Palaeoenvironment and shoreline displacement on Suursaari Island, the Gulf of Finland

The island of Suursaari in the middle of the Gulf of Finland is exceptionally high (175 m a.s.l.). Sediment profiles from one mire and three lakes were investigated using diatom and pollen analysis, radiocarbon dating and levelling of the elevations of ancient shorelines. The pollen stratigraphy of the Lounatkorkiasuo Mire sediment suggests a sedimentary record dating from the late Allerød. The development of late-glacial vegetation went through the same phases as in southern Finland, however these are probably somewhat earlier on the island of Suursaari. There are differences in the Holocene vegetation history of the higher and lower areas of the island. Lake Ruokalahenjärvi was isolated around 10 000 BP during the initial phase of the Yoldia Sea and the diatom assemblage indicates that at that time brackish-water flow had not penetrated into the Gulf of Finland. Diatoms from the isolation sediments of Lake Liivalahenjärvi and Lake Veteljärvi indicate a freshwater environment for the Yoldia Sea final phase at 9500–9600 BP. Levelling of coastal formations on Suursaari Island reveals that the Late Weichselian and early Holocene ancient shorelines are 5–15 m higher than expected from the isobase data for similar land uplift areas on the mainland. The anomalous shoreline levels on Suursaari Island may be explained by irregular land uplift. By the time of the Litorina Sea differences in shoreline altitudes had disappeared