Late‐Holocene shore displacement of the Finnish south coast: diatom, litho‐ and chemostratigraphic evidence from three isolation basins

Abstract

Lake isolation studies provide the most reliable method for reconstructing the shore displacement history of the areas affected by postglacial isostatic land uplift. Due to the slow uplift rate and the presence of numerous lakes, the Finnish south coast is one of the key areas for investigating the Litorina transgression and subsequent shore displacement history of the Baltic. To reconstruct the relative sea‐level dynamics during the late Holocene, three small lakes from the critical altitudes were analysed concerning their diatom, sulphate‐sulphur, and sodium stratigraphies and loss‐on‐ignition values. Comparisons of the records indicate that the availability of nutrients significantly influenced the diatom‐floristic patterns during the lake isolation. In particular, the mass occurrence of Fragilaria spp. seems to be associated with high values of sulphate‐sulphur and sodium during and after isolation. On the basis of the new and old data, a shore displacement curve for the 30m Litorina isobase is compiled. This indicates that since the clear Litorina transgression at 7500–6500 14C BP, no transgressions have taken place but that the shore displacement in the area has been a stable process. The occurrence of a number of raised shore formations between the altitude of the major Litorina shore and the present sea level in south Finland can be attributed to the erosion of soft soil layers by the high‐energy wave action on the exposed shore sections.