ALBERT

Description: Seawater-derived neodymium (Nd) isotope ratios of sediment coatings have increasingly been used in paleoceanographic reconstructions to derive information on the past variability of deepwater sources and mixing. Dissolved Nd in seawater originates from weathering processes of the continental crust. It is delivered to the oceans either through boundary exchange processes or in dissolved or particulate form through riverine input (Frank, 2002). The use of radiogenic Nd isotopes as oceanic tracers is enabled by the average ocean residence time of Nd, which is similar to the global mixing time of oceans (ca 400 – 2,000 years) and by their independence of stable isotope fractionation by biological or physical processes (van de Flierdt et al., 2006). In this study, we investigate Nd isotope ratios of both core-top and Holocene sediment samples (leachates and detritus) and compare them to modern seawater Nd isotope signatures in the eastern Fram Strait, as well as to a multitude of proxy indicators for the climatic and oceanographic variability in the eastern Fram Strait during the past 8,500 years. The West Spitsbergen Current (WSC) represents the major means of northward heat transport to the Arctic Ocean via eastern Fram Strait. While the upper layers of the WSC are fed by warm and saline Atlantic Water derived from the North Atlantic Current, deepwater inflow through eastern Fram Strait is still subject of debate. Our study is primarily intended to reconstruct Holocene variability of bottom water sources in the Arctic Gateway. However, relatively radiogenic Nd isotope ratios of the leached fraction since the late Holocene suggest a contribution of fine lithic grains transported by sediment-laden sea ice from the East Siberian shelves to the Fram Strait (Dethleff and Kuhlmann, 2009). This is in contrast to the less radiogenic Nd isotope ratios of present-day intermediate and deep water in the area documenting an Atlantic source. We therefore suggest an alternative explanation, which is input and transport of sediment material with preformed sediment coatings from source areas on the Siberian shelf to the Central Arctic and the eastern Fram Strait. The eastern Fram Strait has been the bottleneck of Arctic sea ice export and main ablation area for ice-transported (fine-grained) material since approximately 5 cal ka BP. References Dethleff, D. and Kuhlmann, G., 2009. Entrainment and export of fine-grained surface deposits into new ice in the southwestern Kara Sea, Siberian Arctic. Continental Shelf Research, 29, 691-701. Frank, M., 2002. Radiogenic Isotopes: Tracers of Past Ocean Circulation and Erosional Input. Reviews of Geophysics, 40, 1001, 10.1029/2000RG000094. Van de Flierdt, T., Robinson, L.F., Adkins, J.F. and Hemming, S.R., 2006. Temporal stability of the neodymium isotope signature of the Holocene to glacial North Atlantic. Paleoceanography, 21, PA4102

Description: Northward advection of relatively warm and saline Atlantic Water masses keeps the eastern Fram Strait ice-free all year while the western part - occupied by cold and fresh Arctic water - is perennially ice-covered. The Fram Strait, often referred to as the Arctic Gateway, is the only deep-water passage for Atlantic-derived water masses to enter the Arctic Ocean. It therefore plays a crucial role for the heat budget of the Arctic Ocean. Two high resolution sediment sequences from the Western Svalbard margin covering the last ca. 10.000 years have been studied in order to derive information on Holocene variability of the heat transport to the Arctic Ocean. Planktic foraminifer fauna imply a stepwise transition from deglacial/Early Holocene to modern-like conditions in the eastern Fram. Repeated short-term advances of the sea ice margin are superimposed on the generally strong heat transport to the Arctic Ocean during the Early to Mid-Holocene until ca 5 cal ka BP. Strong fluctuations in stable isotope data of the subpolar planktic foraminifer species Turborotalita quinqueloba infer strong variability of summer sea surface conditions probably caused by variable extent of the upper mixed layer and the sea ice margin. The Late Holocene Neoglacial phase was characterized by the dominance of the cold water-indicating planktic foraminifer species Neogloboquadrina pachyderma. Consistent with the decreasing solar insolation, cooler (sub-)surface conditions established after ca 5 cal ka BP most likely related to a weakening of the Atlantic Water advection and strong export of Arctic sea ice through Fram Strait. During that time, N. pachyderma most likely migrated to the subsurface layer due to enhanced freshwater available at the surface. Cool Late Holocene conditions are reversed by a strong warming event likely caused by a significant strengthening of Atlantic heat advection to the Arctic during the present, anthropogenically influenced period.

