The late Middle and Late Pleistocene environmental and glaciation history of Northern Ostrobothnia and Finnish Lapland

The Scandinavian Ice Sheet (SIS) covered Fennoscandia numerous times during the Pleistocene, but the exact number, extent of ice and precise time and duration of glacial and non-glacial phases have not been fully established for the eastern flank of the SIS. At several sites, only incomplete sedimentary records have been preserved owing to different rates of erosion and glaciotectonic deformation during several glacial advances and the intervening non-glacial phases. The most complete sediment successions have been found in northern and west-central Finland, in an area that has escaped erosion due to the vicinity of the ice-divide zone and frozen bed conditions. The main objective of this thesis was to examine the paleoenvironment and glaciation history of the Weichselian (MIS 5–2; ca. 116 000 - 11 700 years ago) and the late Saalian (MIS 6; ca. 160 000 - 130 000 years ago) stages in northern and west-central Finland. This thesis specifically concentrates on pollen stratigraphy and its use in correlation although other stratigraphical and geochronological tools have also been applied. Sediment sequences from three different sites located in northern and central Finnish Lapland and Northern Ostrobothnia, central western part of Finland, were studied using stratigraphical and dating methods. Furthermore, a safer heavy liquid pollen preparation method for minerogenic sediments was introduced and validated. Based on the results, it is suggested that the modified LST Fastfloat method gives comparable results compared to the conventional hydrofluoric acid (HF) method. The thesis provides new geochronologically and biostratigraphically constrained data to establish the more complete stratigraphy for northern and central Finland and beyond. Results indicate that the ice sheet advanced further south than previously thought during the Early Weichselian Herning stadial (MIS 5d). Results also strengthen the view that ice-free conditions prevailed during the Middle Weichselian (MIS 3) stage. Moreover, results from the Kaarreoja site from northern Finnish Lapland indicate that the climate was warmer than previously suggested.