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

Abstract

The temperature dynamics of conjugated soils of a paleocryogenic soil complex on chalk deposits in the Orenburg oblast is studied. Temperature measurements are combined with the study of cryogenic characteristics. The freezing point of soil moisture has been measured in laboratory. The temperature field within the soil complex is most heterogeneous. In the fall–winter season, the soil of microhighs is colder than the soils of microlows and microslopes; however, the microslope warms up faster than the microelevation and microlow in the spring–summer season. The differences between the temperature of soils in the frozen layer of microhighs and microlows reach –4.5°C at the beginning of freezing (December 15, 2019 at a depth of 15 cm), –4.0°C at the end of winter (February 10–11, 2020 at a depth of 5 cm), and –6.5°C during thawing (March 21–23, 2020 at a depth of 5 cm). The differences in the temperature regime along the microrelief are accompanied by the differences in the moisture distribution along the profile and determine the manifestation of cryogenic processes. The soils of microhighs freeze deeper and lens-type cryostructure forms in the entire frozen layer; this is accompanied by frost heaving, cryogenic sorting of coarse fragments, and formation of a crust on the soil surface ensuring preservation of the microtopography and soil cover pattern of chalk polygons. Cryogenic processes determine the formation of a platy soil structure in microhighs. Freezing in microlows is blocked in the middle part of the profile because of relatively high soil temperatures and low soil freezing points. The cryogenic characteristics and processes described in microhighs are unobservable in microlows.