Carbonate Profile of Soils in the Baikal Region: Structure, Age and Formation Conditions
The results of the study of the carbonate profiles of soils in the western (Cis-Baikal) and eastern (the Selenga middle mountains) Baikal region are presented. There is a similarity in their structure: numerous CaCO<sub>3</sub> maxima in the soil profiles are typical, which is associated with the repeated redistribution of carbonates during different stages of pedogenesis. The carbonate profile of soils in the Baikal region is relatively ancient (the youngest accumulations date from the Middle Holocene) and a rather conservative formation that does not undergo any noticeable rearrangements under modern climatic conditions. The accumulations of secondary carbonates are localized in the carbonate-accumulative horizons of the modern surface soils and buried Kargin (MIS-3) soils and Holocene soils. The exception is hypocoatings that are often found beyond the indicated horizons, which is associated with a more active redistribution of carbonates within the root system of vegetation. The solid-phase effect of carbonatization (carbonate neoformations) is observed in the study area mainly in the soils formed on calcareous rocks and their redeposition products and does not depend on the type of soil and the nature of the growing vegetation. Based on the analysis of the composition of stable carbon and oxygen isotopes in secondary carbonate accumulations, it was found that carbonate precipitation in soils of the Cis-Baikal region occurred during the degassing of soil solutions in the course of freezing–thawing cycles, dynamic increase and decrease in the soil biological activity, and alternation of the soil moistening with snowmelt and rainwater with its subsequent freezing that can take place in spring and autumn seasons. Under these conditions, atmospheric CO<sub>2</sub> has the main influence on the isotopic composition of carbon in pedogenic carbonates; the isotopic composition of oxygen was controlled by the fractionation of isotopes upon freezing of the soil solutions. Secondary carbonate accumulations in the Selenga middle mountains were formed during the soil drying as a result of active water consumption for plant transpiration, which is most pronounced in the first half of summer.