Ocean circulation and freshwater pathways in the Arctic Mediterranean based on a combined Nd isotope, REE and oxygen isotope section across Fram Strait

Abstract

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 16pmol/kg in the upper ∼500m 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 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 (<200m) 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] (21–29pmol/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 (<150m) 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 38pmol/kg) in the upper ∼100m. 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.