The Antarctic Circumpolar Current in the glacial Indian Ocean: A multi-site paleoceanographic reconstruction

During the Last Glacial Maximum (LGM), Earth's climate system operated under profoundly different boundary conditions. Ice sheets were larger, and sea level, global temperature and atmospheric CO2 levels were lower. Paleoceanographic studies also indicate that ocean circulation was substantially different from modern. However, gaps remain in our understanding of the under-sampled Southern Ocean. The dynamics of the wind-driven Antarctic Circumpolar Current (ACC) remain poorly constrained, despite its importance as the world's largest current, carrying ~137 Sv eastward around Antarctica. Previous reconstructions have largely focused on the flow of the ACC in the Drake Passage; with little known about its evolution in the Indian Ocean (IO) sector. In the modern Southern IO, the ACC passes south of Africa before meandering around the Kerguelen Plateau, through the deep Kerguelen-St Paul passage, then south of Australia into the Pacific Ocean. ACC volume transport through the Kerguelen-St Paul passage rivals that through Drake Passage. Here, we present reconstructions of near-bottom flow speed variability in key ACC pathways in the IO since the LGM. We utilize sediment core records from the SW- IO (Agulhas Plateau), SE- IO (Amsterdam-St. Paul Plateau and Broken Ridge) and the southern Tasman Rise. We employ the "Sortable Silt" paleocurrent proxy, which yields quantitative reconstructions of near-bottom flow speed by exploiting the sensitivity of the mean size of abiogenic sediments in the 10-63µm fraction to this flow. Initial results reveal a heterogenous glacial-interglacial pattern in ACC flow speeds between the SW- and SE-IO, with faster (slower) 'Sortable Silt' flow speed in the SW (SE) Indian Ocean during the LGM. These results imply that the ACC was stronger or more equator-ward south of Africa, and either weaker or following a drastically different path in the SE Indian Ocean. Paleocurrent records from additional sites in the SE-IO and South Tasman Rise as well as comparisons to paleoclimate model results will help to resolve: (1) how regionally robust these results are; (2) what the underlying causes might be; and (3) what the implications are for large-scale ocean circulation, Southern Westerly Wind placement and the Southern Ocean carbon cycle across the last glacial cycle.