Description: Highlights • First comprehensive seawater Nd isotope and REE data for the Laptev Sea. • Dissolved Nd isotopes, salinity and stable oxygen isotopes trace water masses. • No evidence for REE release from particles of the organic-rich Siberian Rivers. • Preferential estuarine LREE removal follows increasing salinity from 10 to 34. • Formation and melting of sea ice redistribute REEs within water column. Abstract Marine neodymium (Nd) isotope and rare earth element (REE) compositions are valuable tracers for present and past ocean circulation and continental inputs. Yet their supply via high latitude estuaries is largely unknown. Here we present a comprehensive dissolved Nd isotope (expressed as εNd values) and REE data set together with seawater stable oxygen isotope ( O) compositions of samples from the Laptev Sea recovered in two Arctic summers and one winter. The Laptev Sea is a shallow Siberian Shelf sea characterized by extensive river-runoff, sea-ice production and ice transport into the Arctic Ocean. The large variability in εNd (−6 to −17), REE concentrations (16 to 600 pmol/kg for Nd) and REE patterns is controlled by freshwater supply from distinct riverine sources and open ocean Arctic Atlantic Water. Strikingly and contrary to expectations, except for cerium no evidence for significant release of REEs from particulate phases is found, which is attributed to low amounts of suspended particulate matter and high dissolved organic carbon concentrations present in the contributing rivers. Essentially all shelf waters are depleted in light (L)REEs, while the distribution of the heavy REEs shows a deficiency at the surface and a pronounced excess in the bottom layer. This distribution is consistent with REE removal through coagulation of riverine nanoparticles and colloids starting at salinities near 10 and resulting in a drop of all REE concentrations by ∼30%. With increasing salinity preferential LREE removal is observable reaching ∼75% for Nd at a salinity of 34. Although the delayed onset of dissolved REE removal contrasts with most previous observations from other estuarine environments, it agrees remarkably well with results from recent experiments simulating estuarine mixing of seawater with organic-rich river waters. In addition, melting and formation of sea ice leads to further REE depletion at the surface and strong REE enrichment near the shelf bottom as a function of ice melting and brine transfer, respectively. The ice-related processes significantly affect the distribution of dissolved REEs in high-latitude estuaries and likely also similarly contribute to the redistribution of other dissolved seawater constituents.

Description: Pyroxenites are an essential component in petrological and geochemical models for melt formation at mid-ocean ridges and ocean islands. Despite their rarity, their origin has been widely discussed and various processes have been invoked for their formation. Here, we present a detailed study of the microtextures and major, minor and trace element compositions of relatively fresh pyroxenites and associated harzburgites from the ultraslow-spreading Lena Trough, Arctic Ocean. Microtextural and geochemical characteristics suggest an origin by magmatic assimilation–fractional crystallization with a high ratio of mass crystallized to mass assimilated. The major element compositions of pyroxenes suggest that this process occurred at high pressures (〉0·7 GPa), although interstitial plagioclase in two of the pyroxenites indicates that melt–rock reaction continued at lower pressures. The parental melt to the pyroxenites was most probably depleted mid-ocean ridge basalt similar to basalts from the North Lena Trough and westernmost Gakkel Ridge; basalts from the Central Lena Trough cannot have functioned as parental melts. The melt was generated close to the garnet–spinel facies transition by variable degrees of partial melting and reacted with the local refractory harzburgite. Pyroxenites from this study provide further evidence, together with plagioclase-bearing and vein-bearing peridotites, for significant melt stagnation below the Lena Trough that occurred over a range of depths, either continuously or stepwise. Comparison with abyssal pyroxenites reveals common characteristics, suggesting that, consistent with results of high-pressure crystallization experiments, they mark the onset of (reactive) crystallization of melts passing through the deeper parts of the mid-ocean ridge plumbing system.

Description: OS51C-1004 Dissolved radiogenic Nd isotopes (εNd), rare earth element (REE), Ba, and nutrient concentrations combined with oxygen isotopes retrieved along a section between Spitsbergen and Greenland at approximately 79°N during the ARK XXVII/1 cruise in 2012 were measured to characterize the origin and mixing of the water masses in the Fram Strait. Deep waters below 500 m are nearly constant in Nd concentration (CNd) around 16 pmol/kg and εNd signatures range from -9.5±0.2 to -10.9±0.2. The heavy REE to light REE ratio (HREE/LREE) ranges from 4 to 5. Ba concentrations range from 47 to 58 nmol/kg, increasing slightly with depth. These homogeneous signatures do not allow identification of distinct deep water masses. The upper 500 m of the water column close to the Western Svalbard margin including the shelf is relatively warm and saline (T ≤ 5.5°C, S ≤ 35.1) and shares characteristics of Atlantic Water (AW) including low CNd (~15 pmol/kg) and relatively unradiogenic εNd signatures (-12.2±0.2). This water is also characterized by HREE/LREE around 4 and CBa around 50 nmol/kg. Low salinity surface waters on the East Greenland shelf have unradiogenic εNd signatures similar to AW (-12.4±0.3) but in contrast to AW high CNd of up to 37 pmol/kg. At the same time the HREE/LREE ratio is relatively low (~3.5) and CBa reaches 73 nmol/kg. This suggests a significant freshwater contribution either from the McKenzie or the Lena rivers. Eastwards of these freshwater-influenced waters (at ~5°W), admixture of a Pacific component characterized by a more radiogenic εNd (-8.8±0.2) and high nutrient concentrations outcropping at surface was detected. Waters of the same origin are present on the East Greenland shelf at about 150 m depth. Based on these data we use mass balance calculations to determine the fractions of sea ice meltwater, Eurasian run-off, North American run-off, and Arctic seawater and compare these results with our εNd and REE data.

