ИСТИНА

The ice complex (IC) is a unique feature. Despite series of investigations of ice complex conducted in Yakutia, Chukotka and Alaska there is no unified theory of IC genesis is developed up to date. This is mainly attributed to variety in poly-mineral fractions of ice complex and differences in cryogenic features. Yenisey gulf coasts are underlined by permafrost. Upper 10-15 m of geologic profile of the coasts of Yenisey and Yenisey gulf is composed very icy Pleistocene – Holocene continental sediments of polygenetic origin with abundance of syngenetic ice-wedges (SIW). Sediments of Yenisey gulf coasts that can be classified as IC with some unique features and SIW were under investigation in 2004 – 2010 years. Sediments are characterized by high organic content (0.6 - 1.2 percent), mainly silt particles (up to 80 percent). Belt-shaped layered cryostructure and high soil moisture (more 60 – 80 percent) point on syngenetic conditions of sediment freezing. Ice complexes are found at interfluves, slopes and old thermokarst drained basins and form the second alluvial terrace of Yenisey River at elevations around 10-15 m a.s.l. The formation of the terrace finished 10 000 y. a. (Gusev et al. 2011). IC of the second fluvial terrace of Yenisey River had formed at the same time due to regression of Polar Basin more than 300 km north. These conditions occurred in Late Weichselian period (MIS2) around 18 kBP. In water samples taken from melt wedge ice, oxygen (δ18O) and deuterium (δD) content and the ice chemical composition were determined. In the Dikson area, the average content of stable isotopes in the SIW of the IC lower layer is -26.0‰ for δ18O and -198.4‰ for δD. Severe winters of the time of the lower layer SIW formation can be reconstructed according to ice isotopic composition. The predominance of HCO3 - and Ca++ in ice chemical composition is also typical for them. In the Dikson area, SIW of the lower layer formed under the last cryochron (MIS2) conditions. A calcium hydrocarbonate composition of SIW indicates that winter sediments formed over the land which occupied modern shelf after the sea regression during the last cryochron up to the isobath of 120 m. The same indicators of isotopic and chemical composition characterize SIW (up to 10 m thick) in the deposits of alluvial terrace (10-15 m high) of the Yenisei River in the area of Sopochnaya Karga (Streletskaya and Vasilev 2009). We mapped Late Pleistocene – Holocene SIW in Kara Sea cost, islands and shelf using the Atlas of paleographic maps (1991) and R. Stein et al (2002) with additions. Ages of about 3.4-4.6 ka BP were determined for peat layer and wood fragments in ice-rich deposits surrounding SIW of the IC upper layer near Dikson. The average content of stable isotopes in the SIW is - 20.7‰ for δ18O and -154.4‰ for δD. The predominance of CL- among the anions and Na+ among the cations in SIW chemical composition indicates an increasing role of ocean in precipitation composition in the Holocene. The layer comprising ice-wedges is underlain by an autochtonous peat layer 1.0-1.5 m thick containing wood fragments with ages 7.3 ka BP and 8.0 ka BP, that corresponds to the start or early part of the Holocene climatic optimum and beginning termokarst in Arctic. During the Holocene, SIW grew while thermokarst depressions, which formed during the Holocene climatic optimum, were being filled in with silty sediments. The reworked material of the pre-Holocene IC deposits predominates in the sections. Compared to the Late Pleistocene sediments, the Upper Holocene sediments of the IC contain more organic matter. The Western Taymyr IC formed in two stages: in the Late Pleistocene (MIS2) and in the Holocene cold periods. The section structure and the difference in SIW chemical and isotopic composition point at that. A lighter composition of oxygen isotopes (-26-25‰) and the predominance of calcium and hydrocarbonate ions are typical for the Late Pleistocene SIW, while a heavier oxygen isotopic composition (-19-21 ‰) and the predominance of sodium and chloride ions are typical for the Holocene SIW. The Western Taymyr coast and part of Kara Sea shelf are the westernmost area of distribution of the last cryochron IC deposits. In contrast to the Yakutia IC, the Western Taymyr IC is less thick (up to 12 m), includes smaller syngenetic ice wedges (SIW) and formed in the Late Pleistocene/Holocene. The ice complex is contemporaneous to deposits of the Last Glaciation of Taymyr and hence is a good indicator of limited distribution of LGM ice sheet in the region.