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The complex Postglacial Baltic Sea development is a never-ending story. Due to the rapid and differential postglacial uplift and the complex deglaciation of Scandinavia 16000-9000 cal yr BP, the Baltic Sea Basin (BSB) experienced an intricate early postglacial development. This comprised the glacial to postglacial transition, glacial oscillations at potential threshold areas of the BSB, large differences in uplift rates of different threshold areas, leading to large water level changes within the BSB as well as shifts in hydrology and salinity. The first two stages, the Baltic Ice Lake (BIL; 16000-11.700 cal yr BP) and the Yoldia Sea (YS; 11.700-10.700 cal yr BP), were largely influenced by the melting and oscillating Scandinavian Ice Sheet, producing sediments poor in organic matter.
The BIL ended with an abrupt drainage, lowering the Baltic level with 25 m. While the BIL was dammed-up above sea level most of its time, the YS was at sea level but without sustained brackish-marine conditions due to narrow thresholds and a large glacial melt-water influx to the BSB. The early Holocene warming led to a rapid deglaciation, and when the next Baltic Sea stage began, the Ancylus Lake (AL; 10700-c. 10.000 cal yr BP), the ice margin had retreated to northern Scandinavia but sediments were still glacially influenced. Owing to the high uplift rates in south-central Sweden, where the narrow sill areas of the YS were situated, the AL was dammed-up above sea level and the Baltic water body was "tilted" towards the south. Here the resulting Ancylus transgression amounted to c. 20 m, until a new outlet through the Danish area could start operating.
A combination of fluvial erosion of sills, resulting in a regression within the BSB, and the rising global sea level, caused the AS to soon attain sea level. This resulted in the first signs of occasional, but only slight saline conditions at 9800 cal yr BP, when the Littorina Sea stage began (LS; 9800 cal yr BP-present); a transition which was complex and would take >1000 yr before truly brackish conditions began to prevail. Due to the slow uplift rates in the southern Baltic the LS is a perfect laboratory for Holocene sea level changes. The records show periods of increased sea level rise between 8000 and 6000 cal yr BP, possibly related to increased melting rates of remaining ice sheets. The start of the LS also marks the onset of recurring phases of anoxic conditions (hypoxia) in the deeper parts of the BSB.
Such phases are characterized by higher organic content in the sediments and seem to coincide with milder and drier climatic conditions, resulting in decreased freshwater run-off and increased salinities. This suggests that the BSB is sensitive to changes in the climate system, especially the hydrologic cycle, but data also imply that human activities with increasing nutrient inputs to the BSB have been an additional amplifier of hypoxia during the last millennium.
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