Development of anoxia during the Holocene fresh–brackish water transition in the Baltic Sea

Abstract

One of the most pronounced environmental changes during the Holocene Baltic Sea history was the transition from the freshwater Ancylus Lake to the brackish water Litorina Sea. The establishment of brackish conditions during this transition (the A/L) was caused by an interplay of sea level rise and subsidence of sills in the Danish Straits. The northward progression of salt water influence caused the gradual and transgressive development of a halocline which obstructed vertical water circulation in the deep depositional basins. It caused changes in surface water properties (mirrored by diatom flora and productivity levels), in redox conditions of bottom waters, in organic carbon preservation of sediments, and in nutrient cycling. In seven cores from the Arkona, Bornholm and Gotland Basins, the A/L transition was 14C dated and studied in high-resolution samples for minor and major element composition.The earliest marine inflows were small and can only be detected by sedimentary properties in the southern Baltic. Further north the salinity increase was gradual and retarded; only when the connection through the Danish Straits was well established the entire Baltic proper became brackish with a stratified water column. This development took altogether 200014Cyr (c. 9000–7000bp). Diatom analyses indicate a fast increase in salinity c. 700014Cyrbp, which coincides with a transgressive phase in the straits. In the Gotland Basin, deposition of laminated sediments and periodically euxinic conditions were established first at the deepest bottoms, and rose to shallower water depth as the basin was gradually filled with dense brackish water.The laminated sequences have high organic carbon contents, which is attributed to better preservation under anoxic conditions. Litorina sediments from the Arkona and Bornholm Basins are bioturbated even though conditions became more reducing also in these sediments during the A/L transition. The development of reducing conditions during the A/L transition probably caused P re-mobilization from the sediments and a decrease in the rates of denitrification. Both factors increased primary productivity. A comparison between isochronous sediments from different basins shows that certain elements (Mo, Cu, V and Cd) are enriched sediments deposited during predominantly anoxic conditions.