ALBERT

Description: Radiogenic isotopes of hafnium (Hf) and neodymium (Nd) are powerful tracers for water mass transport and trace metal cycling in the present and past oceans. However, due to the scarcity of available data the processes governing their distribution are not well understood. Here we present the first combined dissolved Hf and Nd isotope and concentration data from surface waters of the Atlantic sector of the Southern Ocean. The samples were collected along the Zero Meridian, in the Weddell Sea and in the Drake Passage during RV Polarstern expeditions ANT-XXIV/3 and ANT-XXIII/3 in the frame of the International Polar Year (IPY) and the GEOTRACES program. The general distribution of Hf and Nd concentrations in the region is similar. However, at the northernmost station located 200 km southwest of Cape Town a pronounced increase of the Nd concentration is observed, whereas the Hf concentration is minimal, suggesting much less Hf than Nd is released by the weathering of the South African Archean cratonic rocks. From the southern part of the Subtropical Front (STF) to the Polar Front (PF) Hf and Nd show the lowest concentrations (〈0.12 pmol/kg and 10 pmol/kg, respectively), most probably due to the low terrigenous flux in this area and efficient scavenging of Hf and Nd by biogenic opal. In the vicinity of landmasses the dissolved Hf and Nd isotope compositions are clearly labelled by terrigenous inputs. Near South Africa Nd isotope values as low as epsilon-Nd = -18.9 indicate unradiogenic inputs supplied via the Agulhas Current. Further south the isotopic data show significant increases to epsilon-Hf = 6.1 and epsilon-Nd = -4.0 documenting exchange of seawater Nd and Hf with the Antarctic Peninsula. In the open Southern Ocean the Nd isotope compositions are relatively homogeneous (epsilon-Nd ~ -8 to -8.5) towards the STF, within the Antarctic Circumpolar Current, in the Weddell Gyre, and the Drake Pasage. The Hf isotope compositions in the entire study area only show a small range between epsilon-Hf = +6.1 and +2.8 support Hf to be more readily released from young mafic rocks compared to old continental ones. The Nd isotope composition ranges from epsilon-Nd = -18.9 to -4.0 showing Nd isotopes to be a sensitive tracer for the provenance of weathering inputs into surface waters of the Southern Ocean.

Description: 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 (e-Nd), rare earth element (REE) and stable oxygen isotope (d18O) 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 e-Nd 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 e-Nd 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 e-Nd 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 CE 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 e-Nd (reaching -8.8) and elevated [Nd] (21-29 pmol/kg). These properties are used together with d18O 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 epsilon-Nd 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 epsilon-Nd signatures and REE characteristics in the upper water column provide a reliable basis for assessments of shallow hydrological changes within the AM.