First Data on the Age and Formation Conditions of Secondary Carbonate Accumulations in Upper Pleistocene and Holocene Soils of Upper Angara Region

Аннотация

The first data on the age of carbonate cutans in soils of the Upper Angara Region are given. Three groups of them were identified on the basis of the conditions of their occurrence, morphology, and substance and isotopic composition. The direct ¹⁴C AMS-dating of microlayers of cutans enables us to determine the periods of their formation. Cutans of the first group were formed in the Middle Holocene Period (3600–3300 cal. yr BP). Neoformations of the second and third groups appeared in the second half of MIS-3 (24 100–23 300 and ~34 000–35 000 cal. yr BP, respectively). Paleoecological conditions reconstructed for the identified stages of formation of carbonate cutans correlate well with the trend of climatic changes in the region and in the Northern Hemisphere in general. They reflect the effect of temperature and moisture fluctuations on the dynamics of soil-forming processes. The comparison of the age of neoformations with the age of modern and buried soils shows that pedogenic carbonate cutans in soils of the Upper Angara Region are a relict feature of the previous epochs of pedogenesis (MIS-3) and of the first stages of modern soil formation, which apparently began in the Middle Holocene. The similar ratios between stable carbon and oxygen isotopes in the Middle Holocene and Late Kargin (MIS-3) cutans permit us to suggest that the pedogenic conditions in those periods were similar and were mainly determined by relatively low temperatures of pedogenesis and long-term seasonal soil freezing.