Micromorphological diagnostics of pedogenetic, eolian, and colluvial processes from data on the fabrics of crusty horizons in differently aged extremely aridic soils of Mongolia

The micromorphological and microtomographic studies of differently aged extremely aridic soils in the Trans-Altai Gobi Desert of Mongolia allowed us to determine the rates and sequences of processes associated with sedimentogenesis and pedogenetic transformation of the parent material. In the most developed extremely aridic soils, the topsoil or the surface Aye horizon consisting of the layers of different compositions and geneses (eolian and colluvial sediments) displays vesicular voids, which are virtually absent in the young soils. Two different mechanisms of the origin of vesicular voids are suggested: (a) in the case of the fresh eolian material, the bicarbonate–calcium equilibrium is displaced towards the formation of calcite with the release of CO 2 under the impact of changes in the moisture and temperature; (b) in the case of the dense silty clay material, the release of sorbed air (nitrogen and oxygen) from the dry soil particles takes place under the impact of atmospheric precipitation. The locally acting pedogenesis in these soils specify the development of hypocoatings and typical coatings consisting of the amorphized phyllosilicates with the disordered structure inside the vesicular voids and the formation of iron microconcentrations and iron films on mineral grains.