The last fully fresh water stage in the Baltic basin, the Ancylus Lake ended at 9,800 BP when salt water from the world ocean started entering the Mastogloia Sea in the
Great Belt region commencing an initial Littorina Sea which as a saline or brackish sea continues to this day. These initial
transgressions were incomplete and occurred at different times in different areas of the Baltic basin lasting until 8.5 ka cal. BP. The transition timings from fresh to brackish water that mark the onset of the Littorina Sea are not yet clearly defined. They may have been in the northern Great Belt region around 9.0 ka cal. BP and east of the Darβ Sill which is at the western end of the Baltic Sea, at sites so far studied, are after 8.5 ka cal. BP and this is the definite onset of the Littorina Sea. Between 8 and 6 ka cal. BP the mid-
Holocene relative sea level rise has been studied in more detail than later changes in water level in the Littorina Sea. The
Holocene climatic optimum is now defined as about 8 to 4.8 cal. ka BP and resulted in northern Europe in a period of warm, and dry climate. The
Ångermanland region of northern
Sweden had the highest
post-glacial rebound in northern Europe, so between 9.1 and 7.8 ka cal. BP relative sea level dropped here from above sea level. Transitional regions in the eastern Baltic show positive relative sea level tendencies until between 7.5 and about 7.1 ka cal BP, when they become negative. The final melting of the
Laurentide ice sheet followed by much lower rate of global sea-level rise took place at this general time, between 8 and 7 ka cal. BP. The southern and western Baltic basin has a negative trend in relative sea level throughout the Holocene. So it is known that near the Denmark straits this area was below sea level around 8.5 to 8.0 ka cal. BP, but the area around the
Usedom/
Rügen islands had up to higher relative sea level at the same time. There is only a good fit in timing of relative sea levels with global ice history studies in this later negative relative sea level group, meaning that the calculated contribution of ice loading in the global ice models is likely wrong for the eastern Baltic region. A
transgression of the Baltic widened its ocean link, allowing it to reach a peak of salinity during the warmer
Atlantic period of European climatology. At this peak, the sea bore twice the volume of water and covered 26.5% more land than it does today.
Diatom studies of the sediments of the
Landsort Deep off Sweden suggest that the highest surface water salinities occurred between 7.1 and 5.4 ka BP, about the time of the Littorina Sea high stand. The
halocline developed causing a stratified water column, due to the inflow of North Sea water into the deep waters of the Baltic basin. The transition about 4 ka cal. BP to today's Baltic Sea, which could also be called the late Littorina Sea is ill defined. After the Holocene climatic optimum, land uplift exceeded world ocean sea level rise, and the resulting shallowing of the sills in the Baltic basin resulted in a gradual decrease in salinity. Some have it complete by 3.0 cal ka BP and it is characterised by microfossil changes from those of a typical high salt marine environment. As the period ended, the features of the modern coast appeared, including lagoons, spits, and dunes. Notable exceptions include steep terraces such as the
Øresund where the recession of sea level exposes less dry land. == Ecology ==