Northwest Siberia as a MIS 2 desert? Inferences from quartz morphoscopy and polygonal ice wedges

摘要

Following a major revision of the Sartan ice sheet limits, Velichko et al.'s (2011) view of northwestern Siberia as a cold arid desert during MIS 2 (27.8-11.7 ka) has been prominent in regional scholarship. Our article presents the analysis of a series of sandy sediments associated with MIS 2 polygonal ice-wedge pseudomorphs (PIWPs) located in the Lower Nadym, the Middle Nadym, and the Siberian Uvals regions. Whereas Velichko et al.'s cold desert hypothesis was based on optical morphoscopy, our research relied on the scanning electron microscope technology (in conjunction with sedimentological, soils, and radiocarbon data). Our morphoscopic analysis of three wedge fill - host sediment pairs suggests that none of the MIS 2 surface sediments can be unequivocally associated with desert dune environments, but instead resemble alluvial deposits that have been moderately to substantially reworked by aeolian and cryogenic (freeze-thaw) processes. While the environmental characteristics varied between different sub-regions, northwestern Siberia likely resembled a sparsely vegetated tundra during MIS 2. AMS radiocarbon dates obtained from the organic matter of paleosols discovered inside of PIWPs indicate that the climate warming in northwestern Siberia began around 13.5-13 ka cal BP (end of MIS 2). This climatic shift was associated with a more pronounced warm season, as evidenced by the surface sediments experiencing more intensified freeze-thaw cycles in all of the examined locations. Judging from our analysis of sandy deposits, aeolian activity intensified in the Siberian Uvals in the middle MIS 2, compared to early MIS 2. Aeolian activity also intensified in the Middle Nadym in the second part of MIS 2, compared to the first part of MIS 2. Besides the intensified freeze-thaw processes at the end of MIS 2, surface deposits of the Lower Nadym region remained relatively stable throughout the Sartan cryochron.