Description: 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, 〈9 cal.ka) is distinguished by decreasing total abundance and diversity of microfossils likely due to climate cooling. Growing representation of C. neoteretis indicates strong subsurface ADW inflow and reduced interaction of the open-sea and shelf water masses. References Lubinski, D.J., Polyak, L., Forman, S.L., 2001. Freshwater and Atlantic water inflows to the deep northern Barents and Kara seas since ca 13 14C ka: foraminifera and stable isotope. Quat. Sci. Rev., 20, 1851-1879. Spielhagen, R.F., Erlenkeuser, H., Siegert, C., 2005. History of freshwater runoff across the Laptev Sea (Arctic) during the last deglaciation. Glob. Planet. Change, 48 (1-3), 187-207. Taldenkova E., Bauch H.A., Gottschalk J., Nikolaev S., Rostovtseva Yu., Pogodina I., Ovsepyan Ya., Kandiano E., 2010. History of ice-rafting and water mass evolution at the northern Siberian continental margin (Laptev Sea) during Late Glacial and Holocene times. Quat. Sci. Rev., 29, 27-28, 3919-3935.

Description: The Arctic Ocean has undergone profound changes in the last ca. 50 kyr, reaching from a dense sea ice cover with large numbers of icebergs during the mid-Weichselian glaciation (MWG, 〉45 ka) to a perennial sea ice cover with seasonally open leads in the Holocene. During the main glacial phases (MWG and last glacial maximum (ca. 20 ka)), large parts of the surrounding continents were covered by ice sheets which discharged icebergs to the ocean, leaving traces in the form of ice-rafted debris (IRD) in the bottom sediments. Different lithologies in the source areas of the icebergs allow to reconstruct the pathways of the ice and thus the large-scale drift pattern of the oceanic ice cover. Microfossils and geochemical proxies give evidence of other parameters of the surface-near water masses and their spatial and temporal variability. We use a multiproxy data set from sediment cores obtained between the Alpha (130-160°W) and Gakkel (30-60°E) ridges to reconstruct the paleoenvironment in the central Arctic with emphasis on the intervals with extensive continental glaciations. Sedimentation rates were generally low (1 cm/kyr or less) with the exception of the MWG with several cm/kyr. Coarse fraction content (IRD and microfossils) in sediments from both glaciation intervals is increasing towards the Alpha Ridge, revealing a stronger influence (iceberg discharge) of the North American Arctic ice sheet if compared to the northern Eurasian ice sheet. Planktic foraminifer occurrences in Alpha Ridge sediments from the MWG indicate that seasonally open waters were present occasionally and may have allowed higher melt rates than in the Eurasian subbasin. The paleoenvironmantal picture for the LGM is more ambiguous because of extremely low sedimentation rates or even an interval of non-sedimentation. However, it seems likely that the eastern part of the Eurasian Basin was largely free of icebergs for a few thousand years during the LGM. The different dominating lithologies of IRD found in the analyzed sediment cores point to different regional sources of icebergs for the last 50 kyr and allow to trace the boundary between the ice drift systems in the Amerasian and Eurasian subbasins through time. The MWG sediments at all sites show a minimum in carbonate/dolomite contents, indicating a more southward position of the Beaufort Gyre than today. Increasing proportions of carbonate/dolomite thereafter indicate a continuous iceberg export from the North American Arctic in the last 50 ky.