Abstract:
Two boreholes, BDP-96-1 (200 m thick) and BDP-96-2 (100 m thick), drilled at the top of the underwater Akademichesky Ridge at 53°41′48″N and 108°21′06″, gave two parallel cores which were dated paleomagnetically. The correlation of the paleomagnetic data with the global magnetic scale has shown the age of the sediments exposed at a depth of 200 m to be 5 Ma. The average sedimentation rate was constant, about 4 cm/ka. Earlier palynological studies of core samples from BDP-96-1 were performed at 2 m intervals, which corresponds to the period of ∼45-50 ka. In this paper, we present results of a detailed palynological analysis of the upper 30 m of the core performed at 20 cm intervals (4-5 ka). The age of the investigated core from BDP-96-1 covers most of the Brunhes epoch. Taking into account the lost upper 630 cm, it matches the time range from 170 to 780 ka BP. Palynological analysis of the sediments from the range under discussion revealed 13 epochs in the development of the regional flora structure: seven epochs of predominance of forest plant formations and six epochs of its significant degradation. The epochs are correlated with the stages of changes in the volume of global ice, recorded in the marine oxygen isotope scale. It is difficult to determine the extents of the coolings, because spores and pollen are scarce in the sediments of these epochs. However, it is apparent that forests did not disappear from East Siberia during the coolings. All main arboreal species now growing there persisted during Pleistocene glaciations. The structure of the vegetative cover underwent profound changes. The areas occupied by forests decreased. Larch and spruce north-taiga forests were predominant under the humid cold conditions of the beginning of interglacials and the end of glaciations; they were then replaced by fir and cedar pine forests. Larch-pine and cedar pine middle-taiga forests expanded under the moderately warm and relatively dry climate of the mid-interglacials. Obviously, this was accompanied by a wide spread of steppe vegetation. A comparison of the frequency and habit of changes in vegetation type in the study region 170-780 ka BP with those in various regions of the Northern Hemisphere demonstrates that the East Siberian vegetation responded to global climatic changes synchronously with the vegetations of other regions.