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An international research team led by Kiel University (CAU) has systematically investigated for the first time how the WAIS responded to temperature increases in the Southern Ocean during a natural warm period, Marine Isotope Stage 11 (MIS 11), about 400,000 years ago. The results, published in the journal Nature Communications, provide valuable insights into the conditions under which the WAIS became unstable in the geological past and what parallels this could have for current and future warming scenarios.
The West Antarctic Ice Sheet (WAIS) is one of the most dynamic regions of the Antarctic continent. Much of its bed lies below sea level, making the region particularly sensitive to ocean warming. Understanding the development of the WAIS is central to anticipating future sea level changes. If the WAIS were to melt completely, global sea levels could rise by more than four meters.
"Our results show that the West Antarctic Ice Sheet has been sensitive to warming in the Southern Ocean, especially in the circumpolar deep water, in the past," says first author Lena Jebasinski, a doctoral researcher at the Institute of Geosciences at Kiel University. "This is an important warning signal, as we are currently observing similar trends in temperature development in the region, which could jeopardize the stability of the Antarctic ice sheet in the short or long term. The conditions at that time and today's developments show clear parallels."
Insight into the warm period of the future
The warm phase studied, which occurred around 400,000 years ago, is known as Marine Isotope Stage 11 (MIS 11). Considered one of the longest and most stable warm periods of the past million years, it is often used in climate research as a reference period for the present and future. During this time, global temperatures were up to two degrees Celsius higher than pre-industrial levels, while CO₂ concentrations were similar to those in the pre-industrial period. Sea levels were about six to 13 meters higher than today, suggesting that the West Antarctic ice sheet had significantly receded at that time.
Traces in the ocean floor: clues from fossil micro shells
The international research team from Germany, the US, and the UK examined the chemical composition of fossil foraminifera, tiny single-celled marine organisms preserved in sediment cores from the Pacific sector of the Southern Ocean. These calcareous shells store information about past environmental conditions. The researchers found recurring phases during MIS 11 in which the deep water in the Pacific Southern Ocean had lower oxygen concentrations, known as oxygen minimum events. These phases usually coincided with a warming of the ascending circumpolar deep water. Under certain conditions, this water comes into contact with the Antarctic ice sheet at the base of the ice shelves. Such warming promotes the melting of the ice shelves and thus impairs the hinterland‘s ice stability. The resulting increase in meltwater entering the ocean likely reduced in the formation of dense Antarctic bottom water, a crucial component of global overturning circulation. "This could explain why the West Antarctic ice sheet was significantly smaller during the warm period studied than it is today, and why global sea levels were significantly higher than they are today," explains Lena Jebasinski, a paleoclimate researcher writing her doctoral thesis on the subject.
Innovative methodology provides a glimpse into the past
To reconstruct the environmental conditions in the Southern Ocean during this time period, the research team combined several geochemical methods. They primarily examined foraminifera. Under oxygen-depleted conditions, solid uranium compounds are deposited around the shells of these tiny marine organisms in the sediment. This signature allowed the researchers to draw conclusions about past oxygen deficits in the bottom water of the central Pacific Southern Ocean. Additionally, the researchers determined the neodymium isotope ratios, a biologically independent indicator of water mass circulation. This demonstrates that the observed oxygen minimum events are due to physical changes in the ocean rather than fluctuations in biological activity. Furthermore, the foraminiferal shells were analyzed for indicators of bottom water temperature. By combining these methods, the team was able to create a detailed picture of the interactions between the temperature of circumpolar intermediate waters, oxygen content, and ocean circulation during the warm period 400,000 years ago.
Widespread instability of the West Antarctic ice sheet
"Until now, these oxygen minimum events could only be detected in the Atlantic part of the Southern Ocean. Our data now show for the first time that the Pacific sector may also have been affected at the same time. The instability of the West Antarctic Ice Sheet at that time must therefore have been significantly greater than previously assumed," explains Professor Julia Gottschalk, head of the Paleoceanography and Marine Geology working group at Kiel University and co-author of the study. If current trends in the Southern Ocean warming continue, disturbances in the WAIS mass balance and Antarctic bottom water formation are likely to become more frequent in the future.
These new findings are based on sediment cores obtained during Expedition 383 "Dynamics of the Pacific Antarctic Circumpolar Current (DYNAPACC)", aboard the research vessel JOIDES Resolution in 2019 as part of the International Ocean Discovery Program (IODP). Professor Julia Gottschalk, head of the Paleoceanography and Marine Geology working group at Kiel University, was directly involved in the expedition. "Such long drill cores are unique archives of our climate system. We can read from them how closely the ocean and ice in the southern high latitudes have been intertwined over thousands of years," Gottschalk emphasizes. "Understanding these past processes provides crucial insight into how Antarctica, and thus global sea levels, might respond to warmer climatic conditions than those experienced today," she adds.
In addition to Kiel University the new study includes researchers from the Alfred Wegener Institute for Polar and Marine Research (Bremerhaven), the University of Delaware (Newark, USA), Columbia University's Columbia Climate School (New York, USA), and the University of Portsmouth (UK).
Photos to download:
http://www.uni-kiel.de/de/detailansicht/news/172-westantarktisches-eisschild
Lena Jebasinski
Institute of Geosciences
Paleoceanography and Marine Geology
Christian Albrecht University of Kiel (CAU)
Email: lena.jebasinski@ifg.uni-kiel.de
Jebasinski, L., Frick, D. A., Kapuge, A. K. I. U., Basak, C., Saavedra-Pellitero, M., Winckler, G., Lamy, F. et al. (2025). Southern Ocean evidence for recurring West Antarctic Ice Sheet destabilization during Marine Isotope Stage 11. Nature Communications, 16(1). https://doi.org/10.1038/s41467-025-65002-9
https://joidesresolution.org/expedition/383/ Link to the expedition with the Joides Resolution with further information
https://www.paleoceanography.ifg.uni-kiel.de/de About the CAU Working Group on Paleoceanography and Marine Geology
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