Geochemistry of Carbonates in Small Lakes of Southern West Siberia Exampled from the Holocene Sediments of Lake Itkul

A 1.8 m thick core of the Holocene (7.9 C-14 kyr BP) sediments of Lake Itkul' (Novosibirsk Region) has been studied. Based on the geochemical and lithostratigraphic properties of the bottom sediments, we have established the following stages of the lake evolution: (1) the beginning of sedimentation, 7.8-7.0 C-14 kyr BP; (2) extreme shallowing with a probable complete drying, similar to 7.0-5.5 C-14 ky BP; (3) rise in the water level, similar to 5.5-4.3 C-14 kyr BP; (4) repeated shallowing, 4.3-2.8 C-14 kyr BP; and (5) subsequent watering, 2.8-0 C-14 kyr BP. At present, the lake again tends to shallowing. We have established that Lake Itkul' has mineral sediments with a high content of carbonates (up to 64%). The high concentrations of HCO3- and Ca2- and stable saturation (S/St > 1) of the surface water cause a shift of the carbonate-calcium equilibrium toward the carbonate formation. The studied authigenic carbonates are aragonite and fine-grained aggregates of poorly crystallized calcite particles with different Mg contents. Aragonite is both biogenic (mollusk and ostracode shells) and chemogenic (formed during the lake shallowing). In addition to carbonates, the lacustrine sediments contain mixed-layer aluminosilicates, feldspars, and quartz. The presence of pyrite throughout the sediment section indicates reducing conditions and the activity of sulfate-reducing microorganisms. Carbonates (especially aragonite) are significantly enriched in strontium. Manganese does not form its own minerals but is present as an isomorphic impurity in authigenic carbonates.