Late Weichselian and Holocene shore displacement history of the Baltic Sea in Finland

Abstract

About 62 percent of Finland’s current surface area has been covered by the waters of the Baltic basin at some stage. The highest shorelines are located at a present altitude of about 220 metres above sea level in the north and 100 metres above sea level in the south-east. The nature of the Baltic Sea has alternated in the course of its four main postglacial stages between a freshwater lake and a brackish water basin connected to the outside ocean by narrow straits. This article provides a general overview of the principal stages in the history of the Baltic Sea and examines the regional influence of the associated shore displacement phenomena within Finland. The maps depicting the various stages have been generated digitally by GIS techniques. Following deglaciation, the freshwater Baltic Ice Lake (12,600–10,300 BP) built up against the ice margin to reach a level 25 metres above that of the ocean, with an outflow through the straits of Öresund. At this stage the only substantial land areas in Finland were in the east and south-east. Around 10,300 BP this ice lake discharged through a number of channels that opened up in central Sweden until it reached the ocean level, marking the beginning of the mildly saline Yoldia Sea stage (10,300–9500 BP). As the connecting channels rose above sea level, however, the Baltic Sea became confined once more, to form the Ancylus Lake (9500–8000 BP). During its existence the outflow channel to the ocean shifted to the Straits of Denmark and the major lake systems of central Finland became isolated from the Baltic basin. After the brief Mastogloia transition phase, a greater influx of saline water began to take place through the Straits of Denmark, marking the Litorina Sea stage (7500– 4000 BP), to be followed by a somewhat less saline stage known as the Limnea Sea. After a transgressive period early in the Litorina Sea stage, shoreline displacement in Finland has proceeded at a steadily declining rate.