On the basis of a detailed study of the diatom and aquatic palynomorph assemblages and a detailed radiocarbon chronology of sediment cores obtained from the south-eastern inner Laptev Sea shelf adjacent to the Lena Delta the spatial and temporal variability in the Lena River discharge during the last 6 cal. ka were reconstructed. It was shown that in the area adjacent to the Lena Delta variations in surface water salinities, reconstructed using freshwater diatoms as a proxy, were mainly caused by changes in the volume of the Lena River runoff through the major riverine channels Trofimovskaya, Bykovskaya and Tumatskaya. Several paleohydrological phases are recognized:
(i) establishment of modern-like conditions within the eastern Lena River Valley occurred from 6-4.2 cal. ka;
(ii) Lena River outflow increased in north-eastward direction via the Trofimovskaya or Bykovskaya channels from 4.2-2.7 cal. ka, coeval with a reduction of runoff toward the north via the Tumatskaya Channel;
(iii) generally stable hydrological conditions northward of the Tumatskaya Channel and variations in riverine discharge north-eastward of the Trofimovskaya and Bykovskaya channels prevail since 2.7 cal. ka.
Using indicator species of dinocysts as principle marine proxy, an influence of Atlantic water to the southeast inner Laptev Sea shelf could be inferred, possibly along the Eastern Lena paleovalley, brought into this area by winddriven reversed bottom currents.
Zusammenfassung:
Untersuchungen von Diatomeen- und aquatischen Palynomorphen-Vergesellschaftungen wurden an Radiokohlenstoff datierten Sedimentkernen aus der inneren sudostlichen Laptevsee nahe des Lenadeltas durchgefuhrt. Anhand dieser Daten wurde die zeitliche und raumliche Veranderlichkeit des Flusswasserausstromes der Lena fur die letzten 6 ka (Kalenderjahre) rekonstruiert. Zeitliche Veranderungen in den Haufigkeiten von Suswasserdiatomeen sind begrundet durch Wechsel in der Menge des Lenaausflusses durch die drei grosen Haupkanale im Delta: Trofimovskaja, Bykovskja, sowie Tumatskaja. Darauf basierend konnten prinzipiell drei grose palaohydrologische Phasen unterschieden werden:
(1) heutigen Verhaltnissen vergleichbare Bedingungen wurden ostlich des Lenadeltas zwischen 6 und 4.2 ka etabliert;
(2) Erhohung des Flusswasseraustrags in nordostliche Richtung uber die Kanale Trofimovskaja und/oder Bykovskaja zwischen 4.2 und 2.7 ka bei gleichzeitiger Reduzierung Richtung Norden via Tumatskaja;
(3) Ausbildung relativ stabiler Bedingungen nordlich des Deltas sowie Auftreten wechselhafter Flusswasseraustragen in ostliche Richtung nach 2.7 ka.
Das Auftreten von marinen Dinocysten in den Sedimenkernen belegt den Einfluss von Wassermassen mit vermutlich atlantischem Ursprung. Es ist zu vermuten, dass diese durch windgetriebene Bodenstromungen entlang der alten versunkenen Flusstaler auf den ostlichen inneren Schelf verfrachtet werden.
Climatic events of the last millennia in the northeastern part of the Norwegian-Greenland basin were reconstructed based on micropaleontological studies of sediments from the AMK-6150 core. New results were obtained from analyses of dinoflagellate cysts, diatoms, and benthic and planktonic foraminifers. These results allow us to infer the time of sediment accumulation and the prevailing natural conditions. According to preliminary data on climatostratigraphy based on the composition of microfossils, the period of sediment formation in the AMK-6150 core does not exceed 7 thousand years. The composition of microfossil assemblages and the results of reconstructions indicate repeated changes in marine environmental conditions during this time. Sediments at the depth of 23-24 cm recorded the beginning of a significant growth of temperature and salinity of surface waters due to a possible increase of the Norwegian Current influence. According to the species and quantitative composition of dinocyst and diatom assemblages, sediments in the short depth range of 14-12 cm accumulated during a period of noticeable decrease in temperature and increased influence of Arctic water masses. The analysis of dinocysts by the method of modern analogues allows reconstruction of the quantitative values of summer paleotemperatures of surface waters and the duration of ice cover. It was found that the cooling episodes and probable formation of seasonal sea ice were possible during the accumulation of sediments at the depths of 29-24, 14-12 and 3-1 cm.
