Palaeoenvironmental evolution of the Baltic Sea basin during the Last Interglacial (Eemian, Mikulino stages): a review

This review presents an investigation of the evolution of the Baltic Sea basin and its connections through the Eemian Stage interglacial, based upon sequences of marine and associated deposits from the White Sea to the southwest Baltic, via the Karelian channel. Pollen analyses and foraminiferal and ostracod analyses are combined to provide the evolution of relative sea‐level change, supported by dinoflagellate cyst and diatom records. An independent chronology is established by correlation of the pollen record with that from the Bispingen laminated lake deposits in north Germany. Additional published sites from the area are included in a wider reconstruction of events. The evidence from five selected areas is assessed, with detailed evaluation of two critical time slices, respectively, in the Corylus pollen zone (E4) and that during the Carpinus ‐ Picea zones (E5–E6). Ostracod analyses indicate that freshwater lakes initially existed in the basin prior to the marine inundation. This was followed by an early marine transgression. The timing of the first marine conditions varies across the basin. For example, in the southeast Baltic they were established in pollen zones E2–E3, and in the southwest Baltic they occurred at the base of E3, with fully marine conditions established in zone E4, continuing into E5. In the White Sea area marine conditions occurred in zone E2. In most areas, regression of the sea in the Baltic basin began late in zone E5–E6 and was occasionally followed by freshwater deposition in zone E7. Compared to the Holocene, the salinity of the basin was markedly higher during the period of maximum sea level (zones E4–E5) as shown by the foraminifera and other marine fossils. The Eemian transgression occurred through the Kiel Canal area in addition to the Danish straits‐Kattegat seaway, indicating that a connection to the North Sea was present in the area during the interglacial sea‐level maximum highstand. Moreover, the nature, duration and regional impact of the link to the White Sea via Karelia are discussed.