Tracking postglacial palaeoceanographic change of the Baltic Sea Basin using Osmium isotopes: Transition between the freshwater Ancylus Lake and the marine/brackish Littorina Sea

The Baltic Sea has experienced marked changes in salinity over the Holocene due to a rising global sea level and the isostatic rebound of the area. Here we investigate the temporal transition of the Littorina Sea Transgression (~8000 cal BP) between the freshwater Ancylus Lake stage and the marine/brackish Littorina Sea stage using Osmium isotopes (i.e. 187 Os/ 188 Os values), supplemented by BIT index, TOC content and sedimentology. The 187 Os/ 188 Os values of surface sediments show a clear distinction between the North Sea, Kattegat Bay and Skagerrak Strait (~1) and that of the Baltic Sea Basin (~1.9 to 2.6), with more radiogenic 187 Os/ 188 Os compositions being associated with lower surface salinity values. A 187 Os/ 188 Os record determined from a 4.5 m-long core from the central Baltic (the anoxic Gotland Basin) reveals the transition from the freshwater Ancylus Lake stage into the Littorina Sea stage by a lowering of 187 Os/ 188 Os values coupled with the sharp decrease in BIT index values, and an increase in the TOC content, accompanied by the appearance of brackish water diatoms. Collectively we show that 187 Os/ 188 Os values can be applied to identify large-scale changes between freshwater and marine environments, as previously demonstrated for the Arctic Basin during the Eocene.