ENVIRONMENTAL CHANGES DURING THE BOREAL TRANSGRESSION IN THE NORTH-EASTERN WHITE SEA REGION (DETAILED CASE STUDY OF BYCHYE-2 SEDIMENT SECTION)

Detailed multiproxy (lithology, micropaleontology, palynology) study of a 455 cm thick marine sediment sequence overlying the Moscovian till exposed in Bychye-2 section on the Pyoza River allowed for reconstructing past environmental changes during the Boreal transgression. Stratigraphic subdivision is based on the succession of local palynological zones that were correlated with previously established regional zones. The latter are constrained on the basis of correlation with the West European palynological zones. Marine sediments of Bychye-2 section accumulated from the end of the Moscovian glacial (>131 ka BP) until ca. 119,5 ka BP. Five ecological zones were established in the section in accordance with the upward trends in the changes in lithology and variability in the taxonomic composition of fossil assemblages of benthic foraminifers and ostracods and associations of marine dinocysts and freshwater green microalgae. Taken together, they are indicative of the progressive shallowing of the basin under the improving climatic conditions, which primarily influenced the degree of sea-ice cover extent. Three successive phases in the evolution of the Boreal transgression have been identified: 1) a seasonally sea-ice covered relatively deep freshened basin of the initial phases of flooding (455-360 cm, >131-130,5 ka BP); 2) a deep basin of the maximum phase of flooding with less extensive sea-ice cover (360-290 cm, 130,5-130,25 ka BP); 3) a shallow basin with reduced seasonal sea-ice cover (290-0 cm, 130,25-119,5 ka BP). The flooding of the territory with cold Arctic waters was rapid, as evidenced by the composition of microfossil assemblages represented by river-proximal Arctic species in combination with the species that prefer water depths of at least 40-50 m. The regression in the region started about 130 ka BP, which indicates that the glacioisostatic rebound of the territory was ahead of the global eustatic sea level rise. The most warm-water and taxonomically diverse assemblages of foraminifers and ostracods, containing species typical of the Baltic Sea, were recorded during the regressive stage, especially in the time interval of ~128-124 ka BP. This probably gives evidence for a rather long-lasting connection of the White and Baltic seas.