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Tracking Southern Ocean Sea Ice Extent With Winter Water: A New Method Based on the Oxygen Isotopic Signature of Foraminifera

Lund, David C. and Chase, Zanna and Kohfeld, Karen E. and Wilson, Earle A. (2021) Tracking Southern Ocean Sea Ice Extent With Winter Water: A New Method Based on the Oxygen Isotopic Signature of Foraminifera. Paleoceanography and Paleoclimatology, 36 (6). Art. No. e2020PA004095. ISSN 2572-4517. doi:10.1029/2020pa004095.

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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 CO₂ 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 ¹⁸O/¹⁶O ratio of foraminifera (δ¹⁸O_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 δ¹⁸O_c, implying that paired analysis of planktonic and benthic foraminifera can be used to infer sea ice extent. To demonstrate the feasibility of the δ¹⁸O_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.

Item Type:Article
Related URLs:
URLURL TypeDescription
Lund, David C.0000-0002-4847-2889
Chase, Zanna0000-0001-5060-779X
Kohfeld, Karen E.0000-0001-7241-1624
Wilson, Earle A.0000-0003-2329-5115
Additional Information:© 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 Adkins. In particular, we would like to thank Will Hobbs for generously sharing his knowledge of Southern Ocean sea ice and the processes that govern its modern distribution. Data Availability Statement: The ARGO data depicted in Figure S2 were collected and made freely available by the International Argo Program and the national programs that contribute to it (, The Argo Program is part of the Global Ocean Observing System. The stable isotope data in Figure 8 are summarized in Table S1 and are available on the NOAA Paleoclimatology Data website (
Subject Keywords:foraminifera, sea ice, Southern Ocean, oxygen isotopes
Issue or Number:6
Record Number:CaltechAUTHORS:20210519-141321689
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Official Citation:Lund, D. C., Chase, Z., Kohfeld, K. E., & Wilson, E. A. (2021). Tracking Southern Ocean sea ice extent with winter water: A new method based on the oxygen isotopic signature of foraminifera. Paleoceanography and Paleoclimatology, 36, e2020PA004095.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:109196
Deposited By: George Porter
Deposited On:24 May 2021 15:06
Last Modified:01 Dec 2021 08:05

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