Status: this preprint is open for discussion and under review for Climate of the Past (CP).
Antarctic sea ice over the past 130,000 years, Part 1: A review of what proxy records tell us
Xavier Crosta1,Karen E. Kohfeld2,3,Helen C. Bostock4,Matthew Chadwick5,Alice Du Vivier6,Oliver Esper7,Johan Etourneau1,8,Jacob Jones2,Amy Leventer9,Juliane Müller7,Rachel H. Rhodes10,Claire S. Allen5,Pooja Ghadi11,Nele Lamping7,Carina Lange12,13,14,Kelly-Anne Lawler15,David Lund16,Alice Marzocchi17,Katrin J. Meissner18,19,Laurie Menviel18,20,Abhilash Nair21,Molly Patterson22,Jennifer Pike23,Joseph G. Prebble24,Christina Riesselman25,Henrik Sadatzki7,Louise C. Sime5,Sunil K. Shukla26,Lena Thöle27,Maria-Elena Vorrath7,Wenshen Xiao28,and Jiao Yang29Xavier Crosta et al.Xavier Crosta1,Karen E. Kohfeld2,3,Helen C. Bostock4,Matthew Chadwick5,Alice Du Vivier6,Oliver Esper7,Johan Etourneau1,8,Jacob Jones2,Amy Leventer9,Juliane Müller7,Rachel H. Rhodes10,Claire S. Allen5,Pooja Ghadi11,Nele Lamping7,Carina Lange12,13,14,Kelly-Anne Lawler15,David Lund16,Alice Marzocchi17,Katrin J. Meissner18,19,Laurie Menviel18,20,Abhilash Nair21,Molly Patterson22,Jennifer Pike23,Joseph G. Prebble24,Christina Riesselman25,Henrik Sadatzki7,Louise C. Sime5,Sunil K. Shukla26,Lena Thöle27,Maria-Elena Vorrath7,Wenshen Xiao28,and Jiao Yang29
Received: 24 Mar 2022 – Discussion started: 06 Apr 2022
Abstract. Antarctic sea ice plays a critical role in the Earth system, influencing energy, heat, and freshwater fluxes, air-sea gas exchange, ice shelf dynamics, ocean circulation, nutrient cycling, marine productivity, and global carbon cycling. However, accurate simulation of recent sea-ice changes remains challenging, and therefore projecting future sea-ice changes and their influence on the global climate system is uncertain. Reconstructing past changes in sea-ice cover can provide additional insights into climate feedbacks within the Earth system at different timescales. This paper is the first of two review papers from the Cycles of Sea Ice Dynamics in the Earth system (C-SIDE) Working Group. In this first paper, we review marine- and ice core-based sea-ice proxies and reconstructions of sea-ice changes throughout the last glacial-interglacial cycle.
Antarctic sea-ice reconstructions rely mainly on diatom fossil assemblages and highly branched isoprenoid (HBI) alkenes in marine sediments, supported by chemical proxies in Antarctic ice cores. Most reconstructions for the Last Glacial Maximum (LGM) suggest winter sea-ice expanded all around Antarctica and covered almost twice its modern surface extent. In contrast, LGM summer sea-ice expanded mainly in the regions off the Weddell and Ross seas. The difference between winter and summer sea ice during the LGM led to a larger seasonal cycle than today. More recent efforts have focused on reconstructing Antarctic sea-ice during warm periods, such as the Holocene and the Last Interglacial (LIG), which may serve as an analogue the future. Notwithstanding regional heterogeneities, existing reconstructions suggest sea-ice cover increased from the warm mid-Holocene to the colder Late Holocene, with pervasive decadal-to-millennial scale variability throughout the Holocene. Sparse marine and ice core data, supported by proxy modelling experiments, suggest that sea-ice cover was halved during the warmer LIG, when global average temperatures were ~2 °C above the pre-industrial (PI).
