ИСТИНА

Syngenetic and epigenetic ice wedges, their isotopic composition, radiocarbon age and pollen spectra of the surrounding deposits have been analyzed during long-term study of the “Belenky” key site on the first (6–8 m high) terrace of Chara River (720 m a.s.l.) in Northern Transbaikalia. It has been revealed that the ice wedges are of Holocene age and were formed between 10 to 7.5 ka 14C BP (Figure 1). This period is identified as the first half of the Holocene optimum for the south of East Siberia. The isotopic composition of ice wedges, segregated ice, blister ice, precipitations, river and lake waters has been specified. The isotopic composition (δ18О, δD) of relict ice wedges is the lightest and amounts to –23‰ and –185‰, respectively. The isotopic composition of structured ice from the sandy loam above the ice wedges is –15.7‰ and –133‰; and of small ice wedges in peat and sand is –15.3‰ and –117.9‰, respectively. Syngenetic ice wedges up to 7 m in height were actively formed during the Holocene (dated 10–7.5 ka BP) in sands interbedded with pebbles and gravel of the Chara River valley. The mean value of δ18O is –23‰ and of δD is –180‰. Comparison of the isotopic composition of Holocene and modern ice wedges showed that the δ18O values of the Holocene were more negative by 1–3‰. Conversion of isotopic data to palaeotemperature showed that the average winter temperature of the cold stages of the Holocene optimum (10–7.5 ka BP) could be colder than today by 2–3°C and for the average January by 3–4°C. The sum of winter temperatures could drop to –5100 –5700 degree×days, i.e., it was even lower than the extremely cold modern winters by 300–600 degree×days. At warmer stages of 10–7.5 ka BP, winter temperatures were close to modern ones. The three pollen phases can be divided by analyzing of pollen spectra. The first one corresponds to the 2-meters of sand pack at the bottom. Conditions during the period of this horizon accumulation were the most favorable and characterized by relatively high temperatures and humidity. Likely larch forests were replaced with of pine and birch forests with spruce, larch and dwarf birch. According to the method of summer total temperature sums reconstruction on the composition of pollen spectra [A.Vasil’chuk, 2009], the sum of summer positive temperatures during the first period in the initial phase of the Holocene Optimum accounted as 1600–1800 degree х day, i.e. they were slightly higher than today's total temperature (up to 200 degree х day higher than modern) or close to it. The second pollen phase corresponds to a deterioration of vegetation conditions. Landscapes characterized by the maximum development of fens and birch-pine woodlands were formed during the end of the sand layer accumulation. It is possible to talk about decreasing of the total positive temperatures up to 1300 degree х day (i.e. 300 degree х day below modern). Growth of wood pollen during the third pollen phase indicates the subsequent improvement of the conditions of vegetation and development of birch-pine forests with larch and spruce. This is evidenced by the appearance of Ericaceae and forest species of Lycopodiopsida in the spectra. The total positive temperatures were close to the modern and were about 1550–1600 degree х day. The composition of pollen spectra at depths of 4.3 m, 2.7 m and 1.5 m fix the wide development of polygonal landscapes and possible growth of ice wedges (mostly in width). Indicators of the polygonal tundra development are the domination of Sphagnum and Bryales in these spectra with Betula sect. Nanae, Caryophyllaceae, Artemisia.