Estimating Southern Ocean sea ice extent: A new method based on the oxygen isotopic signature of foraminifera
Abstract
Southern Ocean sea ice plays a central role in the meridional overturning circulation, transforming globally prevalent watermasses through buoyancy loss and gain at the sea surface. Buoyancy loss due to sea ice growth creates bottom water that flows into the Atlantic, Indian, and Pacific basins, while buoyancy gain due to sea ice melt transforms 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 carbon dioxide 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 (δ18O) of foraminifera. In the seasonal sea ice zone, surface cooling and sea ice formation creates a cold surface mixed layer, known as winter water, which persists in the subsurface (~50-150 m) during austral spring and summer. The inverted temperature profile yields a unique equilibrium δ18O profile that can be reconstructed using planktonic and benthic foraminiferal δ18O. We will present a compilation of N. pachyderma and Cibicidoides spp. results from the Atlantic sector of the Southern Ocean that yields an estimate of winter sea ice extent consistent with modern observations. Despite having the highest shell fluxes during austral summer, N. pachyderma appears to calcify in sub-surface winter water, meaning its oxygen isotopic signature reflects the influence of surface freezing during sea ice formation. We will also present a preliminary estimate of Southern Ocean sea ice extent for the LGM and discuss strategies for improving sea ice reconstructions using foraminiferal δ18O during the both the LGM and deglaciation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMPP0290004L
- Keywords:
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- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 0429 Climate dynamics;
- BIOGEOSCIENCES;
- 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCES;
- 4273 Physical and biogeochemical interactions;
- OCEANOGRAPHY: GENERAL