ИСТИНА

Lake Galich (58°24'N, 42°17'E, 101 m above sea level) is one of the largest natural lakes in the middle part of the Volga River basin. Although it is situated outside the area of the Late Valdai/Weichselian ice-sheet, it is believed that during the Last Glacial Maximum (LGM) it was a part of the vast Kostroma proglacial lake connected with other similar lakes. According to this hypothesis, during the LGM the shoreline of the Kostroma proglacial lake was at about 145 m a.s.l., and the present-day Lake Galich formed a narrow bay over 60 m deep. According to the age model based on radiocarbon dating, 69 m of Lake Galich sediments penetrated by coring accumulated for almost 60 thousand years. This sediment thickness provides a continuous archive of the Valdai Glaciation environment and climate history, from the end of the Marine Isotope Stage (MIS) 4 to the beginning of the Holocene, which is unique for the region. The studies of Lake Galich sediments are of great importance for resolving the question of the existence of large proglacial lakes in the central part of the Russian Plain during the LGM. Palynological data indicate that during the Early and Middle Valdai (MIS 4 and 3) the vegetation in the area was of a periglacial forest-steppe type, with open woodlands of spruce, pine and birch. Grain-size analyses of lake sediments indicate very stable sedimentation conditions during MIS 3, with particle size gradually increasing from clay to medium loam and an average accumulation rate of 1.4 mm per year. At the boundary of MIS 3 and 2 the sediment accumulation rate did not change, but the fraction of coarse silt, the main component of loess deposits, reached its maximum content (60%). At the LGM, the periglacial-steppe communities with Artemisia prevailed in the area due to increasing aridity of climate. Development of erosion on the slopes poorly protected by sparse vegetation is marked by presence of spores reworked from the Jurassic and Early Cretaceous deposits. After the LGM, the mean accumulation rate of lake sediments decreased to 0.8 mm per year, in spite of the considerable increase in particle size. Therefore, the studies of bottom sediments of Lake Galich showed no traces of a manifold increase in the depth of the lake during the proposed damming up to 145 m a.s.l. at the LGM, which casts doubt on the very possibility of such proglacial lakes existence in the region at the time. The research was carried out with support of the Russian Science Foundation project 17-17-01289 «Runoff rerouting and migration of the Main Divide of the Russian Plain during the last glaciation».