Subantarctic jet migrations regulate vertical mixing in the Southern Indian

In the Indian sector of the Southern Ocean, the main Antarctic Circumpolar Current (ACC) transport is associated with the subantarctic jet and deviates around several subantarctic islands and volcanic plateaus. This forms regional eddies that are hotspots for productivity, nutrient and CO2 cycling. Though ACC jet migrations related to past climate variations have been reported in previous studies, little is known about their effect on vertical mixing where fronts interact with bathymetry. Here, we investigate past frontal shifts and their impacts on water column structure in the Del Cano Rise, Crozet Archipelago, and Conrad Rise regions over the last 160,000 years by reconstructing surface and subsurface temperatures from siliceous microfossils assemblages in four sediment cores. Our results suggest that the ACC core was displaced equatorward during the last glacial period (70-18 kyrs), away from bathymetric features of the Indian sector of the Southern Ocean. This equatorward shift probably reduced front/bathymetry interactions and, with it, mixing of the upper layer of the ocean in the lee of subantarctic islands and plateaus. We propose that this process participated in the regional isolation of Antarctic Surface Water from deeper water masses. During Marine Isotope Stage (MIS) 5e (132-118 kyrs), an interglacial episode with a warmer-than-modern climate, the reverse situation is reconstructed, whereby the tightening of the ACC fronts against volcanic plateaus probably resulted in enhanced vertical mixing.