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Abstract Southern Ocean sea ice plays a central role in the oceanic meridional overturning circulation, transforming globally prevalent watermasses through surface buoyancy loss and gain. Buoyancy loss due to surface cooling and sea ice growth promotes the formation of bottom water that flows into the Atlantic, Indian, and Pacific basins, while buoyancy gain due to sea ice melt helps transform the returning deep flow into intermediate and mode waters. Because northward expansion of Southern Ocean sea ice during the Last Glacial Maximum (LGM; 19–23 kyr BP) may have enhanced deep ocean stratification and contributed to lower atmospheric CO2levels, reconstructions of sea ice extent are critical to understanding the LGM climate state. Here, we present a new sea ice proxy based on the18O/16O ratio of foraminifera (δ18Oc). In the seasonal sea ice zone, sea ice formation during austral winter creates a cold surface mixed layer that persists in the sub‐surface during spring and summer. The cold sub‐surface layer, known as winter water, sits above relatively warm deep water, creating an inverted temperature profile. The unique surface‐to‐deep temperature contrast is reflected in estimates of equilibrium δ18Oc, implying that paired analysis of planktonic and benthic foraminifera can be used to infer sea ice extent. To demonstrate the feasibility of the δ18Ocmethod, we present a compilation ofN. pachydermaandCibicidoidesspp. results from the Atlantic sector that yields an estimate of winter sea ice extent consistent with modern observations.
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This content will become publicly available on July 28, 2026
Increase in Deglacial Ocean Heat Content Linked to Contrasts in Extratropical Warming
Abstract Proxy‐based reconstructions suggest that equilibrium changes in global mean sea surface temperature (ΔGMSST) are nearly equivalent to changes in mean ocean temperature (ΔMOT) on glacial‐interglacial timescales over the past 900,000 years. However, the underlying mechanisms responsible for this relationship remain poorly understood. Here we use simulations from Paleoclimate Modeling Intercomparison Project Phase 3 and 4 (PMIP3/4) to investigate equilibrium ΔMOT and its linkage to sea surface temperature changes between the Last Glacial Maximum (LGM, ∼21,000 years ago) and pre‐Industrial. Results show that PMIP3/4 simulations generally underestimate proxy‐based ΔMOT. Regression analysis reveals that LGM MOT is strongly modulated by mid‐latitude SST cooling, with the Southern Ocean having a greater influence compared to other oceanic regions, thus helping explain why models with similar ΔGMSSTs exhibit significantly different ΔMOTs. Additionally, we find a strong relationship between simulated Antarctic sea‐ice coverage and Southern Ocean SST changes, with implications for constraining sea‐ice reconstructions.
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- Award ID(s):
- 2103032
- PAR ID:
- 10645332
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 52
- Issue:
- 14
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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