Carbonates in the soils of the Aral Sea dried bottom

Abstract

The paper considers the first stage of salt deposition, namely, the features of calcium carbonate formation in the soils (solonchak-like formations) of the Aral Sea dried bottom, its south-eastern part. The studied soils (sandy and silty-clay soils) are significantly saline and enriched with calcium carbonate. The content of only mollusk shells can reach 10-20%. The content of mineral CaCO3 in the soils is quite uniform, its content varies by no more than an order of magnitude, which indicates relatively close conditions for its formation in soils with different particle size distribution. However, the types of aggregations of calcium carbonate (with a close size ca. 250 µm) and the mechanisms of their formation in the soils are different. In sandy soils, ooids (CaCO3 cutans on the surface of sand-sized particles) are formed by the template mechanism. In silty-clay soils with a large number of crystallization centers (the predominant particle size is < 50 µm), the precipitation of calcium carbonate leads the formation of CaCO3 microaggregates from silty-clay particles. Separate crystallites and mesocrystals of calcium carbonate (up to 40 µm in size) are found singly.