Stable Carbon Isotopic Composition of Organic Matter of the Late Pleistocene and Holocene Soils of the Baikal Region

Abstract

The results of studying stable carbon isotopes of the organic matter of Late Pleistocene and Holocene soils of the Baikal region are presented. The δ13С values in soils of different ages varies from –30.00 to –21.36‰, which indicates the formation of their organic matter from C3 plants at least in the past 35 ka. Fractionation of isotopes during the organic matter mineralization leads to an increase in δ13С values with the depth. At the same time, in soils of steppe and forest-steppe landscapes, an increase in δ13С is about 3‰, whereas in forest soils it can reach 4–5‰, which is most likely associated with a slow transformation of organic residues in the upper horizons of forest soils. In the Late Glacial and Early Holocene soils, an elevated content of organic carbon is noted, in which the stable carbon isotopic composition is lighter than that in the recent soils. This may be explained by the influence of permafrost slowing down the transformation of organic matter. In the sequence starting with rather dry soils of the Ol’khon region and the Selenga Mountains to the forest-steppe soils of the Upper Angara region with its more humid conditions and, further, to the taiga soils in the south of Central Siberia, there is a marked decline of δ13С values in the organic matter. Thus, we deal with a bright example of the δ13С dependence on the soil water supply, i.e., on precipitation during the growing season. Reconstruction of precipitation based on δ13С values attests to a gradual aridization of the climate starting from the Late Glacial period with a maximum drying in the Middle Holocene, while in the Late Holocene the level of humidization increases. A tendency towards a gradual decrease in humidity was also observed during MIS-3. It is probable that the most humid climate was during the formation of the Early Karga soils. In the range of 36–31 ka BP, climatic humidity decreased and became comparable with the modern one. At the end of MIS-3, climatic humidity was below the current level.