Holocene history of the Baltic Sea as a background for assessing records of human impact in the sediments of the Gotland Basin

Sediment cores from the Gotland Basin were studied for their siliceous micr ofossil assemblages and organic carbon content to compare recent environmen tal changes in the Baltic Sea with its natural long-term history. Age model s were constructed using Pb-210, Cs-137 and corrected and calibrated C-14 d ates. The transgression that marks the onset of the Ancylus Lake stage is r ecorded in the sediments as a small increase in organic carbon coinciding w ith a peak in diatom abundance and increased diatom diversity. A minor occu rrence of brackish-freshwater diatoms is recorded in the Ancylus Lake c. 99 50-9750 cal. yr BP (c. 8900-8800 C-14 yr BP), correlating with the onset of the Initial Litorina Sea in the Bornholm Basin. A high-productivity event is recorded in the end of the Post-Litorina Sea and corresponds to the Medi aeval warm event. An alteration in the diatom assemblage contemporaneous wi th a decrease in organic carbon, interpreted as representing a deterioratio n in the climate, correlates with the start of the 'Little Ice Age' about 8 50-700 cal. yr BP. A change dated to ad 1950-1960 is probably an effect of increased nutrient availability in the open Baltic Sea. This effect of eutr ophication was probably caused by increased discharge of nutrients deriving from fertilizers, as the responding diatom species partly indicate a cold climate rather than a warm one, as would have been expected if this had bee n only a response to the warmer climate documented during the last 100 year s or so.