Abstract

—We present results of a study of carbonate coatings formed at the lower surfaces of pebble inclusions in Holocene–Upper Pleistocene deposits of the Irkutsk–Cheremkhovo plain. The coatings resulted from the leaching of carbonates from the overlying deposits during pedogenesis. They are composed of pure and magnesian calcite with minor inclusions of quartz and feldspars. The coatings form morphologically and compositionally different microlayers, which reflect a successive change in the pedogenesis conditions during their formation. The stable-isotope composition is as follows: δ13C is –6.80 to –2.05‰, and δ18O is –16.33 to –10.15‰. It reflects the precipitation of carbonates during the degassing of soil solutions in the course of periodic freezing–thawing processes, dynamic increase and decrease in the biological activity of soils, and alternating moistening of soil with meltwater and rain water and its subsequent freezing, which could occur in the region in spring and autumn. A joint analysis of the carbon isotope composition of the organic matter of soils and carbonate coatings shows that the latter formed in the phytocoenosis environment with predominant C3 vegetation. The differences in the morphology and occurrence of the coatings permit them to be divided into three groups. The coatings of the first group formed in the Middle Holocene (3.6–3.3 cal. ka BP), and the coatings of the second and third groups, in the second half of MIS 3 (24.1–23.3 and ~34–35 cal. ka BP, respectively). The paleoecologic conditions reconstructed for the established stages of carbonate coating formation are in satisfactory correlation with the general course of climatic fluctuations in the region and in the Northern Hemisphere as a whole. They reflect the influence of temperature and humidity fluctuations on the dynamics of soil formation processes. Comparison of the age of the coatings with the age of recent and buried soils shows that the pedogenic carbonate coatings in the Upper Angara soils are a relict feature of the previous epochs of pedogenesis (MIS 3) and the first stages of recent soil formation, which began, most likely, in the Middle Holocene. Based on analysis of the rates of coating formation and comparison of the isotope composition of the coatings of different age groups, we assume that the climate in the Karga (MIS 3) megainterstadial was more humid than that in the Middle Holocene, with the temperatures of pedogenesis being the same. Both in the Middle Holocene and in the Karga epoch, the regional soils received little heat and were seasonally frozen for a long time.

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