Palaeoclimate and palaeoceanography of the Skagerrak-Kattegat since the Late Weichselian based on diatom records

Abstract: Diatoms from the surface sediments and sediments cores in the Skagerrak-Kattegat were analyzed and the results were presented in four papers and discussions. The broad aims of this thesis are twofold. The first aim is to correlate diatom data from the surface sediments to modern sedimentary environments, and try to find out the relationships between environmental variables such as salinity, temperature, currents and water depth, and between diatom assemblages and environmental variables. The second one is to reconstruct palaeoclimate and palaeoceanography of the area since the Late Weichselian. Paper I deals with diatoms in the surface sediments of the Skagerrak-Kattegat and their correlation with the spatial changes of eight environmental variables. The distribution of diatom assemblages reflects the interactions of currents in this area. The results of canonical correspondence analysis show that summer and winter sea surface salinities are the most important environmental variables affecting diatom distribution. Paper II presents the environmental changes of the Late Weichselian in the southern Kattegat. The results show that freshwater diatoms dominate the assemblages throughout the core. Salinities, particularly the sea surface salinity, were much lower than that of today. Sea ice diatoms were main marine species in the assemblages. A climatic amelioration at about 10600 BP was registered. The results presented in papers III and IV show a two-step climatic warming and consequently oceanographic change during the Younger Dryas/Preboreal transition. An abrupt warming in surface water took place at about 10100 BP, which resulted from the inflow of warm Atlantic surface water. Summer sea surface temperature rose about 8 degrees centigrade. This greatly influenced the surface water of the Skagerrak-Kattegat and the shallow areas, but had little effect on the bottom water in the deeper parts. A great difference in temperature along the water column was formed, which resulted in a pronounced stratified water column with warm water at the surface and cold water at the bottom. The warming of bottom water in the deep parts of the region occurred a few hundred years after the first warming when the inflow of warm Atlantic water became stronger and/or the Scandinavian ice sheet had no large influence on the area. A warm period of 7700-5100 BP and a climatic deterioration at about 5100 BP were also documented by the changes in current patterns in paper IV. The thesis also discusses the influences of meltwater from the Baltic Ice Lake and the stratification of water column in the Kattegat on the regional palaeoceanography and palaeoclimate.