ALBERT

Beschreibung: Nearly half the inflow of warm and saline Atlantic Water (AW) to the Arctic Ocean is substantially cooled and freshened in the Barents Sea, which is therefore considered a key region for water mass transformation in the Arctic Mediterranean (AM). Numerous studies have focused on this transformation and the increasing influence of AW on Arctic climate and biodiversity, yet geochemical investigations of these processes have been scarce. Using the first comprehensive data set of the distributions of dissolved radiogenic neodymium (Nd) isotopes (expressed as ɛNd), rare earth elements (REE) and stable oxygen isotope (δ18O) compositions from this region we are able to constrain the transport and transformation of AW in the Barents Sea and to investigate which processes change the chemical composition of the water masses beyond what is expected from circulation and mixing.Inflowing AW and Norwegian Coastal Water (NCW) both exhibit distinctly unradiogenic ɛNd signatures of −12.4 and −14.5, respectively, whereas cold and dense Polar Water (PW) has considerably more radiogenic ɛNd signatures reaching up to −8.1. Locally formed Barents Sea Atlantic Water (BSAW) and Barents Sea Arctic Atlantic Water (BSAAW) are encountered in the northeastern Barents Sea and have intermediate ɛNd values resulting from admixture of PW containing small amounts of riverine freshwater from the Ob (〈~1.1%) to AW and NCW. Similar to the Laptev Sea, the dissolved Nd isotope composition in the Barents Sea seems to be mainly controlled by water mass advection and mixing despite its shallow water depth. Strikingly, the BSAW and BSAAW are marked by the lowest dissolved REE concentrations reported to date for the AM reaching 11 pmol/kg for Nd ([Nd]), which in contrast to the Nd isotopes, cannot be attributed to the admixture of REE-rich Ob freshwater to AW or NCW ([Nd] = 16.7, and 22 pmol/kg, respectively) and instead reflects REE removal from the dissolved phase with preferential removal of the light over the heavy REEs. The REE removal is, however, not explainable by estuarine REE behavior alone, suggesting that scavenging by (re)suspended (biogenic) particles occurs locally in the Barents Sea. Regardless of the exact cause of REE depletion, we show that AW transformation is accompanied by geochemical changes beyond those expected from water mass mixing.This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Beschreibung: Highlights • Observations show that formation of sediment-laden sea ice occurs in coastal polynyas in winter. • Sea ice rafted sediments are a significant component of the Laptev Sea’s sediment budget. • No observational evidence for sediment entrainment into sea ice in mid-shelf polynyas at water depth greater than 20 m. Abstract Sea ice is an important vehicle for sediment transport in the Arctic Ocean. On the Laptev Sea shelf (Siberian Arctic) large volumes of sediment-laden sea ice are formed during freeze-up in autumn, then exported and transported across the Arctic Ocean into Fram Strait where it partly melts. The incorporated sediments are released, settle on the sea floor, and serve as a proxy for ice-transport in the Arctic Ocean on geological time scales. However, the formation process of sediment-laden ice in the source area has been scarcely observed. Sediment-laden ice was sampled during a helicopter-based expedition to the Laptev Sea in March/April 2012. Sedimentological, biogeochemical and biological studies on the ice core as well as in the water column give insights into the formation process and, in combination with oceanographic process studies, on matter fluxes beneath the sea ice. Based on satellite images and ice drift back-trajectories the sediments were likely incorporated into the sea ice during a mid-winter coastal polynya near one of the main outlets of the Lena River, which is supported by the presence of abundant freshwater diatoms typical for the Lena River phytoplankton, and subsequently transported about 80 km northwards onto the shelf. Assuming ice growth of 12 to 19 cm during this period and mean suspended matter content in the newly formed ice of 91.9 mg l-1 suggests that a minimum sediment load of 8.4x104 t might have been incorporated into sea ice. Extrapolating these sediment loads for the entire Lena Delta region suggests that at least 65% of the estimated sediment loads which are incorporated during freeze-up, and up to 10% of the annually exported sediment load may be incorporated during an event such as described in this paper.

Beschreibung: The water masses passing the Fram Strait are mainly responsible for the exchange of heat and freshwater between the Nordic Seas and the Arctic Ocean (the Arctic Mediterranean, AM). Disentangling their exact sources, distribution and mixing, however, is complex. This work provides new insights based on a detailed geochemical tracer inventory including dissolved Nd isotope (εNd), rare earth element (REE) and stable oxygen isotope (δ18O) data along a full water depth section across Fram Strait. We find that Nd isotope and REE distributions in the open AM primarily reflect lateral advection of water masses and their mixing. Seawater-particle interactions exert important control only above the shelf regions, as observed above the NE Greenland Shelf. Advection of northward flowing warm Atlantic Water (AW) is clearly reflected by an εNd signature of -11.7 and a Nd concentration ([Nd]) of 16 pmol/kg in the upper ∼500 m of the eastern and central Fram Strait. Freshening and cooling of the AW on its way trough the AM are accompanied by a continuous change towards more radiogenic εNd signatures (e.g. -10.4 of dense Arctic Atlantic Water). This mainly reflects mixing with intermediate waters but also admixture of dense Kara Sea waters and Pacific-derived waters. The more radiogenic εNd signatures of the intermediate and deep waters (reaching -9.5) are mainly acquired in the SW Nordic Seas through exchange with basaltic formations of Iceland and SE Greenland. Inputs of Nd from Svalbard are not observed and surface waters and Nd on the Svalbard shelf originate from the Barents Sea. Shallow southward flowing Arctic-derived waters (〈 200 m) form the core of the East Greenland Current above the Greenland slope and can be traced by their relatively radiogenic εNd (reaching -8.8) and elevated [Nd] (21 to 29 pmol/kg). These properties are used together with δ18O and standard hydrographic tracers to define the proportions of Pacific-derived (〈 ∼30 % based on Nd isotopes) and Atlantic-derived waters, as well as of river waters (〈 ∼8 %). Shallow waters (〈 150 m) on the NE Greenland Shelf share some characteristics of Arctic-derived waters, but exhibit less radiogenic εNd values (reaching -12.4) and higher [Nd] (up to 38 pmol/kg) in the upper ∼100 m. This suggests local addition of Greenland freshwater of up to ∼6 %. In addition to these observations, this study shows that the pronounced gradients in εNd signatures and REE characteristics in the upper water column provide a reliable basis for assessments of shallow hydrological changes within the AM.

