Postglacial environmental changes at the eastern Laptev Sea continental margin: evidence from benthic and planktic foraminiferal assemblages

In order to reconstruct the late glacial to Holocene history of Atlantic-derived water (ADW) inflow into the Arctic Ocean along the northern Eurasian margin, past freshwater inputs and variability in sea-ice extent we carry out investigation of the distribution and abundance of benthic and planktic foraminifers in the AMS14C-dated sediment core PS2458-4 from the eastern Laptev Sea continental slope (78°10.0’N, 133°23.9’E) located at 983 m water depth in front of the position of the former Lena and Yana river mouths on the shelf break (Spielhagen et al., 2005). The 822 cm long sediment sequence was sampled continuously in 2 cm thick slices. The core is well-dated from 201 cm down to 625 cm, this sediment interval corresponds in time to 8.9-14.6 cal.ka (Spielhagen et al., 2005), whereas the basal and uppermost parts of the core lack sufficient amount of datable material. Based on preliminary data on taxonomic composition and abundance of benthic and planktic foraminifers several intervals were distinguished corresponding to certain changes in past environments. The oldest lateglacial period (822-660 cm) is characterized by very low abundance of microfossils suggesting harsh environmental conditions with heavy sea-ice cover. However, high percentages of ADW-indicative benthic foraminifer Cassidulina neoteretis and the presence of diverse planktic subpolar species give evidence for the strong subsurface inflow of ADW along the Laptev Sea continental slope. The increasing representation of Elphidium clavatum, higher foraminiferal abundance and species diversity imply growing fluvial influence during early deglaciation (660-600 cm, until 14.2 cal.ka). The abundance of subpolar planktic foraminifers is the highest although their diversity considerably decreases compared with the earlier lateglacial times. Enhanced ADW inflow to the core site is manifested by the sharp dominance of C. neoteretis in the record during the Bølling-Allerød period (600-520 cm, 14.2-12.9 cal.ka). Transition to the Younger Dryas period (520-504 cm, ~12.9-12.6 cal.ka) is marked by a drastic decrease in the total abundance of all microfossils coincident with the previously recorded sharp negative excursion in the oxygen isotope composition of planktic foraminifers implying surface water freshening (Spielhagen et al., 2005). This layer appeared to be also enriched in authigenic vivianite concretions indicating anaerobic conditions at the sea floor. Previously, vivianite accumulations were recorded in lateglacial sediments from the western Laptev Sea and were provisionally related to the meltwater-induced stratification of the water column (Taldenkova et al., 2010). Generally, the Younger Dryas period in the studied core (520-380 cm, 12.9-10.8 cal.ka) is characterized by the dominance of typical arctic species (Cassidulina reniforme, E.clavatum, Islandiella helenae/norcrossi) and decreasing foraminiferal abundance. The core site was probably located close to the summer sea-ice marginal zone with increased seasonal productivity as evidenced by the appearance of Nonion labradoricum. The Early Holocene (380-215 cm, 10.8-9 cal. ka) stands out as a period of sharply increasing abundance and diversity of benthic foraminifers together with other benthic fossil groups (ostracods, bivalves, gastropods). However, the abundance of planktic foraminifers and representation of C. neoteretis decrease. Altogether this might be a result of surface water warming, diminished sea-ice cover, strong mixing of open-sea waters with freshened water masses formed on the flooded outer shelf. The latter conditions were unfavorable for planktic foraminifers and C. neoteretis (Lubinski et al., 2001). Mid-Late Holocene time interval (~215-35 cm,