Biostratigraphy of Eemian sediments at Mertuanoja, Pohjanmaa (Ostrobothnia), western Finland

The till-covered clay and silt deposits at Mertuanoja, Pohjanmaa (Ostroboth nia), western Finland, have been investigated in great detail. The Eemian i nterglacial environment is reconstructed here on the basis of pollen, diato m and dinoflagellate analyses. The pollen stratigraphy shows an interglacia l vegetational succession reflecting stable climatic conditions typical of the Eemian Stage in the Pohjanmaa area. The initial Betula forests were fol lowed by Pinus-Betula forests with Quercus. The next successional phase was dominated by Betula, Pinus and Alnus; temperate deciduous trees and Corylu s. also grew in the area. Later, Picea advanced and temperate deciduous tre es declined. Some Corylus was, however, still present and thermophilous Osm unda thrived in wet places. The diatom record indicates that the sediments were deposited first in a freshwater basin, then in the Eemian Baltic Sea, and finally in a freshwater basin once more. The presence of dinoflagellate s demonstrates that the Eemian Baltic Sea, when at its maximum extent, was connected to the Atlantic Ocean, which brought northern cool-temperate surf ace waters to Finland as far north as Mertuanoja. Mertuanoja is the first i nterglacial site at which numerous dinoflagellate cysts were encountered in Finnish Quaternary sediments.