Was there a marine transgression on the Laptev Sea Shelf during the last interglacial? Evidence of continental climate in the Siberian Arctic by terrestrial plant macrofossils

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Kienast, F. , Schirrmeister, L. , Grosse, G. , Tarasov, P. , Siegert, C. and Hubberten, H. W. (2006): Was there a marine transgression on the Laptev Sea Shelf during the last interglacial? Evidence of continental climate in the Siberian Arctic by terrestrial plant macrofossils , 36th International Arctic Workshop, 15-18 March 2006, INSTAAR, University of Colorado, Boulder, Colorado, US. .
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Terrestrial plant macrofossils from permafrost deposits at the Bol'shoy Lyakhovsky Island, New Siberian Archipelago, Arctic Siberia, revealed the existence of subarctic forest tundra with dominating Alnus fruticosa, Betula fruticosa, B. nana, and Ledum palustre during the last interglacial (Fig. 1). The reconstructed palaeovegetation resembled today's southern subarctic shrub tundra near the tree line occurring about 350 km SW of the study site. This low shrub tundra was however interspersed with arctic/alpine upland communities and grasslands in a more open landscape and thus differing from modern tundra in this region. Accordingly, the climate was characterised by increased evaporation, which resulted in the local persistence of steppe and meadow vegetation, the occurrence of halophytic plants reflecting salt accumulation, and littoral pioneer species, indicating fluctuating lake levels. The massive presence of southern, boreal, today extralimital plants, especially among aquatics, indicates an extended growing season and summer temperatures during the last interglacial of at least 12 °C, thus 8 °C higher than today. Such an increase in temperature in contrast to the current warm stage in arctic Siberia is not explainable by the global warming during the Eemian Interglacial alone but must be superposed by regional effects. All the climatic implications point to more continental conditions and consequently to a lower oceanic influence. Accordingly, the interglacial marine transgression possibly advanced during the Eemian not as far as it did during the Holocene.Considering an Eemian global sea level exceeding the current one by 5.5 to 6 m (Dumas et al., 2006), there are only two possible causes of that difference: a tectonic shelf subsidence or a subsidence due to thermoerosion of the ice-rich permafrost both occurring after the last interglacial(Eemian). Thermoerosion took place during both interglacials the current and the last one (Andreev et al., 2004) and thus it fails to be the only reason for an inundation happening no earlier than during the early Holocene about 7500 years BP (Bauch et al., 2001; Romanovskij et al., 2000). As continuation of the Gakkel Ridge, the Laptev Shelf region undergoes an intense neotectonic spreading, which involves a quick subsidence (Drachev et al., 1998; Nikolaev et al., 1985). The subsidence of the extremely shallow shelf region (less than 50 m water depth over hundreds of km offshore) resulted obviously for the first time during the Holocene in such a far reaching coastline shift. The shift of the main runoff direction of the Lena River in the Delta from W to NE and E within the Late Quaternary is evidence of the high speed of tectonic shelf subsidence (Schwamborn et al., 2002).The Holocene marine transgression, up to 600 km to the south, had a decisive impact on the climate of the whole region, which was the most important refugium of tundra steppe and the associated mammal fauna during interglacials (Boeskorov, 2006; Sher, 1997; Yurtsev, 1982). Due to the maritime influence reaching then far inland, cloudiness and precipitation rose, resulting in decreased summer temperatures and soaked soils and causing the displacement of steppes and meadows by tundra wetlands.

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