Mid-to Late Holocene changes in the hydrographic conditions in the Baltic Sea, as inferred from dinoflagellate cysts assemblages

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University of New Brunswick
Dinocyst assemblages from marine sediments in the south-central Baltic Sea provide a record of changes in the Holocene Baltic Sea hydrographic system for the last 6000 years. A low diversity assemblage of four genera of autotrophic dinocysts is observed throughout, dominated by the species Operculodinium centrocarpum. Dinocyst assemblages, especially process length variation in O.centrocarpum are used to interpret qualitative paleosalinity. The Littorina Transgression, a period of increasing salinity was already underway in the eastern Gotland Deep by 6000 years BP, and continued until 4750 years BP. The highest dinocyst concentrations were observed during Mid Holocene Warming 6000 - 4000 years BP. Littorina Sea A Phase (6000 - 4750 years BP) is defined by an inferred major increase in salinity. From 4750 - 1000 years BP, only minor changes in salinity are reflected in the dinocyst assemblage. From 4000 - 1000 years BP, stable, low concentrations of dinocysts are interpreted as a result of shorter blooming seasons during Neoglacial summers. Littorina Sea B Phase (4750- 1000 years BP) is defined by inferred minor fluctuations in salinity and stable concentrations of dinocysts throughout. Increased dinocyst abundance is observed from 1000 - 800 years BP before declining from 800 - 100 years BP to low concentrations (25 000 dinocysts/g sediment). This is interpreted as caused in part by longer, warmer summers during the Medieval Warm Period (MWP) and shorter, cooler summers during the Little Ice Age as well as increased nutrient runoff into the Baltic Sea from increased anthropogenic land use during the MWP. Dinocyst assemblages indicate gradually decreasing salinity from 1000 - 100 years BP. Late Littorina Sea B Phase (1000 - 100 years BP) is defined by an inferred minor decrease salinity and elevated concentrations of dinocysts during the MWP.