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

The till‐covered clay and silt deposits at Mertuanoja, Pohjanmaa (Ostrobothnia), western Finland, have been investigated in great detail. The Eemian interglacial environment is reconstructed here on the basis of pollen, diatom and dinoflagellate analyses. The pollen stratigraphy shows an interglacial vegetational succession reflecting stable climatic conditions typical of the Eemian Stage in the Pohjanmaa area. The initial Betuta forests were followed by Pinus‐Betula forests with Quercus. The next successional phase was dominated by Betula, Pinus and Alnus ; temperate deciduous trees and Corylus also grew in the area. Later, Picea advanced and temperate deciduous trees declined. Some Corylus was, however, still present and thermophilous Osmunda 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 dinoflagellates demonstrates that the Eemian Baltic Sea, when at its maximum extent, was connected to the Atlantic Ocean, which brought northern cool‐temperate surface waters to Finland as far north as Mertuanoja. Mertuanoja is the first interglacial site at which numerous dinoflagellate cysts were encountered in Finnish Quaternary sediments.