Neodymium isotope data measured on mixed species of uncleaned planktic foraminifera, planktic (G. bulloides) and benthic (C. kullenbergi) stable carbon and oxygen isotopes, mean sortable silt size fraction measurements and CaCO3 contents of sediments from giant gravity core TT1811-34GGC, located within the SE Indian/Southern Ocean. Data span the last glacial cycle, from 118,000 years B.P. to the late Holocene. The chain of events surrounding the initiation and intensification of the last glacial cycle remain relatively poorly understood. In particular, the role of Southern Ocean paleocirculation changes is poorly constrained, in part, owing to a paucity of sedimentary records from this region. In this study we present multiproxy data – including neodymium isotope and sortable silt measurements – for paleocirculation changes within the deep (3167 m water depth) Indian sector of the Southern Ocean from a new sediment core, TT1811-34GGC (41.718°S, 80.163°E). We find a tight coupling between circulation changes, Antarctic climate, and atmospheric CO2 concentrations throughout the last 118,000 years, even during the initial stages of glacial inception of Marine Isotope Stage (MIS) 5.4 to 5.1. We find that periods of cooling correspond to reductions in the entrainment of North Atlantic-sourced waters within the deep Southern Ocean, as evidenced by more radiogenic neodymium isotope values of deep water bathing our core site. Cooling also corresponds to generally slower bottom water flow speeds, as indicated by finer sortable silt size fractions. A reduction in entrainment of North-Atlantic sourced waters occurred during MIS 5.4-5.1, when Atlantic circulation was strong, suggesting a Southern hemisphere control on paleocirculation changes at that time. We hypothesize that expanded Southern Ocean sea-ice during MIS 5.4 increased the density of the deep Southern Ocean, reducing the ability of Atlantic-sourced waters to mix into the deep Southern Ocean. This led to an expanded contribution of Pacific Deep Water within the lower circulation cell and increased stratification within the deep Southern Ocean. These paleocirculation changes can help account for the reduction in atmospheric CO2 across the MIS 5.5 to 5.4 transition, and in doing so help explain the chain of events surrounding the decent into the last glacial period.