Beschreibung: During the ARCTIC '91 expedition aboard RV Polarstern (ARK VIII/3) to the Central Arctic Ocean, a box corer sample on the Gakkel Ridge at 87 degrees N and 60 degrees E yielded a layer of sand-sized, dark brown volcanic glass shards at the surface of the sediment core. These shards have been investigated by petrographic, mineralogical, geochemical and radiogenic isotope methods. The nearly vesicle-free and aphyric glass shards bear only minute microphenocrysts of magnesiochromite and olivine (Fo(88-89)). Most glasses are fresh, although some show signs of incipient low-temperature alteration. From their shapes and sizes, the glass shards most likely formed by spalling of glassy rinds of a nearby volcanic outcrop. Geochemically, the glasses are relatively unfractionated tholeiites with E-MORB trace element compositions. Thus, they are quite similar to the previously investigated ARK IV/3-11-370-5 basalts from 86 degrees N. The Nd and Sr isotopic ratios of PS 2167-2 glasses are significantly lower than for ARK IV/3-11-370-5 basalts and suggest an isotopically heterogeneous mantle source of Gakkel Ridge MORE between 86 degrees and 87 degrees N. The positive Delta-8/4 Pb value (similar to 16) and high Sr-87/Sr-86 ratio (0.70270), found for PS 2167-2 glasses are similar to that of ARK IV/3-11-370-5 basalts and show the influence of the DUPAL isotopic anomaly in the high Arctic mantle. These results argue against the presence of an 'anti-DUPAL anomaly' in the mantle below the North Pole region and simple models of whole-mantle convection.

Beschreibung: The record of glacier fluctuations in western Scandinavia, as reconstructed from continental data, has been correlated with records of ice-rafted detritus (IRD) from well-dated sediment cores from the Norwegian Sea covering the past 150,000 yr B.P. The input of IRD into the ocean is used as a proxy for ice sheet advances onto the shelf and, thus, for the calibration of a glaciation curve. The marine results generally support land-based reconstructions of glacier fluctuations and improve the time-control on glacial advances. The Saalian ice sheet decayed very rapidly approximately 125,000 yr B.P. In the Early Weichselian, a minor but significant IRD maximum indicates the presence of icebergs in isotope substage 5b (especially between 95,000 and 83,000 yr B.P.). Reduced amounts of calcareous nannofossils indicate that surface waters were influenced by meltwater discharges during isotope substages 5d and 5b. An extensive build-up of inland ice began again during isotope stage 4, but maximum glaciation was reached only in early stage 3 (58,000-53,000 yr B.P.). Marine sediments have minimum carbonate content, indicating strong dilution by lithogenic ice-rafted material. Generally, the IRD accumulation rate was considerably higher in stages 4-2 than in stage 5. A marked peak in IRD accumulation rates from 47,000 to 43,000 yr B.P. correlates well with a second Middle Weichselian ice sheet advance dated by the Laschamp/Olby paleomagnetic event. Minimum ice extent during the Ålesund interstade (38,500-32,500 yr B.P.) and several glacial oscillations during the Late Weichselian are also seen in the IRD record. Of several late Weichselian glacial oscillations on the shelf, at least four correspond to the North Atlantic Heinrich events. Ice sheet behavior was either coupled or linked by external forcing during these events, whereas internal ice sheet mechanisms may account for the noncoherent fluctuations.

Beschreibung: Sediment dynamics on the Laptev Sea shelf (Siberian Arctic), which is ice-covered for about 9 months a year, were studied for a I-year period. Two oceanographic bottom-mooring stations equipped with an Acoustic Doppler Current Profiler (ADCP) and a Conductivity Temperature Depth meter (CTD) were deployed on the eastern Laptev Sea shelf between August 1998 and September 1999. Thus, for the first time information on current, suspended particulate matter (SPM), and bottom temperature variations were provided throughout one seasonal cycle for a Siberian shelf sea. The data set indicates that during and shortly after the river-ice breakup (June-early July) sediment transport is dominated by riverine input and transport onto the eastern Laptev Sea shelf within the surface layer beneath the fast ice. Under ice-free conditions (mid-July to September), SPM is mainly trapped on the eastern Laptev Sea shelf SPM discharged by the Lena River is transported within the surface layer to the mid-shelf where it sinks through the water column into the bottom nepheloid layer only to be carried back onto the inner shelf again together with newly resuspended bottom material. On the inner shelf the material is partly transported back into the surface layer by turbid mixing and carried out onto the mid-shelf again. During freeze-up (October), SPM in the surface layer on the inner shelf is incorporated into newly formed ice and transported with the ice over the continental margin into the deep Arctic Ocean. Beneath the ice cover (November to June-July) SPM slowly sinks and sediment transport is of minor importance on the inner shelf. However, beneath the polynya, bottom material is resuspended during storm events and transported onto the inner shelf where it temporarily settles. The study suggests a quasi-estuarine sediment circulation on the eastern Laptev Sea shelf, especially during ice-free conditions, and sediment export dominated by ice rather than bottom transport