Sea Surface Conditions and Antarctic Bottom Water Formation Offshore Cape Darnley, East Antarctica, Over the Last 600 ka: New Insights From the Diatom Record

Abstract

The Cape Darnley marine region of East Antarctica is one of only four sectors where Antarctic Bottom Water (AABW) is produced. AABW formation at Cape Darnley is highly dependent on surface ocean processes—specifically, the high production of frazil ice. Here, we investigated the sea surface conditions at Cape Darnley over the past 600 thousand years, with a focus on previous late Pleistocene interglacials using micropaleontological and geochemical proxies from a marine sediment core from Armand Ridge. Relative diatom abundances revealed two key intervals of interest: Marine Isotope Stage (MIS) 13 (533–478 ka) and 1 (11–0 ka). During MIS 13, an increase in Eucampia antarctica indicated increased micronutrients to the region. Among other mechanisms, this indicated ice shelf melt reflecting retreat of the East Antarctic Ice Sheet through an extended period of muted warmth across the Mid‐Brunhes Transition (MIS 15–MIS 13). While during MIS 1 an increase in the sea ice related diatom species Fragilariopsis curta and Thalassiosira antarctica revealed a signal consistent with frazil ice formation, indicating the late onset of the modern Cape Darnley Polynya and conditions conducive to modern AABW formation. There is no evidence of the polynya during warmer than present interglacials MIS 11 or MIS 5e. This suggests that the formation of AABW at Cape Darnley requires very specific sea ice and oceanographic conditions and is therefore at risk under future climate change.