Evolution of the Valley Ecosystems in the Lower Reaches of the Bikin River in the Late Pleistocene and Holocene

Abstract

Reconstructions of changes in vegetation, climate and sedimentation conditions in the valley of the lower reaches of the Bikin river lower reaches (Northern Primorye) for the last 70 thousand years were made from results of palynological and diatom analyses, radiocarbon dating of peat samples, plant detritus and woody remains contained in the alluvial and biogenic deposits of floodplain and above the floodplain terraces exposed by profiles. New evidence was obtained for multiple and significant transformations of natural environment of the region for several climatic cycles during the Late Pleistocene and Holocene corresponding to marine isotope stages MIS 4–1. It was established that the final phase of MIS 4 was dominated by ecosystems of mari and dark coniferous taiga, the interglacial stage of MIS 3 by communities of birch forests with the involvement of broad–leaved plants, and the glacial stage of MIS 2 by birch open woodland with larch and spruce as well as by sphagnum bogs and dwarf birch. In the Early Holocene (Q1IV) there occurred a rapid expansion of deciduous plants. During the Holocene Optimum, about 6.5 ka, the ecosystems were dominated by polydominant forests. The maximum development of oak–deciduous forests with conifers is recorded about 5 ka. The transition from the Middle to Late Holocene (Q2IV–Q3IV) is characterized by a reduction in the proportion of pine and broad–leaved vegetation and by the development of small–leaved and dark coniferous plant communities. It is found that in the second half of the Late Holocene (Little Climatic Optimum), pine–oak forests began to dominate the landscapes, and the end of that period developed the contemporary ecosystems.