Late H olocene B altic S ea outflow changes reconstructed using C 37:4 content from marine cores

The B altic S ea is an intra‐continental brackish water body. Low saline surface water, the so‐called B altic outflow current, exits the B altic S ea through the K attegat into the S kagerrak. Ingressions of saline oxygen‐rich bottom water enter the B altic S ea basins via the narrow and shallow K attegat and are of great importance for the ecological and ventilation state of the B altic S ea. Over recent decades, progress has been made in studying H olocene changes in saline water inflow. However, reconstructions of past variations in B altic S ea outflow changes are sparse and hampered because of the lack of suitable proxies. Here, we used the relative proportion of tetra‐unsaturated C 37 ketones ( C 37:4 %) in long‐chain alkenones produced by coccolithophorids as a proxy for outflowing B altic S ea water in the S kagerrak. To evaluate the applicability of the proxy, we compared the biomarker results with grain‐size records from the K attegat and M ecklenburg B ay in addition to previously published salinity reconstructions from the K attegat over the last 5000 years. All S kagerrak records showed an increase in C 37:4 % that is accompanied by enhanced bottom water currents in the K attegat and western B altic S ea over the past 3500 cal. a BP , indicating an increase in B altic S ea outflow. This probably reflects higher precipitation in the B altic S ea catchment area owing to a re‐organization of N orth A tlantic atmospheric circulation with an increased influence of wintertime W esterlies over the B altic catchment from the mid‐ to the late H olocene.