Benthic foraminifera in the surface sediments of the southern Baltic Sea – paleoenvironmental proxy perspectives

The Baltic Sea developed to its modern settings as a result of complex processes initiated by the retreat of the Fennoscandian Ice Sheet around 15,000 cal BP. The formation of the Baltic Sea went through a series of stages, each of which was characterized by changes in environmental conditions, especially in terms of spatial extent, salinity and water levels. Although the history of sea level and salinity changes in the Baltic Sea has already been fairly well described, different parts of the current basin have been subjected to changes of varying speed, extent and consistency with the general trend for the entire Baltic, as exemplified by foraminifera based reconstructions from more saline western region of the sea.

The aim of the study, was to investigate whether foraminifera could also inhabit the low saline southern Baltic Sea and whether there would be indicator species among them, preferring e.g. a certain ranges of temperature or salinity of the bottom water. For this purpose, surface sediments were examined from 19 locations following the salinity gradient, from the more saline Bornholm Basin to areas characterized by gradual water freshening, i.e. the Slupsk Furrow, the Gdansk Basin, up to the Gotland Basin. The obtained results indicate that the foraminifera biodiversity is low (5 species), which is common in the low saline environment of the Baltic Sea. The foraminifera fauna showed pattern in species distribution from the western to the eastern part of the studied transect. The muddy sediments of the Bornholm Basin (bottom salinity of 16.58) were rich in Ammonia beccari, Elphiudium excavatum clavatum and Elphidium incertum. The sandy-gravel sediments of the Slupsk Furrow were barren or single individuals of Miliammina fusca occurred randomly, which may be related to the strong hydrodynamics of the bottom currents at this area. The Gdansk Basin up to the Gotland Basin (bottom salinity around 12) was dominated by Reophax dentaliniformis, which also accounted for more than 70% of the total foraminifera fauna of the southern Baltic Sea in terms of abundance. The conducted statistical analysis indicates a strong correlation of A. beccari and Elphidium spp. with temperature and salinity of the bottom waters, whereas R. dentaliniformis with sea depth, the knowledge that can be used in palaeoceanographic reconstructions of the Baltic Sea.