Relief structure and postglacial evolution of natural and climatic conditions of the central part of the Oka Plateau (Eastern Sayan)
A.A. SHCHETNIKOV1,2, E.V. BEZRUKOVA2
1Institute of the Earth’s Crust, Siberian Branch, Russian Academy of Sciences, Irkutsk, Russia
2A.P. Vinogradov Institute of Geochemistry, Siberian Branch, Russian Academy of Sciences, Irkutsk, Russia
Keywords: lacustrine sediments, palynology, environmental records, Holocene, climate change
Abstract
The article presents the results of a comprehensive geomorphological and paleogeographic study of the central part of the Oka Plateau (Eastern Sayan). The relief structure of the region is characterized in detail. A special place in it is occupied by elements of the paleohydronetwork - basins of drained moraine-dammed reservoirs and abandoned fragments of valleys, including tiered ones. A comprehensive record was obtained from the bottom sediments of Lake Sagan-Nur, which made it possible to reconstruct a consistent picture of changes in the natural environment in the lake basin in the Holocene. It is shown that in the period of about 8600-7100 years ago the lake was fed by glaciers, large flood events occurred in its basin, the water level in the lake was higher than today, and it was probably connected to a moraine-dammed paleo-reservoir, traces of which are recorded in the estuary part of the Aynak River valley. Later, about 7100-5500 years ago, a sharp drop in the rate of lacustrine sedimentation and a gradual attenuation of the relative amplitude of flood events occurred. After 5500 years ago, the warming of the regional climate, especially in winter, and the disappearance of the “remnants” of the late Pleistocene glaciers caused a gradual expansion of dry areas at lower hypsometric levels favorable for the regional development of pine forests and the rise of the upper boundary of pine on the slopes. At the end of this time interval, the last restructuring of the local hydrographic network occurred and the modern river system was finally formed. Reconstructions for the period of the last 5500 years show a much calmer hydrological regime and sedimentation pattern. Palynological indices also suggest a warming of the regional climate and a decrease in soil moisture, which took place following a change in the influx of solar radiation in the temperate latitudes of the Northern hemisphere.
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