Humus component of pedosediments at the end of the Sartan cryochron in the Nadym River basin (West Siberia)

Abstract

The aim of the study. To assess the potential of humus substances from the pedosediments in the polygonal ice wedges pseudomorphs as indicators of the paleoecological conditions of their formation during the period of their functioning within the surface soil, using the case of the Razduv object of the end of the Sartan cryochron (corresponding to MIS-2) in the basin of Nadym River (West Siberia). Important comment: In this paper the terms “humus”, “humus component of soils” and the “system of humus substances” are used as synonyms. Location and time of the study. The research was conducted in the Nadym district of the Yamalo-Nenets Autonomous Okrug (YaNAO), at the Razduv site 15 km to the north-east of the Nadym city, during the late summer 2018. Methods: The study is based on a new methodological approach to the diagnostics of the bioclimatic conditions at the end of Sartanian cryochron in one of the northern territories of West Siberia, based on the humus substances of pedosediments which serve as indicators of paleoenvironment. Main results. It was established that the applied indicator features of the formation conditions of the studied pedosediments, based on indicators of the composition and spectral properties of their humus component, correlated well with each other, in their totality reflecting the cold humid conditions of soil formation, similar to those for modern soils of different tundra subzones of the West Siberia. Conclusions. The study of the specific features of composition, structure and properties of humus substances from the pedosediments in the pseudomorphs after polygonal ice wedges, dated back to the end of the Sartanian cryochron at the key section of Razduv and revealing the indicators of their formation conditions, demonstrated that the characteristics of the humus substances system and in particular one of its components, i.e. humic acids, obtained by independent methods, have high diagnostic significance for reconstructing paleoclimatic and paleoecological conditions of their formation. Characteristics of modern soils and paleosols from various parts of North Siberia, from northern Trans-Urals to the coastal lowlands in the north of Yakutia, were used as a recent reference dataset for diagnostics and reconstruction of the bioclimatic conditions of paleosol development on the land surface during MIS 2. It was confirmed that the studied samples, attributed to redeposited soil materials, i.e. pedosediments, due to their morphology in fact had undergone the phase of pedogenesis that is evidenced by the presence of humus substances, including humic acids with their typical outline of FTIR-spectra with a characteristic set of the absorption bands, individual ratio of the main structural elements and similar variation of the quantitative parameters of the fluorescence emission. The closest similarity of the composition and proportion of the main components of the humus substances system was found between the samples of the Sartanian pedosediments and the modern soils formed in the tundra conditions of West Siberia. The similarity of the main humus substances ratio, as well as the main structure-forming elements and values of all fluorescence characteristics of humic acids, especially for the samples with higher humus content and those affected by permafrost, was revealed. These results confirm that at the end of the Sartanian cryochron the pedogenesis in the vicinities of Razduv section was similar to that is now going on in the region located to the north of the study site.