Mineralogical Indicators of Climate Changes in Southwestern Siberia in Holocene Sediments of Bolshie Toroki Lake

We present the results of studying the Holocene sediments of Bolshie Toroki Lake, a shallow brackish lake with carbonate sedimentation and high bioproductivity, located in the eastern part of Baraba lowland (southwestern Siberia) . The mineral component of bottom sediments was studied by such methods as X-ray diffraction (XRD) analysis, IR spectroscopy, laser granulometry, elemental analysis, and others. By the mathematical modeling of complex XRD patterns, Mg-calcites with different Mg contents and aragonite have been established in the assemblage of carbonate minerals. Their structural and crystallochemical features and quantitative ratios were determined. The obtained high-resolution carbonate record was compared in the dated section with the distribution of the ash content, which is determined by the bioproductivity of the basin, and with a number of geochemical indicators of climate changes. The use of such a complex approach allowed us for the first time to recognize four stages in the evolution of Bolshie Toroki Lake; these stages were caused by cycles of drying/wetting of the regional climate and fluctuations of the lake level in the second half of the Holocene. Stage I (middle of the Atlantic) corresponded to the formation of the lake; stage II (second half of the Atlantic) was characterized by shallowing of the lake in a dry and warm climate; stage III (most of the Subboreal), shallow basin in a dry and cool climate; and stage IV (from the end of the Subboreal until the present) was marked by an increase in the water level of the lake and climate humidization.