Diatom stratigraphy in five offshore sediment cores from the northwestern Baltic proper implying large scale circulation changes during the last 8500years

Five Holocene sediment cores from the northwestern Baltic proper were analy sed for lithology, siliceous microfossil assemblages and geochemical parame ters. The data indicate that surface water salinity and redox conditions be low the halocline have changed drastically at least four times since the Ba ltic Sea changed from a fresh water lake (the Ancylus Lake) to a semi-enclo sed brackish water sea (the Litorina Sea) c. 8500 yrs BP. These variations appear to be mainly effects of changes in water depth at the thresholds of inlet areas. Based on these changes, and earlier studies of the shoreline d isplacement in the inlet areas, we propose a tentative model for changes of large scale water circulation in the Baltic Sea during the last c. 8500 yr s. At the transition from fresh to brackish water 8500 C-14 yrs BP, upwelling of nutrient rich bottom water started to occur, causing a slight increase i n primary production. Diatom assemblages in sediments indicate a slow rise in surface water salinity during this period. At 7000-6500 C-14 yrs BP, surface water salinity and primary production sim ultaneously increased, as anoxic bottom conditions were established at dept h below the halocline. We suggest that high primary production was caused b y increased input of oceanic water, leading to increased upwelling of nutri ent rich bottom water. At the anoxic bottoms laminated sediments formed unt il 5000-4500 C-14 yrs BP. This period (c. 7000-4500 C-14 BP) was contempora neous with the post-glacial transgression maximum in Oresund, and we sugges t it represents the most saline phase of the Baltic Sea post-glacial histor y. Due to a regression in Oresund starting 4500 C-14 yrs BP, upwelling decreas ed and the halocline was lowered, resulting in decreased primary productivi ty and hence oxic deep water conditions. The diatom assemblages of the sedi ments indicate a lowering of salinity at the beginning of this period. We suggest that the second period of anoxic bottom conditions c. 2000-1500 C-14 yrs BP was caused by a change of dominating inflows from the Oresund t o the Belt Sea. This resulted in decreased salinity of the inflowing water which did not penetrate to the deepest parts of the basin as frequently as before. The diatom record indicates both a second lowering of salinity and a change in the large scale water circulation at the beginning of this peri od.