Geochemical characteristics of the sapropel sediments of small lakes in the Ob’–Irtysh interfluve

More than 20,000 lakes with sapropel sediments are located in southern West Siberia. The work is aimed at an integrated geochemical study of ten sapropel lake systems: a geochemical description of the lake waters, hydrobiological analysis, and analysis of the sapropels (phase and chemical analyses of inorganic matter and elemental analysis of organic matter). The geochemical landscape and low-flow regime of the waters of Siberian small lakes favor massive growth of living organisms. The anaerobic medium at the water/sediment boundary promotes the burial of organic matter as terrigenous material builds up. Based on the results, the sapropels are classified by chemical and mineral compositions into organic, organic siliceous, organic carbonate, organic carbonate-siliceous, and carbonate-siliceous. The sapropels inherit the contents of elements in soils and parental rocks, but the presence of considerable amounts of organic matter in the sediment determines their lower contents with respect to those in the soils of water catchment areas (except U, Cd, Hg, Ca, Sr, and Mn). Biochemical formation of pyrite and calcite is observed in the lake bottom sediments. The calcite formation in the sapropel sediments of Lakes Barchin, Bergul’, Kankul’, and Itkul’ determines their high Ca, Sr, Mn, and Mg contents. Comparison of data on the anion composition of the waters and the organic and mineral compositions of the sapropels has shown that the formation of low-ash biogenic sapropel takes place in lakes whose waters contain a considerable amount of dissolved organic matter and in which HCO3− is a predominant anion. No effect of the cation composition of the waters (Na, Ca, and Mg) on the sapropel composition has been detected. Therefore, Lakes Kankul’, Kachkul’nya, Bergul’, Barchin, and Kambala are the sapropel lake systems with the highest potential.