Transformation of clay minerals (smectite-zeolite, illite, kaolinite, and bentonite) and admixtures of iron oxides (hydroxides) under the action of an alkaline cyanobacterial community was studied. The results demonstrate that the processes of transformation of clay minerals such as intensification of removal of exchange bases and dissolution of silicates and iron oxides occurred in the presence of the alkaliphilic cyanobacterial community. The main factor that determines resistance of a mineral to biochemical weathering is its composition. Transformations of clay minerals in the course of active cyanobacterial photosynthesis (up to 14 days) and at decomposition of organic matter (OM) (28–60 days) are different. For smectite-zeolite and illite, these processes are dissolution of silicates and oxides (removal of Si and Fe) and removal of exchange bases (K), which were observed at both the of biomass production and OM destruction stages. For two other clays, the processes of neosynthesis are more typical: formation of carbonates (most probably siderite for bentonite clay and Mg-calcite for kaolin clay) and transformation of ferrihydrite into the more thermodynamically stable goethite.

Paleosols buried under steppe kurgans of the Golden Horde period (13th–14th centuries AD) in the Lower Volga basin are characterized by an increased humus content, lower salinity and gypsum content, and higher magnetic susceptibility of the soil material in comparison with the paleosols buried in the preceding period and the background surface soils. A comparative analysis of the morphological, chemical, and magnetic properties of the buried and surface soils allows us to conclude that an increase in climatic humidity within this dry region took place in the period of the high Middle Ages, with a peak in the 13th–14th centuries AD. The climatic change was manifested in the soil evolution at the taxonomic levels of soil genera and soil subtypes (in the ecotone zones). On the basis of measured magnetic susceptibility values, the mean annual precipitation levels in the Golden Horde period have been reconstructed. According to our estimates, the mean annual precipitation in the Lower Volga basin in that time was 30–80 mm higher than at present. The favorable paleoenvironmental and paleosol conditions of the Golden Horde period were important factors that affected the ethnic and political situation in the Lower Volga region.

Information about the past evolutionary changes in the soil cover is essential as a retrospective basis for long-term forecasts of soil development under the changing climatic conditions, and also for understanding the present-day problems of desertification and land degradation in the south of the East European Plain. The principal task of the present study was to estimate quantitatively the shift of the boundary between the steppe and desert-steppe zones in this region during the late Holocene under the impact of global climate variations. The properties of about 100 paleosols of archaeological sites widely represented in the region were used for quantitative reconstruction of paleoprecipitation, paleotemperature, and climate aridity. To obtain regional calibrations, the dependences of the magnetic properties of modern soils on climatic parameters in the south of the East European Plain were analyzed in detail. Samples of 35 soil profiles along the 1500-km-long Voronezh–Volgograd–Astrakhan–Elista–Voronezh transect were used. This transect crosses several natural climatic zones with gradients in the annual precipitation of about 500 mm/yr and in the annual temperatures of about 3°C, which allowed us to obtain reliable relationships between the climatic parameters and the properties of recent soils. Limitations for paleoclimate reconstructions based on the magnetic properties of soils in the range of mean annual precipitation <600 mm/year were identified. Cartographic reconstruction of changes in the De Martonne aridity index (IDM) using GIS modeling methods and paleopedological data indicates that the Volga–Don interfluve has undergone repeated changes in the climatic situation over the past 5000 years resulting in the shifts of the boundaries between soil-geographical zones to distances of up to 200–300 km.