The effects of redeposition phenomena on the reconstruction of palaeoenvironment in the southeastern Baltic Sea

Abstract

The coastal waters of the southeastern Baltic Sea, up to a depth of approximately 60–65 m, experienced the significant basin regression at the very end of the existence of the Baltic Ice Lake and the beginning of Yoldia Sea, as well as some transgressions during the Ancylus Lake and Litorina Sea stages. In the Lithuanian waters, two 5-m cores, taken from different depths and sedimentary zones were analysed. The radiocarbon dating, diatom and pollen analysis, loss on ignition and magnetic susceptibility methods were employed in order to reconstruct the environmental conditions that influenced the sedimentation process in the coastal waters. The prevalence of fresh-brackish benthic diatoms throughout the sediment sequences indicates that shallow freshwater environments existed in the coastal waters of the various Baltic Sea basins from the Late Glacial until the Middle Holocene. The results of the integrated studies revealed radiocarbon age reversals and inconsistencies between sediment ages and the results of diatom and pollen studies. The frequent stratification of sandy sediments with organogenic sediment interlayers indicates the redeposition of sediments of different origins and ages during the transgressions of the Ancylus Lake and Litorina Sea. The highly variable content of freshwater planktonic-benthic diatoms, as well as presence of brackish diatoms in organogenic sediments, confirm an unstable environment in the coastal waters during the basin's transgressional phases. In summary, the studies indicate that the stratigraphic identification of sediments in coastal waters is problematic due to the specific sedimentary environment in which they are deposited. This also complicates the reconstruction of the palaeogeographic conditions of entire basins during the different stages of the Baltic Sea development. When interpreting the radiocarbon ages and palaeobotanical data, the specific sedimentary environment of coastal waters should be taken into account.