9 novembre : Séminaire de Matthieu Civel-Mazens

ACC fronts migrations and their impacts in the Indian sector of the Southern Ocean assessed from radiolarian assemblages

Mathieu Civel-Mazens, Postdoc LEMAR

jeudi 9 novembre, 14h30, Amphi A (Francheteau), IUEM

In the Southern Ocean (SO), the wind driven and eastward flowing Antarctic Circumpolar Current (ACC) is divided by several hydrological fronts. These fronts, associated with jet currents that concentrate most of the ACC transport, generate eddy fields when deviated by bathymetry or rough seafloor topography. They are known to have shifted in position according to past climate variations. Impacts on the physical and biological carbon pump as long as the extent of these shifts remain poorly constrained, particularly where fronts interact with topographic highs. Through subsurface and surface temperature reconstructions from radiolarian and diatom assemblages-based transfer functions, the past positions of ACC fronts are assessed for the last couple glacial-interglacial cycles in the western Indian sector of the Southern Ocean (ISO), strewn with obstacles to the ACC flow. We estimate that warmer (colder) conditions during interglacials (glacials) resulted in southward (northward) front migrations, causing more (less) generation of Eddy Kinetic Energy (EKE) and, thus, forming more (less) vigorous eddy fields downstream of volcanic plateaus in the ISO. Eddy activity may have played a non-negligeable role in carbon and nutrient cycling at a glacial/interglacial time scale. Curious productivity behaviors remain unexplained and will be investigated at the IUEM through δ³⁰Si measurements on three radiolarian species.

Figure 1: Siliceous microfossils found in Southern Ocean sediments. Left: centric diatom, Thalassiosira lentiginosa; Right: Polycystine Radiolarian, Stylochlamidyum asteriscus.