There are limited marine (14) and ice core (4) sea-ice proxy records covering the complete 130,000 year (130 ka) last glacial cycle. The glacial-interglacial pattern of sea-ice advance and retreat appears relatively similar in each basin of the Southern Ocean. Rapid retreat of sea ice occurred during Terminations II and I, while the expansion of sea ice during the last glaciation appears more gradual, especially in cores data sets. Marine records suggest that the first prominent expansion occurred during Marine Isotope Stage (MIS) 4 and that sea ice reached maximum extent during MIS 2. We however note that additional sea-ice records and transient model simulations are required to better identify the underlying drivers and feedbacks of Antarctic sea-ice changes over the last 130 ka. This understanding is critical to improve future predictions.
The paper by Crosta et al. reviews the current knowledge in terms of i) the use of both marine and ice core sea ice proxies in Antarctica, ii) reconstructions so far produced on Antarctic sea ice changes for the last 130,000 years. This review is more than welcome and useful for a broad audience. The introduction brings the reader into the Southern Ocean setting and Antarctic sea ice is described as one of the players in this environment. The interplays between sea ice and the ocean, atmosphere, biosphere and the cryosphere are presented. The mechanisms for sea ice formation are described in Section 2.1, along with the geographical sea ice distribution during the modern time. The authors then describe the satellite-era sea ice trends (1979-present) and the difficulty of climate models to reproduce them, hence highlighting the difficulty to isolate the main forcings responsible for such trends (Sect. 2.2). One way to go is look at the past, and the authors do that by first introducing the marine and ice core proxies (Section 3). A review of the available sea ice reconstructions are presented for the Last Glacial Maximum (Sect. 4.1), the Holocene (Sect 4.2), and for the Last Interglacial (Sect. 4.3). A shorter section presents the results during the glaciation and deglaciation shorter periods (Sect. 4.4). The work ends the authors recommendations on how to use and combine the records together to squeeze out further knowledge and fill the spatio-temporal gaps in Antarctic sea ice reconstructions.
My general comment is that this review is very well structured and I appreciated the combination between marine and ice core data. The phrasing is also very clear and well balanced in highlighting the assumptions behind the proxies and their limitations. It was a great pleasure to read.
Below a list of mostly minor suggestions and typos:
- Decide on “sea-ice” or “sea ice”. I’d use the latter but that’s just my taste.
- L75: “cores” should be “core”.
- L120: in the Figure caption I’d define again the acronyms.
- L141: I’d add “sea ice” after “observed”.
- L144: I’d remove “in ice area trends”.
- L152: I’d insert a new line or a new paragraph here to introduce the paper. Here or at the end of L149.
- L185: 18.10 should be 18 x 10. Also at L201.
- L200: I’d replace “decay” with “retreat”.
- L255: after reading Sect. 3 I asked myself whether it would be beneficial to have a table summarizing the various proxies with their salient features, as well as pros and cos. I redirect the question to you.
- L291: it should probably read “produced in this way”.
- L448: I would consider at least mentioning iodine and its linkages to both sea ice and productivity.
- L512: “in the Dome C”.
- L529: I’d rephrase in “This pattern was attributed to the..”, “..location of the first year sea ice”.
- L534: in this paragraph, maybe here, I’d specifically state that one of the hot topics in Brenr is the relative differences between ssNa and Br transport mechanisms, given the latter is also present in the gas phase.
- L615: I would consider showing the SSI lines in red in the upper panels and the F. obliquecostata as red shading in the panel below. I would also increase the lat,lon font. A question I had while reading the figure is why EDML and PS1768-8 records specifically ? Maybe it could be worth adding a sentence on that somewhere. I would be also curious to compute a (normalized) ensemble of all the Winski et al. (2021) ssNa curves to compare to the EDML one - maybe an ensemble would be more representative on a spatially integrated sea ice signal over Antarctica ? That’s just a curiosity.
- L622. Insert a full stop before “Upper”.
- L744: I suspect the main reason is because the ice core signal integrates a wide region ?
Despite its importance in global climate, our knowledge of Antarctic sea-ice changes throughout the last glacial-interglacial cycle is extremely limited. As part of the Cycles of Sea Ice Dynamics in the Earth system (C-SIDE) Working Group, we here review marine- and ice core-based sea-ice proxies to provide insigths in their applicability and limitations. By compiling published records, we provide information on Antarctic sea-ice dynamics over the past 130,000 years.
Despite its importance in global climate, our knowledge of Antarctic sea-ice changes throughout...
