Subantarctic jet migrations regulate vertical mixing in the Southern Indian
Résumé
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 Ca˜no 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.