Tatyana S. Klyuvitkina

Hydrological changes in the Laptev Sea during the Holocene inferred from the studies of aquatic palynomorphs

Based on the study of aquatic palynomorph assemblages in two Holocene sediment cores obtained from the eastern Laptev Sea shelf, the main stages in the postglacial transgression are reconstructed for the last 11.3 ky. During that period, the inner shelf of the Laptev Sea (sea depth of 51 m) was already flooded representing an area of intense freshwater sedimentation in the immediate proximity from the river mouth in the period of 11.3 to 10.3 ky B.P. Approximately 1.0–1.5 ky later, the inner shelf (sea depth of 32 m) was flooded, although it remained under the influence of the river runoff up to 7.4 ky B.P. The period of 10.3–7.4 ky B.P. was marked by the dominant dinoflagellate cysts Operculodinium centrocarpum among the aquatic palynomorphs, the appearance of more thermophilic dinoflagellate species, and elevated values of the AH ratio, which indicates an enhanced influx of relatively warm North Atlantic waters to the Laptev Sea shelf. The environment close to the present-day one became dominant on the outer and inner shelf of the Laptev Sea approximately 8.6 and 7.4 ky B.P., respectively.

Environmental evolution of the southern Chukchi Sea in the Holocene

The molluscan shells from three drill and two piston cores obtained in the southern Russian sector of the Chukchi Sea are dated by the AMS 14C measurement method back to 0.8–3.5 and 9.2–10.5 ka. The period of 9–10 ka was marked by increased sedimentation rates related to the transgression onset. The fossils in the lower Holocene section exhibit the successive upward replacement of brackish-water organisms by their marine counterparts. After the opening of the Bering Strait in the middle Holocene, the sedimentation was under influence of the increased bioproductivity of the waters. The climatic optimum in the Chukotka region corresponds to the early Holocene, while the late Holocene was characterized by the wider development of the ice cover on the shelf.

Changes in the Lena River runoff during the Holocene

The spatial and temporal changes in the Lena River runoff over the last 9 thousand years are reconstructed through studying the freshwater microfossils in sediment cores obtained from the Laptev Sea inner shelf immediately adjacent to the Lena delta and subject to the freshening effect of river water inflowing the sea through the main arms of the delta (the Trofimovskaya, Bykovskaya, and Tumatskaya arms), the sediments having been thoroughly AMS 14C dated. The freshwater species of diatoms (predominantly the river ones) and green algae that enter the shelf with river water served as indicators of river runoff. The reconstruction of paleosalinity of the sea surface water in the regions under study is based on the relationships (established earlier) between the distribution of freshwater diatoms in the surface layers of sediments in the Arctic seas and the gradients of water salinity in summer. Data on variations in the composition of aquatic microfossil associations in sediments and the reconstructed paleosalinity in the regions of the eastern and western paleovalleys of the Lena River are used to determine the main paleohydrologic events that controlled the variations in the Lena runoff into the shelf zone of the Laptev Sea during the Holocene.