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Isotopic Composition of Massive Ground Ice Exposure at the Kharasavey Coast, West Yamal
N.G. lova & V.I. Solomatin H. Meyer
Faculty of Geography, Lomonosov Moscow State University, Moscow, Russia Alfred Wegener Institute for polar and marine research, Potsdam, Germany 1 SUBJECT OF INVESTIGATION Massive ground ice and icy sediments have been studied near the Kharasavey settlement at the West Yamal coast of Kara Sea from the 1980es. Several points of view on its origin have been suggested, the main are intrasedimental origin of ice, redeposited and deformed by glacier marine sediments or buried fast ice. In order to clarify this question one massive ice exposure were described and sampled in 2008. 2 HOST SEDIMENTS 2.1 Composition, stratigraphy and dates At the Kharasavey Cape area fine grained frozen sediments are exposed in coastal bluffs of 3-14 m height along 10 km coastal section. These sediments are presented mostly by saline clays and clayey sands, which often contain shells. Massive ground ice lies inside the sandy stratum that rises above the sea level at some parts of the coast. In some costal sections sands compose all the bluff. Massive ice tent to be confined at the contact of sandy and clayey strata. The oldest known radiocarbon dates of the sediments are 43600±700, 39700±1000 and 35404±340 (GIN-2649; 2650; Ri-284), than there is a gap up to 9560±130 (GIN-2651, peat in the upper part of the cliff) (Vasil'chuk Yu.K.). 2.2 Position of the studied massive ice bed The studied outcrop was formed in a coastal bluff of 10 m height a.s.l.. Massive ice is exposed at 3 m depth from the surface and overlaid by sandy-clayey stratified sediments. On its sides massive ice is replaced by sands, so we can say the ice bed lies in the upper part of sandy strata in the place where the top of the sands rises above the sea level. The upper contact of massive ice is in general dome-shaped. Upper boundary can't be defined clearly ­ at 2-3 m down the surface ice lenses of 2-20 cm thick subparallel to the top of the ice arise; down the bluff they gradually turn to massive ice body. The lower boundary haven't been exposed. Massive ice bed apparent thickness is 1 m or 1.5-2 m together with ice lenses and icy sediments, it forms thermocirque of 7 m diameter. 3 MASSIVE GROUND ICE 3.1 Description of ice, chemical composition Massive ice body consists of separate ice lenses 5-35 cm thick which are divided by 1-10 cm ground layers. The predominant type of ice is transparent with inclusions of the sediments, bubbly ice is rare. Conductivity of ice is low (15-122 S/cm), which disprove idea of buried fast ice, Cl and Na are predominant ions, chemical composition is in general of the same type with intrasedimental ice veins in adjacent coastal sections. In the center of ice bed the values are higher thanks to prevalence of Ca. 3.2 Isotopic composition 34 samples of massive ice and lenses above it and few samples of intrasedimental ice in clays were analyzed in Isotope Laboratory of AWI. The values of 18O vary from -18.6 to -26.3 (average -21.9) in massive ice and from -15.3 to 20.9 in ice lenses above massive ice. In general isotopic composition is lighter in central part of ice bed and become heavier towards the upper ice lenses. Intrasedimental reticulate ice veins in clays in other coastal sections have, for example, -13.8 to 21.5 18O values, which are close to one in massive ice and lenses above it. But d excess is higher in massive ice and ice lenses (average 8.4 and 6.0) than in intrasedimental ice veins (4.4). 4 CONCLUSIONS Small dimensions of the ice bed, isotopic and chemical composition of ice close to those into intrasedimental ice veins and apparently saline overlaying sediments points to intrasedimental origin of this massive ice bed. References
Vasil'chuk Yu.K. at al. (ed.) 2006. Cryosphere of Kharasavey gas condensate field. St.-Petersburg: Nedra.
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Third European Conference on Permafrost