Holocene environmental changes recorded by diatom stratigraphy in the southern Baltic Sea

Abstract: Sediment cores from the southern Baltic Sea and the Oder River estuary are analysed for their siliceous microfossil assemblages and organic carbon content. Long piston cores from the Bornholm and Gotland Basins provide data on the long-term Holocene history of the Baltic proper and serve as a natural background when evaluating the most recent environmental changes recorded in the short gravity cores. Corrected and calibrated 14C dates for the Holocene part, together with 210Pb and 137Cs dates for the sediments deposited during the last century, are used as a basis for constructing chronologies and age models. The main results are:The brackish phase of the Yoldia Sea stage in the Bornholm Basin is recorded by a diatom assemblage of periphytic taxa indicating a shallower water depth than today and more marine conditions than are recorded by the planktonic assemblage found in the Gotland Basin.The transition between the Yoldia Sea and the Ancylus Lake stages is distinguished in the offshore facies and recorded in the sediments in the form of a small increase in organic carbon coinciding with a peak in the diatom abundance and increased diatom diversity.The onset of the Initial Litorina Sea stage, recorded about 10,100 calendar years BP (c. 8900 14C years BP) in the Bornholm Basin, indicates a complex transition with different phases of brackish-water inflow.Six periods with major marine water inflow into the Baltic basin are recorded during the Holocene. Once an open connection was established with the North Sea, climatic influence seems to have been the most important factor controlling such inflow events.Comparison between the organic carbon content, the absolute abundance of siliceous microfossils and a regional climate proxy, the Greenland (GRIP) ice core (18O record, shows a tentative link between primary production in the Baltic proper and climate. A high temperature on Greenland corresponds to high organic carbon content and increased diatom abundance in the Baltic proper.Climate change is recorded in both the siliceous microfossil assemblages and the organic carbon content. A sediment sequence indicating high productivity c. 950-800 calendar years BP (AD 1000-1150) correlates with the Medieval warm period. An alteration in the diatom assemblage interpreted as being due to a deterioration in the climate correlates with the start of the Little Ice Age about 850-700 calendar years BP (AD 1100-1250).The effects of eutrophication are recorded AD 1850-1860 in the southwestern Baltic proper, AD 1900 in the Oder estuary and AD 1950-1960 in the Gotland Basin. These effects were probably caused by increased discharge of nutrients deriving from fertilisers, as the responding diatom species partly indicate a cold climate rather than a warm one, as would have been expected if this had been only a response to the warmer climate documented during the last 100 years or so.