| Summary: | 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 COâ‚‚ levels, reconstructions of sea ice extent are critical to understanding the LGM climate state. Here, we present a new sea ice proxy based on the ¹â¸O/¹â¶O ratio of foraminifera (δ¹â¸O_c). 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 δ¹â¸O_c, implying that paired analysis of planktonic and benthic foraminifera can be used to infer sea ice extent. To demonstrate the feasibility of the δ¹â¸O_c method, we present a compilation of N. pachyderma and Cibicidoides spp. results from the Atlantic sector that yields an estimate of winter sea ice extent consistent with modern observations. © 2021 American Geophysical Union. Issue Online: 31 May 2021; Version of Record online: 31 May 2021; Accepted manuscript online: 16 May 2021; Manuscript accepted: 06 May 2021; Manuscript revised: 05 May 2021; Manuscript received: 25 August 2020. We would like to thank three anonymous reviewers for their constructive suggestions that markedly improved the initial submission. This work has also benefitted from discussions with Steve Rintoul, Liz Sikes, and Jess ...
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