Natalia Leonidovna Bazina

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 article considers changes of paleoenvironment on the western edge of continental Europe using the example of the Nantua section deposits, the formation period of which falls on the Middle Pleistocene cryochron corresponding to MIS 6 and, possibly, the transition to MIS 5e. The approach used is based on a set of characteristics of specific carbonaceous (humus) substances contained in the deposits, including its most reflective component – humic acids (HA). It is shown that the correlation coefficients of the indicator parameters of the composition and properties of modern humic acids in soils, obtained using different methods for studying their molecular features, are within 0.78–0.90. Based on the changes in the characteristics of the humus component of the deposits, it was established that the latter were formed in changing conditions, when, against the background of general coldness of the climate, there were five relative warmings of varying severity, which caused the appearance of clear signs of pedogenesis and primitive paleosols formation. A trend towards increasing moisture conditions was revealed, which is manifested in an increase in the proportion of fulvic acids relative to humic acids and a decrease in the absolute values of Cha:Cfa. Preliminary calculations of the average long-term air temperatures and the duration of the biological activity period (BAP) showed that their possible limits, respectively, lay between minus 5.9°C and –7.8°C and 52–80 days. The terms “humus” and “humus substance system” are used as synonyms in this paper.