Warming evidences recorded in loess deposits of the Last Glaciation and dynamics of loess accumulation in North-West Cis-Salair (South-East of West Siberia)
In the framework of this work, loess sections represented by subaerial deposits of the MIS 2 - beginning of MIS 1 age of Cis-Salair plain within the Iskitim district of the Novosibirsk region, were studied. Great interest in the section was caused by the presence of three buried paleosols and carbonized organic matter in it, suitable for 14C dating. Despite the wide distribution of the Sartan loess cover in the south of Western Siberia, finds of interstadial paleosols in the loesses of this time are very rare. Previously, Late Glacial paleosols were not found in loess deposits. There is only the Suminskaya paleosol (~ 19.6 - 16.3 thousand years ago) is distinguished, separating the Bagan and Eltsovka loess in the stratigraphic scheme of Western Siberia. However, the Suminskaya paleosol is most often absent in loess sections. In the work, a complex approach to the study of the section was used, which in addition to field observations included grain size and petromagnetic (magnetic susceptibility) analyses, morphoscopy of sand quartz grains and radiocarbon dating. Grain size composition and magnetic susceptibility were measured for the subaerial part of the section. The morphology of quartz grains was studied for all layers of the section. The deposits of the Ust'-Chem section give an idea of the stages of the formation of the loess stratum of the final of the Late Pleistocene in the area of the left bank of the Elbash river valley in the Cis-Salair. The Bagan and Eltsovka loess-like loams lay on subaquatic sediments, its thickness varies and depends on the underlying relief, determined in this place by the form of mother rock outcrops. According to the grain size analysis, it was shown that the main fraction in the sediment composition is coarse silt, which is a characteristic feature of loess deposits. Data on the morphology of quartz grains confirmed that aeolian processes were the leading factor in the accumulation of deposits of Bagan and Eltsovka loess, and traces of cryogenic weathering also were noted. According to the structure of sections Ust'-Chem-1 and Ust'-Chem-2, wide subaerial sedimentation in the considered section of the Elbash river valley began from Sartan time, Karga deposits also were preserved. No traces of more ancient loess accumulation were revealed; they may have been denudated. According to the data of grain size, magnetic susceptibility, and morphoscopy of quartz grains, the accumulation of the Eltsovka loess occurred as a result of the conjugation of the input of dusty material as a result of deposition from the atmosphere and the participation of local transport of sand particles as a result of the entrainment of sediments of water genesis and eluvium of Paleozoic rocks. In the accumulation of Bagan loess, the role of local transport is reduced to a minimum. The section reflects short-period events of the last glaciation, traces of which are very rarely found in loess sections of the southeastern part of Western Siberia. The obtained dates are important for restoring the history of paleogeography of the Novosibirsk region. Earlier, no signs of Late Glacial warming were found in loess deposits of the south-east of Western Siberia. Evidences of two warmings in the Late Glacial period of the northwestern Salair region were established in the range from 14.9 to 12.3 kya: the section contains two weekly developed paleosols, the absolute age of which showed that they can be analogs of the Belling and Allered interstadials in Northern Europe and the Greenland curve interglacials GI-1e and GI-1a, b, c, respectively. Besides, the section revealed the presence of a "warm" event at the base of the Eltsovka loess (MIS 2), which is correlated by the time of it with the formation of Suminian paleosol. It is suggested to indicate that this warming was not developed locally and was occurred in different regions of the south of Western Siberia. Apparently, the warming in the Late Glacial in the south of Western Siberia was uneven and depended on regional palaeocli-matic conditions and relief. The paleosol-3 in the Ust'-Chem-2 trench could also be formed during this warming, but at this stage of the research, there is no data confirming this. In the other studied sections of the Cis-Salair, located within the Novosibirsk Ob region, the Bagan and the Eltsovka loesses (both MIS 2) lie on one another without obvious signs of soil formation between them. Traces of pale-osols formed in the Late Glacial period are also not observed. Possibly, the formation of weakly developed paleosols during periods of short warming during MIS 2-MIS 1 in the study area is associated with more favorable conditions in the foothill regions. At this stage of the study, the lower paleosol from the Ust'-Chem-1 section (from layer 4) does not have a univocal interpretation. According to the obtained 14C dating, its age is 21631 ± 814 cal BP (S0AN-9706). However, given that it has clearly undergone the impact of slope processes, this dating is most likely rejuvenated, considered approximate and requires clarification. Preliminarily, the authors link the formation of this paleosol with the Karga interstadial (MIS 3). Loess deposits of the Elbash river valley formed in the cold intervals of MIS-2 under conditions of a drier paleoclimate, and humus interbeds during short-term Late Glacial warmings observed in different regions of the Northern Hemisphere. A comparison of the established events in the Cis-Salair with global climate records allows us to conclude that in loess deposits in the south of West Siberia, responses to short-period global climatic changes, including centuries ones, can be recorded.