Cryohydromorphic paleosols in the north of West Siberia as indicators of the warm stages of the Late Pleistocene and their link to the cryogenesis development

Abstract

The aim of the study. The aim of the study was to find and justify specifics of the paleosol development in the north of West Siberia, revealing soils levels, associated with climate warming during sea isotope stages (SIS), namely MIS-5, MIS-3 and at the end of MIS-2. Location and time of the study. The research was conducted during the last 10 years in the representative area between the Polar Circle and the sub-latitudinal sector of the Ob river all over the area. We carried out paleopedological and paleocryological investigations with fieldwork in the basin of the Vakh River (right tributary of Ob), the Taz and Nadym Rivers, where the soil pits were studied, and paleosols were discovered for the first time. Methods. The methodology has been focused on detailed study of the paleosols discovered by the present authors based on the investigation in respect to the interaction of paleopedogenetic and cryogenetic processes. Also, the encountered paleosols have been, as well compared to the surface Holocene soils that comprises an important element of analysis of the paleopedological records. For age determination we applied radiocarbon dating of organic residues and humus, contained in the paleosols, as well as U/Th-dating in paleosols. Results. Several paleosol levels were identified in the study area. The paleosol of the early substage of MIS-5 (MIS-5e) had signs of forest pedogenesis without permafrost, whereas the soils of late substages of MIS-5 and MIS3 showed evidence of gleying and cryogenesis; the paleosol corresponding to the end of MIS-2 was best represented by gleyic pedosediments in the ice-wedge pseudomorphs. The studied paleosols indicate the permafrost state of the soil-forming sediments in all phases of the Late Pleistocene with the exception of MIS-5e; the finding is important for the paleoenvironmental reconstruction. The article presents the data about wide distribution in the study region of the polygonal structures represented by the ice-wedge pseudomorphs formed in the coldest phases MIS-4 and MIS-2. The pseudomorphs were developed syn-cryogenically when permafrost still existed and were closely related to the formation of cryohydromorphic paleosols. Within the entire sequence of MIS-3 deposits initially ground wedges are common, also indicating syn-cryogenic development, however, under less strong freezing conditions than during MIS-2. Yet at the end of MIS-3 in the upper part of the sequence cryohydromorphic soils appeared again, as a sign of approaching to the MIS-2 cryochron. It was shown that the fills of the ice wedge pseudomorphs contain well preserved redeposited materials of the cryohydromorphic paleosols, including fragments of the humus horizons. It was suggested that the material from such redeposited pedosediments could be used for the radiocarbon dating and for sampling for the paleobotanical investigations. Conclusions. The obtained results cast doubt on the hypothesis of domination of the cold deserts and ice sheets in the study region during the Pleistocene and hence the formation of only extremal soils. There is evidence indicating the existence of well-developed vegetation and soil cover during the cryochrons with sufficient and in some areas excessive soil moisture, due to shallow permafrost table. The interaction of the paleopedogenetic and paleocryogenic processes resulted in rather informative relict objects, and their studies provide highly reliable results about the factors of past pedogenesis.