The Laptev Sea constitutes the central part of the wide Siberian shelf and is regarded as a key area for the freshwater and sea-ice balances of the Arctic Ocean. More than a quarter of the total continental freshwater runoff to the Arctic Ocean is discharged into the Laptev Sea, mainly via the Lena River (Gordeev, 2000). Furthermore, the Laptev Sea polynya is a major source area for sea ice, which is transported to the Siberian branch of the Transpolar Drift (e.g., Zakharov, 1996). Therefore, knowledge of the Holocene variability of the Laptev Sea hydrology is essential for understanding Arctic Ocean water circulation in the past.
Our paleoenvironmental reconstructions based on a detailed study on diatom and aquatic palynomorph assemblages and a detailed radiocarbon (AMS 14 C) chronology of sediment cores obtained from the south-eastern Laptev Sea adjacent to the Lena River Delta. Established linkages between hydrographical parameters (e.g., summer surface-water salinity, sea-ice conditions, Polyakova, 2003; Klyuvitkina, Bauch, 2006) and the composition of surface-sediment diatom and aquatic palynomorph assemblages were used to reconstruct environmental conditions under postglacial sea-level rise and climatic changes. Our records give evidences for inundation of the outer Laptev Sea shelf (> 51 m water depth) approximately 11.3 cal. ka. River-proximal environment characterized by avalanche-like precipitation of river-loaded matter (“marginal filter”) under water salinity <9. The time interval 10.7-9.2 cal. ka was marked by enhanced influence of Atlantic Water on the Laptev Sea hydrology. Because a continuously rising sea level resulted in the southward retreating coastline, surface-water salinities on the outer shelf approached modern values of about 15-16 around 8.6 cal. ka. Approximately 8.9-8.5 cal. ka, when the sea-level reached the position of the present-day isobaths of about 32-30 m, the inner-shelf was flooded. On the inner Laptev Sea shelf, modern-like environmental conditions were reached about 1 to 1.5 ky later, around 7.4 cal. ka. During the last 6 cal.ka in the area adjacent to the Lena Delta variations in surface water salinities, reconstructed using freshwater diatoms as a proxy (Polyakova, 2003) indicate the changes in the volume of the Lena River runoff through the major riverine channels Trofimovskaya, Bykovskaya and Tumatskaya. It was shown, that the Lena River outflow increased in north-eastward direction via Trofimovskaya and Bykovskaya channels 4.2-2.7 cal. ka. A general increasing trend in riverine discharge is observed between 2.7 and 1.2 cal. ka, and a steep decrease in outflow recorded for the last 1.2 cal. ka.
References
Gordeev V.V., 2000. River input of water, sediment, major ions, nutrients and trace metals from Russian territory to the Arctic Ocean. In: E.L.Lewis et al. (eds.) The freshwater budget of the Arctic Ocean, Kluwer, Amsterdam, 297-322.
Klyuvitkina T.S., Bauch H.A., 2006. Holocene changes in paleohydrological conditions in the Laptev Sea based on aquatic palynomorph assemblages. Oceanology, 6, 911-921.
Polyakova, Ye.I., 2003. Diatom assemblages in the surface sediments of the Kara Sea (Siberian Arctic) and their relationship to oceanological conditions. In: Stein, R., Fahl, K, Futterer, D. K., Galimov, E. M., Stepanets, O. V. (Eds.), Siberian River Run-off in the Kara Sea: Characterization, Quantification, Variability, and Environmental Significance, Proceedings in Marine Sciences, Elsevier, Amsterdam, 375-399.
Zakharov, V. F., 1996. Sea Ice in the Climatic system, St.-Petersburg, Gidrometeoizdat, 213 pp. (In Russian).
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