Lena Thöle, Peter Dirk Nooteboom, Suning Hou, Rujian Wang, Senyan Nie, Elisabeth Michel, Isabel Sauermilch, Fabienne Marret, Francesca Sangiorgi, Peter Bijl

Dinoflagellate cyst assemblages present a valuable proxy to infer paleoceanographic conditions, yet factors influencing geographic distributions of species remain largely unknown, especially in the Southern Ocean. Strong lateral transport, sea-ice dynamics and a low and uneven geographic coverage of surface sediment samples have limited the use of dinocyst assemblages as a quantitative proxy for paleo-environmental conditions such as sea-surface temperature (SST), nutrient concentrations, salinity and sea ice (presence). In this study we present a new set of surface sediment samples (n=66) from around Antarctica, doubling the number of Antarctic-proximal samples to 100 (dataset wsi_100) and increasing the total number of Southern Hemisphere samples to 655 (dataset sh_655). Additionally, we use modelled ocean conditions and apply Lagrangian techniques on all Southern
Hemisphere sample stations to quantify and evaluate the influence of lateral transport on the sinking trajectory of microplankton and through that, to the inferred ocean conditions. K-means cluster analysis on the wsi_100 dataset demonstrates the strong affinity of Selenopemphix antarctica to sea-ice cover conditions, and Islandinium spp. to low-salinity conditions. For the entire Southern Hemisphere, the k-means cluster analysis identifies nine clusters with a characteristic assemblage. In most clusters a single dinocyst species dominates the assemblage. These clusters correspond to well-defined oceanic conditions in specific Southern Ocean zones or along the ocean fronts. We find that when lateral transport is predominantly zonal, the environmental parameters inferred from the sea floor assemblages mostly correspond to those of overlying ocean surface. In this case, the transport factor can thus be neglected and will not represent a bias in the reconstructions. Yet, for some individual sites, e.g., deep water sites or sites under strong current regimes, lateral transport can play a large role. The results of our study further constrain environmental conditions represented by dinocyst assemblages and the location of Southern Ocean frontal systems.