Fossil assemblages of molluscs, ostracods and foraminifers have been studied in AMS¹⁴C dated cores from the Laptev Sea continental margin obtained during TRANSDRIFT V expedition in 1998 (Bauch et al., 2001). A sediment core from the western upper continental slope recovered at 270 m water depth dates back to >15.8 cal. kyr (extrapolated age of core base 17.6 cal. kyr) and represents so far the longest age-controlled record of postglacial to Holocene events in the region. Three cores from the outer-central shelf originate from the submarine river paleovalleys of the Khatanga, Lena and Yana and at their bases dated to 12.7, 11.2, and 11.3 cal. kyr, respectively.

On the upper continental slope, high relative proportions of benthic foraminifer species Cassidulina neoteretis, planktic foraminifers and relatively deep-living ostracods with North Atlantic affinities provide evidence on the past inflows of Atlantic-derived waters. Freshwater inputs, downslope sediment movements and ice-rafting are documented by the presence of euryhaline, brackishwater and freshwater ostracods, river-proximal benthic foraminifers and by the low planktic/benthic ratio. Atlantic-derived waters reached the studied site already 15.8 cal. kyr, i.e prior to the establishment of the pathway through the Barents Sea shelf in the west (Lubinski et al., 2001). Cold, low-nutrient marine environments with recurrent coastal open-water polynya and Atlantic-derived waters in the subsurface water layer are reconstructed for the period 15.8 to 14.0 cal. kyr. Gradually increasing freshwater influence during the subsequent period reached its maximum between 12.7 and 11.2 cal. kyr, when the outer shelf was flooded. Taxonomic composition and abundance/preservation pattern of microfossils from sediment layers corresponding in age to the early and mid-Holocene give evidence for considerably reduced Atlantic-derived water inflow, decreasing freshwater influence, separation of shelf and outer water masses due to the inland coastline retreat, seasonal ice-free conditions and high bioproductivity.

During the earliest stage of the shelf flooding, the pioneer brackish water assemblages of bivalves and ostracods inhabiting the estuarine parts of river paleovalleys were subject to periodical advection of saline offshore waters, as indicated by the presence of deep-living ostracods and planktic foraminifers. Following the pattern of sea-level rise, these strongly fluvially-affected assemblages rapidly transformed into shallow-water marine ones (Taldenkova et al, 2005).

After 3.5-3 cal. kyr well-pronounced changes in the composition of benthic assemblages from both, the outer shelf and upper continental slope, indicate climate cooling combined with the intensification of surface and bottom water circulation. On the outer shelf, this is manifested by the increase in the number of euryhaline ostracods ice-rafted from the inner-shelf regions, and re-introduction of deep-living species due to the advection of Atlantic-derived water with reversal bottom currents. On the upper continental slope, peak values of planktic foraminifers, Cassidulina neoteretis, euryhaline ostracods, river-proximal foraminifers, and ice-rafted debris, all point to the increase in subsurface Atlantic-derived waters inflow, climate cooling and intensification of the wind-driven surface water circulation.

Reference:

Taldenkova E.E., Bauch H.A., Stepanova A.Yu., Ovsepyan Ya.S., Pogodina I.A. Detailed reconstruction of postglacial and Holocene events in the Laptev Sea. Correlation of Pleistocene Events in the Russian North. International Workshop Abstracts. 4-6 December 2006. Saint-Petersburg, 2006, p. 102.