Jaakko Auri, University of Turku (Finland)
Matti Räsänen, University of Turku (Finland)
Aleksis Klap, University of Turku (Finland)
Keijo Nenonen, Geological Survey of Finland (Finland)
Biostratigraphy of till-covered lake sediments exposed on the island of Björkö in the Kvarken Archipelago, Western Finland, is used for vegetational, environmental and chronological reconstruction. The conformable succession is defined and named here as the Svedjehamn Formation, which has four members: the Västerön, Backuddgloet, Gröthamn and Backträsket Members.
The studied section is classified into 7 biostratigraphic units in terms of fossil pollen taxa. The record shows a vegetational succession with two forested phases related to interstadials, the first having high values for Betula pollen and the latter for Picea and Betula, and two treeless phases related to stadials with high NAP (Non-Arboreal Pollen). These are interpreted as representing deposition during the Herning (MIS 5d), Brørup (MIS 5c), Rederstal (MIS 5b) and Odderade (MIS 5a) chrons, respectively. No glaciers reached the area during the stadials.
The marked differences in vegetation relative to other Early Weichselian interstadial sites in Ostrobothnia may be explained by differences in pollen depositional environment and the coastal location of the site. Björkö lies closer to present sea -level than any of the other documented interstadial sites in Finland.