ИСТИНА

It was supposed that the contamination with modern 14C is the main factor for obtaining invalid 14C ages. However, while this is correct for an open system, the array of syncryogenic permafrost sediments is not a true open system. Accumulation and simultaneous freezing of the sediments isolates the permafrost deposits surely. We suppose that contamination with old organic material in permafrost is of importance in aging of the 14C dates. The main problem of radiocarbon dating within permafrost is the uncertain reliability of the 14C ages. Syngenetic sediments contain allochthonous organic deposit that originated at a distance from its present position. To establish ice-wedge formation ages the strategy for the most authentic radiocarbon age selection for syngenetic sediments is considered on the base of a model of yedoma accumulation and distribution of reworked material related to the flood and aeolian transport. The re-deposition of organic material discussed in terms of cyclic syngenetic sedimentation of yedoma. Microcycles are associated with the seasonal periodicity of changes in the depth of an active layer and the accumulation of thin sediment layers. The duration of microcycles is estimated from several years to hundreds of years. The vertical scale of microcycles is several centimeters or tens of centimeters. Mesocycles are conditioned by the pulsing change of the water level of a reservoir, on the coast or shallows of which ice wedges are being formed. The duration of mesocycles is usually estimated from tens of hundreds to several thousand years. The vertical scale of mesocycles is several meters. For 14C dating of ice, wedge complexes it is important to take into account the mesocycles due to the essential difference of the organic material re-deposition at the subaerial and subaqueous stages. For syngenetic ice wedges two stages can be distinguished mainly growth of ice (the subaerial stage), and mainly accumulation of sediments (the subaqueous stage). The growth of syngenetic ice wedges proceeds subaerially during the accumulation of peat or peaty sediments (Vasil’chuk, 2013). Periodically, when gravel, sand, sandy loam, loam, silt, and clay are deposited under subaqueous conditions, ice wedge growth decreases or stops. In real situation in permafrost during syngenetic accumulation at the subaqueous stage , the participation of old organic material may consist of 90-95%. There are many examples of age reversal from peat from subaerial stage that is known to be autochthonous without any signs of re-deposition (Payette et al., 1986). Ancient methane bubbled from the bottom of thermokarst lakes, as shown by Zimov et al. (1997) and Walter et al. (2006) in the permafrost area. Therefore, methanotrophic bacteria, which provide Sphagnum mosses with carbon (Kip et al., 2010), could use ancient methane together with modern. Ancient soil carbon in permafrost soils may be metabolized upon thawing also. The radiocarbon ages of heterotrophically respired carbon ranged from less than 50 yr to 235 yr BP in July mineral soil samples and from 1,525 to 8,300 yr BP in August samples (Nowinski et al., 2010). As syngenetic ice wedge is a closed system, microbial activity is excluded. The dating of organic microinclusions from ice wedges allows us to obtain the age of the ice wedge directly. However, results of the AMS 14C dating of organic inclusions in ice wedges have demonstrated that the problem of an inhomogeneous concentrate also occurs. The comparison of the 14C ages of different fractions from the samples of organic material in the syngenetic ice wedges of a 24-meter terrace near the village of Seyaha demonstrates that the ages of the organic micro-inclusions (more than 200 µm) are the youngest. The concentrations of tritium in the ice were measured in order to evaluate the possibility of modern water participation in the ice wedge. It was shown that modern water did not penetrate into the ice. Micro-inclusions at a depth of 1.8 m are dated as 14,550 yr, and at a depth of 12 m as 14,720 yr BP. The ages of alkaline extracts from the same samples are respectively 19,920 yrs and 23,620 yr. Thus, the differences of 5 kyr and 9 kyr between the ages of the micro-inclusions and alkali extracts may be explained only by a very intensive process of ice wedge accumulation over about 14–15 kyr BP. The advantages and the complications of dating of ice wedges from ice wedges by the accelerator mass spectrometry (AMS) method are discussed applying to true age of dated material search. Radiocarbon ages of different organic materials from the same samples are compared, it is demonstrated that the difference between ages of the fractions from the ice wedges consist of about 9 ka in Seyaha ice-wedge complex in Yamal Peninsula and about 5 ka in Bison yedoma, Kolyma River valley. The principle of the choice of the youngest 14C age from the set and from the layer is proposed for yedoma (Figure 1). Approaches for the choice strategy are,: a) meso- and macro-cyclic model of thick syngenetic ice wedge formation (Yu.Vasil’chuk, 2006, 2013) taking into account; b) modern re-deposition of organic material at subaqueous syngenetic conditions used as pattern for the past syngenetic accumulation of yedoma deposits; c) possible re-working of organic material at syngenetic subaerial or subaerial accumulation, d) comparison number of AMS 14С ages of organic micro-inclusions, alkali extract and pollen concentrate in the ice wedges; e) comparison of the 14C ages from various materials from the same samples both in the ice wedges and in the host sediments. The younger age of the yedoma from cross-sections of Duvanny Yar in Kolyma River valley (35–37 ka BP to 13–10 ka BP), Bison in Kolyma River valley (33.5 ka BP to 26 ka BP), Zelyony Mys in Kolyma River mouth (37 ka BP to 13.6 ka BP), Mamontova Gora in Aldan River valley (19 ka BP to 17 ka BP) etc. is substantiated due to the principle of the choice of the youngest 14C from the set (Vasil'chuk & Vasil'chuk, 1997, 1998, 2014). The ice wedges are considered as key subjects for 14C dating of yedoma, as there are no any exchange processes between the environment and the ice wedges. References Vasil'chuk, Y.K. & Vasil'chuk, A.C. 1997. Radiocarbon dating and oxygen isotope variations in Late Pleistocene syngenetic ice-wedges, northern Siberia. Permafrost and Periglacial Processes, 8(3): 335–345. Vasil'chuk, Y.K. & Vasil'chuk, A.C. 1998. 14С and 18O in Siberian syngenetic ice wedge complexes. Radiocarbon, 40(2): 883–893. Vasil'chuk, Y. K. & Vasil'chuk, A. C. 2014. Strategy of valid 14C dates choice in syngenetic permafrost. The Cryosphere Discuss., 8: 5589-5621.