Late Holocene Baltic Sea outflow changes reconstructed using C37:4 content from marine cores

Abstract

The Baltic Sea is an intra‐continental brackish water body. Low saline surface water, the so‐called Baltic outflow current, exits the Baltic Sea through the Kattegat into the Skagerrak. Ingressions of saline oxygen‐rich bottom water enter the Baltic Sea basins via the narrow and shallow Kattegat and are of great importance for the ecological and ventilation state of the Baltic Sea. Over recent decades, progress has been made in studying Holocene changes in saline water inflow. However, reconstructions of past variations in Baltic Sea outflow changes are sparse and hampered because of the lack of suitable proxies. Here, we used the relative proportion of tetra‐unsaturated C37 ketones (C37:4 %) in long‐chain alkenones produced by coccolithophorids as a proxy for outflowing Baltic Sea water in the Skagerrak. To evaluate the applicability of the proxy, we compared the biomarker results with grain‐size records from the Kattegat and Mecklenburg Bay in addition to previously published salinity reconstructions from the Kattegat over the last 5000 years. All Skagerrak records showed an increase in C37:4 % that is accompanied by enhanced bottom water currents in the Kattegat and western Baltic Sea over the past 3500 cal. a BP, indicating an increase in Baltic Sea outflow. This probably reflects higher precipitation in the Baltic Sea catchment area owing to a re‐organization of North Atlantic atmospheric circulation with an increased influence of wintertime Westerlies over the Baltic catchment from the mid‐ to the late Holocene.