Cryogenic processes in the soils of Northern Asia

Abstract

Cryogenic processes, those occurring in soils below freezing temperatures much of each year, are widespread in northern Asia. The processes result in specific soil properties and the development of unique soil types, subtypes and taxa at lower levels. Two major groups are involved: frostgenic soils and coldgenic soils. In frostgenic soils, the seasonally frozen and thawed layers contact permafrost tables. In coldgenic soils, either the permafrost tables occur below the maximum depths of seasonal freezing and thawing or there is no permafrost. The term “cryogenic soils” should be used to cover both frostgenic and coldgenic soils. Although the rates and intensities of cryogenic processes differ considerably in different parts of northern Asia, the general nature of these processes is similar throughout. This is illustrated by data from investigations of frostgenic meadow-forest soils of the extra-continental forest steppe of the Trans-baikal area of the U.S.S.R. The soils are characterized by processes typical of frostgenic soils: frost heaving, swelling, cryoturbation, thixotropy, vein ice formation and development of platy structure. Frost cracks are very changeable, with annual amplitudes in width on 10–15 cm. Suspensions of humified fine earth periodically enter these cracks and gradually fill them with permeable materials. During downpours, water carrying materials in suspension and solution flows downward through these cracks and then diffuses into the parent material at considerable depth. Thus, an A-C-B profile is developed. Discontinuous layers of organic matter 5–30 cm thick at various depths are characteristic features of these soils. Such layers are thought to originate as a result of the concentration of solutes as the soil freezes and ice lenses form at low temperature gradients.