Alexey Valentinovich Rusakov

The discovery of Pleistocene paleosols, including a level corresponding to the previous interglacial (MIS 5e), in the north of Western Siberia is of fundamental importance for Quaternary stratigraphy and paleoecology. This paper presents, for the first time, the results of a study of a pedocomplex found in the Belaya Gora section (Valley of the Vakh River). The period of its formation, established through uranium-thorium dating and taking into account paleobotanical and paleontological data, spans from the end of MIS 6 to MIS 4 inclusive. Morphogenetic analysis at the macro-, meso-, and micro-levels, as well as data on granulometry, mineralogy, and humus composition — including the humic acid/fulvic acid ratio and the proportions of the main structure-forming elements (C, H, N) in humic acids — made it possible to reconstruct the succession of pedogenetic, sedimentary, and cryogenic processes. Based on the obtained comprehensive geological record and considering previously published paleontological data, a paleoecological reconstruction was carried out, along with the chronostratigraphic attribution of all pedocomplex levels. A paleosol subprofile corresponding to the warmest substage of MIS 5e — the Kazantsev thermochron was identified. At the same time, it was shown that the permafrost-gley (cryohydromorphic) stage, which marks the end of the formation of each paleosol level and took place during the cold substages of MIS5, exerts a dominant influence on paleosol morphology and masks the features of preceding, warmer phases. An unexpected finding of neoformed gypsum crystals in combination with ferruginous nodules was explained by the oxidation of previously formed pyrite and was also attributed to the cryohydromorphic phase of paleopedogenesis. An interregional correlation of the studied profile with pedostratigraphic schemes of the loess formation in adjacent regions — the East European Plain and the south of West Siberia — was carried out. For most elements of the studied pedocomplex and the overlying sedimentary and cryogenic units, correlative levels in loess-paleosol sequences were identified, with the exception of the paleosols from the end of MIS 6 — the Taz cryochron, indicating a gap in the South Siberian loess-paleosol record.

The lithological matrix of surface soil profiles in the northern Mologo-Sheksna Lowland, within the wide valley of the lower Sheksna River and the foothills of the Andoga Ridge, reveals the final stage of Late Pleistocene aeolian sedimentation, reflected in the composition of the surface soils. Morphological and genetic analysis of soil profiles at the macro- and microlevels, supported by analytical characteristics and particle size distribution data, allowed us to identify and substantiate at the regional level a series of four contiguous layers of different ages and lithologically contrasting features. The base of the studied soil profiles is the Moscow glacial basal till (layer I, which displays consistent features of Mikulino pedogenesis (Bt horizons). The final stage of loess sedimentation in the upper part of the soil profiles during the Late Glacial (layer III) has been dated using the OSL method and falls within the range of 17,000–22,000 years ago. Layer II, represented by gravelly-sandy sediments in the middle part of the profiles of several of the studied soils, reflects a local stage of active sedimentation after the Mikulino period, during which the upper part of the glacial till may have been truncated (eroded). Thus, the studied soils represent a pedocomplex formed in the Late Pleistocene (a 150,000-year macrocycle). Identifying the final stage of loess sedimentation in the Late Glacial will allow us to assess the contribution of soil and lithological processes to the formation of the profile of loamy soddy-podzolic soils. Establishing the final stage of eolian sedimentation in the Late Glacial is also of great importance for paleogeography, as it reflects the impact of the global climatic rhythm at the Late Pleistocene-Holocene boundary on the modern landscape. The mantling of the Sheksna River valley by glacial basal till confirms V.V. Dokuchaev's (1877) concept of the role of glaciations in the structure of river valleys on the Russian Plain. The terrace levels in the studied areas represent ancient basement steps and do not belong to the modern river valley.