Salt Neoformations in the Soils of the Aral Sea Dried Bottom
Abstract
Salt neoformations of seaside solonchaks (different in their granulometric composition) of the dried bottom in the southwestern part of the Aral Sea were studied by X-ray fluorescence, electron microscopy, the sedimentation method, and laser diffractometry. The surface layers of soils of sandy and silty clay composition were considerably saline and enriched with CaCO3. The distribution of CaCO3 over the area was quite uniform, indicating relatively similar conditions for its formation during drying. Salt content varied from 3.1 to 5.8% in silty loam soils and 0.4 to 1.2% in sandy soils. Silty loam soils were characterized by the highest salinity level. In addition to gypsum and halite, easily soluble salts were formed in them: eugsterite (2Na2SO4·CaSO4·2H2O), astrakhanite (Na2SO4·MgSO4·4H2O), and mirabilite (Na2SO4·10H2O). The template mechanism of CaCO3 accumulation in soils was revealed. The result of the interaction of CaCO3 with the solid phase of deposits depended on their granulometric composition, which determined the predominant particle size and, accordingly, the number of crystallization centers. Under the conditions of a small number of crystallization centers in sandy soils, the formation of CaCO3 cutans characterized by varying levels of stability and coverage of the sand grain surface occurred. In silty-clay soils with a large number of crystallization centers under constrained conditions for crystal growth, CaCO3 precipitation led to the formation of molecular clusters that bound clay and silty particles into CaCO3 microaggregates. The cause of the increased activity of salt neoformations in the processes of aeolian transport was established. This transport and, therefore, salinization of adjacent territories was facilitated by the blocking of adsorption centers by calcite in clays, which prevented the binding of soluble salts in clay–salt microaggregates.