Freezing of Chalk Cryomorphic Soil Complexes of the Orenburg Region: Temperature Regime and Cryogenic Processes in the Soil Profile

The dynamics of the temperature of conjugated soils of a paleocryogenic soil complex on chalk sediments in the Orenburg region is studied. Temperature measurements were combined with the investigation of cryogenic features. The freezing point of soil moisture was determined in the laboratory. A significant heterogeneity of the temperature field within the soil complex was revealed. During the autumn–winter, the soil of micro-elevation was colder than the soils of micro-depression and micro-slope, and in the spring-summer period, the micro-slope warmed up faster than the micro-elevation and micro-depression. The differences between the temperature of soils in the frozen layer at micro-elevation and in the micro-depression reached –4.5°C at the beginning of freezing (15.12.2019 at a depth of 15 cm), –4.0°C at the end of winter (10–11.02.2020 at a depth of 5 cm), and –6.5°C during thawing (21–23.03.2020 at a depth of 5 cm). Differentiation of temperature regime along the microrelief were accompanied by differences in the profile distribution of moisture and determined the manifestation of cryogenic processes. Micro-elevations froze deeper, a cryogenic texture was formed across the whole zone of freezing, and was accompanied by frost heaving, cryogenic sorting of coarse fragments, formation of a porous crust on the soil surface, what ensures the maintenance of the microrelief and the structure of the soil cover of chalk polygons. Cryogenic textures determine the formation of a platy soil structure on micro-elevations. In the micro-depressions, freezing was blocked in the middle part of the profile due to relatively high soil temperatures and low soil freezing temperatures. Cryogenic features and processes described for micro-elevations are not expressed in micro-depressions.