Paleosols in the Red-Colored Sediment Complex of the Cis-Sayan Basin (Southeastern Siberia)

The studied red-colored sediments of the Mikhailovka section (south of the Irkutsk amphitheater) were formed in the course of discontinuous filling of an erosional landform dating back to the main phase of neotectonic uplift in the Late Pliocene–Early Quaternary. A pedocomplex of three paleosols with different degrees of preservation has been identified in this section. The paleosols developed during phases of decreasing activity of exogenous relief-forming processes. The analysis of the micromorphological features of the paleosols makes its it possible to identify the signs of biogenic structuring and biogenic forms of secondary carbonates in the upper horizons of the paleosols. Their middle-profile horizons have clear features of striated optical orientation of clay particles and contain carbonate pendants attesting to the mobility of ferruginous-clayey and carbonate substances in periods with different moistening conditions. The analysis of δ13C values of soil organic matter, as well as δ13C and δ18O values of carbonate nodules, indicates a significant role of pedogenesis in their origin and suggests that paleosol formation in the red-colored sediments could take place under hydrothermal conditions comparable to the modern ones in the studied area. At the same time, the observed isotopic equilibrium between the carbonate material and the paleosol organic matter suggests a less contrasting hydrothermal regime in the intra-annual cycle and a longer duration of the growing season. Soils and pedosediments formed within the red-colored strata also contain the signs of cryogenesis at the macro- and microlevels, which indicates pronounced tendencies toward climate cooling in the phases of transition from pedogenic to morpholithogenic stages.