Tracking Southern Ocean sea ice extent with winter water: a new method based on the oxygen isotopic signature of foraminifera

• Published in: Paleoceanography and Paleoclimatology
• Main Authors: Lund, DC, Chase, Z, Kohfeld, KE, Wilson, EA
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley-Blackwell Publishing Inc. 2021
• Subjects: Earth Sciences; Oceanography; Chemical oceanography; Austral; Indian; Pacific; Southern Ocean; Sea ice
• Online Access: https://doi.org/10.1029/2020PA004095; http://ecite.utas.edu.au/150278

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; 1923 kyr BP) may have enhanced deep ocean stratification and contributed to lower atmospheric CO 2 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 18 O/ 16 O ratio of foraminifera (δ 18 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 δ 18 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 δ 18 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.