the EASM region, where the comparison then turns out to be not as conclusive (compare L296ff). I strongly suggest to revise this section, and focus on a more integrative approach comparing first to records from the wider North Atlantic realm in more detail. There are e.g., some high-resolution records from across the Americas (for example Travis Taylor et al., 2025, Strikis et al., 2015, and others) that are much closer to the centers of action in the North Atlantic. There are also probably some more from Western/Central Europe… (e.g., NALPS, Li et al., 2021, Luetscher et al., 2015, …). Incorporating more records from the wider Atlantic region could support the discussion of potential “atmospheric responses”, that is at this stage only briefly mentioned in L252 or 290, but I regard this is as not irrelevant in this discussion. Also, in a subnote, I wonder about the absence of a drop into the Younger Dryas at the end of the record. I know this is not the focus of the manuscript but it still makes me wonder... Many other stalagmite records show a very sharp and clear drop in δ¹⁸O values around 12.9ka BP (e.g., Cheng et al., 2020, Affolter et al 2019, Li et al., 2021 …) , but this one, that should presumably be very sensitive to meltwater and temperature in the north Atlantic realm, does not really show a clear signal, at least not in δ¹⁸O... What does this mean?

Minor comments along the text:
L65 Please provide more support that Sr/Mg is indeed only growth rate and not related to sea spray
Figure 2 What do the open and filled symbold mean? What is the correlation between Sr/Mg and growth rate?
L171 does the number 382 years correspond to the length of the transition? Please indictae the timing and uncertainties in the Figure, it is hard to see for some of the breakpoints where they are exactly identified.
L189 Also here, I am not sure which “smaller transient event” is meant. Possibly a second plot that zooms only into the most interesting section of the record would be helpful?
Figure 3 please include time markers of identified events in Figure 3. I also cannot see the temperature changes in d13C as mentioned in the text... The relevance of the maps in the uppermost panel are unclear to me.
L225 I cannot see a “significant cooling” in d13C at that time in Figure 3, I think the y-axis not large enough to really see this drop? A clear marker would be also helpful.
L226 what is the uncertainty of the 850 years?
L231 Again, unsure to which specific feature this refers to
L245 I think its vice versa? First the meltwater, then 850 years later the temperature drop?
L276 The “extended discussion” only discusses two other records from Iberia and the Mediterranean, and no ice core records.
L278 Before jumping to the EASM I would have expected a more comprehensive comparison to other records that are more closer to the North Atlantic realm. Is there a specific reason why this is missing?
L290 Any suggestions which “atmospheric patterns”? A more comprehensive discussion could elucidate this possibly?
L292 This is also another reason why records from that region would be worth to compare with!
L299 It would be interesting to see if the American records are better to compare with? Studies have suggested a close link to AMOC and NA temperatures - your record could provide the possibility to test this (compare eg Travis Taylor et al., 2025,Warken et al., 2020...)
L301 what is the uncertainty of the ice record here?
L402 Heading does not fit to content, its only one record also from Iberia discussed
References
Affolter, S., Häuselmann, A., Fleitmann, D., Edwards, R. L., Cheng, H., & Leuenberger, M. (2019). Central Europe temperature constrained by speleothem fluid inclusion water isotopes over the past 14,000 years. Science advances, 5(6), eaav3809.
Cheng, H., Zhang, H., Spötl, C., Baker, J., Sinha, A., Li, H., ... & Edwards, R. L. (2020). Timing and structure of the Younger Dryas event and its underlying climate dynamics. Proceedings of the National Academy of Sciences, 117(38), 23408-23417.
Li, H., Spötl, C., & Cheng, H. (2021). A high‐resolution speleothem proxy record of the Late Glacial in the European Alps: extending the NALPS19 record until the beginning of the Holocene. Journal of Quaternary Science, 36(1), 29-39.
Luetscher, M., Boch, R., Sodemann, H., Spötl, C., Cheng, H., Edwards, R. L., ... & Müller, W. (2015). North Atlantic storm track changes during the Last Glacial Maximum recorded by Alpine speleothems. Nature Communications, 6(1), 6344.
Stríkis, N. M., et al. (2015), Timing and structure of Mega-SACZ events during Heinrich Stadial 1, Geophys. Res. Lett., 42, 5477–5484, doi:10.1002/2015GL064048.
Travis-Taylor, L., Medina-Elizalde, M., Pajón, J.M. et al. Hydroclimate variability in the northern Caribbean during the last deglaciation was modulated by large-scale atmospheric circulation and climate events. Commun Earth Environ 6, 498 (2025). https://doi.org/10.1038/s43247-025-02465-0
Warken, S.F., Vieten, R., Winter, A., Spötl, C., Miller, T.E., Jochum, K.P., Schröder-Ritzrau, A., Mangini, A. and Scholz, D. (2020), Persistent Link Between Caribbean Precipitation and Atlantic Ocean Circulation During the Last Glacial Revealed by a Speleothem Record From Puerto Rico. Paleoceanography and Paleoclimatology, 35: e2020PA003944. https://doi.org/10.1029/2020PA003944

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Citation: https://doi.org/10.5194/egusphere-2025-3911-RC1

Laura Endres, Carlos Pérez-Mejías, Ruza Ivanovic, Lauren Gregoire, Anna L. C. Hughes, Hai Cheng, and Heather Stoll

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Short summary

Stable isotope data of a precisely dated stalagmite from northwestern Iberia indicate gradual North Atlantic meltwater input during the last glacial maximum, followed by abrupt surges early in the last deglaciation. The first abrupt surge was followed by cooling about 850 years later – unlike later events – which reveals that the Atlantic circulation’s sensitivity to meltwater is variable and related to the evolving background climate boundary conditions.


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