Chemical weathering in semi-arid soils of the Russian plain

Abstract

A 700 km long soil transect in the southern Russian plain was studied. The difference in the average annual precipitation within the transect was 240 mm/year, and the average annual temperature was 3 °C. The objective of this study was to understand the behaviour of chemical elements and features of chemical weathering in semi-arid soils that developed on loesses and loess-like sediments. It was found that the semi-arid soils of the Russian plain are characterized by a low intensity of present chemical weathering. In an alkaline environment with a low amount of precipitation, the rate and direction of silicate weathering are determined not by the climate, but by the geochemical environment of the soil formation, which had already taken shape by the beginning of the Holocene. The alkaline oxidative environment hinders the hydrolysis and prevents the migration of chemical elements in ionic form. As a result, the removal of chemical elements from primary silicate minerals to soil solutions is not a determining process for vertical geochemical migration. The degree of weathering of present soils is mostly inherited from the parent materials of the Quaternary. Conditions for acid hydrolysis occur only in the upper horizons during increased biochemical activity and lower pH values. However, soil processes (carbonatization, salinization, illitization, and loessivage) make the behaviour of alkali (Na2O, K2O) and alkaline-earth (MgO, CaO, Sr) metals more complex, involving them in recycling in the upper soil horizons, and hinder the quantitative characterization of weathering of silicate minerals. Therefore, the use of CIA geochemical indicator as a quantitative measure of chemical weathering for semiarid soils formed in the Holocene becomes problematic. This study will improve the understanding of the geochemical processes that took place in modern soils in semi-arid regions and clarify the possibility of using geochemical weathering ratios in saline sediments.