Morphological and isotopic study of pedogenic carbonate coatings from steppe and forest-steppe areas of Baikal region, South-Eastern Siberia

Abstract

In this study we investigated different age carbonate coatings in the surface steppe and forest-steppe soils and paleosols of the Baikal region and its relationship with environmental fluctuations of the Late Pleistocene and Holocene. The 14C age of thin pedogenic carbonate laminae indicate four periods of coating formation: the second half of MIS-3 (24.1–23.3 and ~34.0–35.0 kyr BP), from the warm phases of the Late Glacial to the Middle Holocene (12.9–4.6 kyr BP) and the final of the Middle Holocene (3.6–3.3 kyr BP). Thus, studied carbonate coatings are mainly relict feature of the previous epochs of pedogenesis and Middle Holocene stage of surface soil formation. Paleoecological conditions reconstructed for the identified stages of formation of carbonate coatings correlate well with the trend of climatic changes in the Baikal region and adjacent areas. They reflect the effect of temperature and moisture fluctuations on the dynamics of soil-forming processes. Distinct δ18O and δ13C values suggest that the coatings of Transbaikalian steppes and forest-steppes of Fore-Baikal region have been formed under contrast environmental conditions. It is assumed that the formation of pedogenic carbonates in Transbaikalia is mainly confined to the summer season, when the soil is most warmed up, and the plants actively consume water and emit a large amount of CO2. This agrees well with the slightly higher temperatures of the warm season and the greater aridity of this region. In turn, carbonate precipitation in Fore-Baikal region occurred during the degassing of soil solutions in the course of periodic freezing and thawing, alternation of phases of snowmelt and rainwater infiltration into the soil and its subsequent freezing, which can be observed in the region in spring and autumn. A significant CaCO3 content in the studied soils contributed to the high growth rates of pendants. Holocene coatings are characterized by maximum growth intensity, which correlates with the highest carbonate content in the slope sediments where they are found. At the same time, MIS-3 coatings formed mainly on significantly less carbonate-rich alluvial deposits, which apparently affected their lower growth intensity. Based on an analysis of the growth rates of coatings, comparison of the isotopic composition of different age groups of pendants and soil micromorphology the more humid climatic conditions of the MIS-3 is assumed in comparison with the Middle Holocene. The temperature conditions of pedogenesis may be similar. Both for the Middle Holocene and for the MIS-3 relatively low temperatures of pedogenesis and long-term seasonal soil freezing are supposed.