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Sedimentation history in the Lena Delta, N-Siberia

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Schwamborn, G. , Rachold, V. , Grigoriev, M. N. and Krbetschek, M. (2012): Sedimentation history in the Lena Delta, N-Siberia , Russian-German cooperation in the scientific exploration of northern Eurasia and the adjacent Arctic ocean. Leopoldina Symposium in cooperation with St. Petersburg State University and the Russian Academy of Sciences, St. Petersburg, 10 September 2012 - 12 September 2012 .
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Abstract:

Core and outcrop analysis from Lena mouth deposits are used to reconstruct the Late Quaternary sedimentation history of the Lena Delta. Sediment properties (heavy mineral composition, grain size characteristics, organic carbon content) and age determinations (14C AMS and IR-OSL) are applied to discriminate the main sedimentary units of the three major geomorphic terraces, which form the delta [1]. The development of the terraces is controlled by complex interactions among the following four factors: (1) Channel migration. According to the distribution of 14C and IR-OSL age determinations of Lena mouth sediments, the major river runoff direction shifted from the west during marine isotope stages 5-3 (third terrace deposits) towards the northwest during marine isotope stage 2 and transition to stage 1 (second terrace) [2], to the northeast and east during the Holocene (first terrace deposits). (2) Eustasy. Sea level rise from Last Glacial lowstand to the modern sea level position, reached at 6-5 ka BP, resulted in back-filling and flooding of the palaeovalleys. (3) Neotectonics. The extension of the Arctic Mid-Ocean Ridge into the Laptev Sea shelf acted as a halfgraben, showing dilatation movements with different subsidence rates. From the continent side, differential neotectonics with uplift and transpression in the Siberian coast ridges are active. Both likely have influenced river behaviour by providing sites for preservation, with uplift, in particular, allowing accumulation of deposits in the second terrace in the western sector. The actual delta setting comprises only the eastern sector of the Lena Delta. (4) Peat formation. Polygenetic formation of ice- rich peaty sand (“Ice Complex”) was most extensive (7-11 m in thickness) in the southern part of the delta area between 43 and 14 ka BP (third terrace deposits). In recent times, alluvial peat (5-6 m in thickness) is accumulated on top of the deltaic sequences in the eastern sector (first terrace). The Late Pleistocene and Holocene history of Lake Nikolay on the second terrace is reconstructed using shallow seismic and ground-penetrating radar (GPR) profiles and sedimentary analyses including granulometry, total organic carbon, δ13C, pollen analysis and layer dating [3]. The main objective for studying this lake is directed to the controversy about a glacial or a periglacial origin of the second terrace (Arga Island) and a glacial or periglacial origin of the numerous lakes located there. Determining the age and genesis of Lake Nikolay, which is the largest amongst the lakes, might mirror large parts of the history of Arga Island. Shallow seismic profiles (25 km total 1ength) of basin fills and complementing GPR profiles (23 km total length) of frozen shallow margins and a transect of sediment cores taken from one of Lake Nikolay’s depo-centers to the lake margin provide evidence for lake evolution since Early Holocene time and its existence until modern times. Prior to lake evolution uppermost sediments of the second sandy terrace of the Lena Delta holding Lake Nikolay accumulated at the end of the Late Pleistocene (14.5 to 10.9 ka BP). Sediment properties suggest a fluvia1 environment with riverbed characteristics. After initial wet ground stages lake basin formation in the sandy environment was established at 7 ka BP due to thermokarst. The onset of the thermokarst processes coincides with the regional Holocene climatic optimum according to pollen analyses [4]. Seismic profiles reveal that under deep lake basins closed zones of unfrozen deposits (i.e. taliks) exist [3][5]. This interpretation is confirmed by mathematical modelling of talik expansion. In fact, neither geological nor geophysical results are obtained, which support the hypothesis of a glacially caused morphology of the area as deduced from remote sensing techniques according to some authors. The occurrence of massive underground ice proposed by others cannot be proven until now, either.

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