Migration of deep convection center in subpolar North Atlantic around 6 ka suggested by a dinocyst proxy of mixed layer depth

This study explores the relationship between dinoflagellate cyst (dinocyst) assemblages and the mixed layer depth (MLD) using canonical correspondence analyses. Our results demonstrate that wintertime deepening of MLD, in response to deep convection events, can influence dinocyst assemblages and especially the relative abundance of Nematosphaeropsis labyrinthus, which can thus be used as a proxy of MLD and the associated deep convection intensity. We analyzed a total of 24 Holocene records of N. labyrinthus percentage in the subpolar North Atlantic, along with quantitative reconstructions of MLD from 16 of them using the entire assemblage. The results reveal a westward migration of potential deep convection region around 6 ka BP, from the Nordic Seas and eastern subpolar gyre (SPG) during the Early Holocene, to the western to central SPG during the Middle and Late Holocene. The intensification of deep convection in the Labrador Sea towards a modern-like situation started during the Late Holocene, one or two thousand years later than the major transition around 6 ka BP in other parts of the subpolar North Atlantic. These results strengthen the hypothesis of reduced deep-water formation in the eastern North Atlantic from the Early to Late Holocene. Next, we will attempt to simulate the MLD in the North Atlantic at 6 ka BP using a regional configuration of the NEMO (Nucleus for European Modelling of the Ocean) ocean model, which includes the Arctic and the Northern Hemisphere Atlantic (ANHA) at 1/4 degree. The comparison of numerical simulations with various proxy reconstructions will allow us to explore potential drivers of this westward migration pattern around 6 ka BP.