Hydrography and mollusc faunas of the Baltic and the White Sea–North Sea seaway in the Eemian

Palynologically dated mollusc and cirriped faunas from a region extending from the North Sea through the Baltic and Karelia to the Arkhangelsk region show that the hydrography of the Baltic was very different from the Holocene. For 2–2.5 ka in the Early Eemian a seaway existed from the Barents to the North Sea through Karelia, until it was severed at the present continental watershed to the north of Lake Onega. After this the Ladoga–Onega trough remained an arm of the Baltic for several millennia. The benthic faunas are comparable to the Holocene, but their boundaries were displaced much further into the Baltic. Notable differences from the Holocene are the absence or rarity of the Macoma balthica biocoenosis, and the presence of cold Portlandia-dominated biocoenoses in Karelia. In the Belt Sea and western Baltic winter sea surface temperatures and salinity were higher than now by ca. 6°C and 15‰, and the distinctly brackish top layer was missing. At the same time cold bottom water (≈2.5°C) with a tendency to anoxia characterised the Karelian arm of the Baltic. Water exchange through this area was inhibited by the constriction to the north of Lake Onega, and the basin and threshold bathymetry. Water transport through the White Sea–Baltic seaway was too sparse to play an active role in the North Atlantic surface circulation or climatic change in the region. The high salinity and temperature in the Belt Sea and western Baltic persisted throughout the Eemian, and are explained by wider and deeper passage from the North Sea to Kattegat, wider straits through Denmark, higher salinity in the North Sea, higher evaporation, as well as more dispersed fresh water supply. The advection of oceanic water into the Baltic culminated in the Early Eemian, before the Carpinus zone, and probably resulted in an oceanic climate in the Baltic region, while at the same time cold winters produced cold bottom water in the Karelian arm.