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 (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.

Description: The warming of the Arctic region in the recent past has proceeded at rates double that of the global average and has been accompanied by rapid sea ice retreat and increased heat and freshwater fluxes to the Arctic Mediterranean (i.e. the Arctic Ocean and the Nordic Seas, AM). Further warming will have strong impacts on ocean circulation, freshwater pathways, and marine ecosystems in the AM. Disentangling the sources, distribution and mixing of water masses involved in the transport and transfer of heat and freshwater is therefore critical for the understanding of present and future hydrological changes in the high-latitude and polar regions and their consequences. This study refines the knowledge of water mass circulation and mixing in the AM and provides new insights into the processes occurring on the Arctic shelves and in high-latitude estuaries. A multi-proxy approach is used combining dissolved radiogenic Nd isotopes (εNd), rare earth elements (REEs) and stable oxygen isotopes (δ18O) together with standard hydrographic tracers. The sources, distribution and mixing of water masses that circulate in the AM and pass the Fram Strait are assessed through evaluation of dissolved εNd and REE, and δ18O data obtained from samples recovered in 2012 along a full water depth section extending between Svalbard and Greenland at ~79 °N, and through a compilation and reassessment of literature Nd isotope and concentration data previously reported for other sites within the AM. The Nd isotope and REE distribution in the central Fram Strait and the open AM primarily reflects the lateral advection of water masses and their mixing, whereas seawater-particle interactions exert important control only above the shelf regions. For example, on the NE Greenland Shelf, remineralization of biogenic and/or release from detrital particles is recorded in bottom waters. Advection of warm Atlantic Water (AW) in the upper water column of the eastern and central Fram Strait is clearly reflected by an εNd signature of -11.7 and a Nd concentration ([Nd]) of 16 pmol/kg. Freshening and cooling of the AW on its way through the AM are accompanied by a continuous change towards more radiogenic Nd isotope compositions (e.g. -10.4 of dense Arctic Atlantic Water). This change results from mixing with intermediate waters but also mirrors the admixture of dense Kara Sea waters and Pacific-derived waters. Exchange with basaltic formations of Iceland and southeastern Greenland is suggested to impart the intermediate and deep waters of the AM with more radiogenic εNd signatures, which reach -9.5 in the Fram Strait. Significant inputs of Nd from Svalbard are not observed and surface waters and Nd on the western Svalbard Shelf originate in the Barents Sea. Shallow (〈 200 m) waters of Arctic origin form the core of the East Greenland Current above the Greenland slope and have relatively radiogenic εNd (reaching -8.8) and elevated [Nd] (21-29 pmol/kg), which together with δ18O and standard hydrographic tracers are used to determine the proportions of Pacific-derived (〈 30 % based on Nd isotopes) and Atlantic-derived seawater, as well as of river waters (〈 8 %). A change in the Nd isotope compositions to less radiogenic values (-12.4) and an increase in [Nd] (38 pmol/kg) are observed at water depths above 100 m near the Greenland coast documenting addition of Greenland-sourced freshwater (GFW). The amount of GFW contained in the upper water column on the NE Greenland Shelf reached 6 % in 2012. Data obtained for the years 2014 and 2015 for the northern and southern NE Greenland Shelf suggest similar fractions of GFW for shallow waters in the Norske Trough and east of Ob Bank, indicating southward and northward propagation of GFW along the Greenland coast assuming that the NE Greenland Ice Stream is the freshwater source. The Nd isotope compositions of Arctic-derived waters (εNd ~ -9) and other water masses were essentially constant over the time period 2012-2015, which provides a solid basis for quantitative estimates of GFW admixture. The GFW distribution suggests that future increased GFW supply forced by global warming will lead to additional freshening of shallow Arctic waters, which, once these waters have traversed the Nordic Seas, may ultimately affect overturning strength in the northern Labrador Sea. Overall, the results obtained from the Fram Strait demonstrate that the pronounced gradients in εNd and REE contents in the upper water column provide a reliable basis for assessments of short-term shallow hydrological changes within the AM. New insights into the processes occurring in high latitude estuaries are provided by dissolved Nd isotope and REE compositions together with δ18O data for the Laptev Sea based on filtered samples collected during two summers (2013 + 2014) and one winter (2012). The Laptev Sea is a Siberian Shelf sea characterized by extensive river-runoff, sea-ice production and ice transport into the Arctic Ocean. The broad range in εNd (-6 to -17), REE concentrations (16 to 600 pmol/kg for Nd) and REE patterns found in the frame of this study is attributed to freshwater supply from the Siberian rivers and advection of open ocean Arctic Atlantic Water. Strikingly and contrary to expectations, there is no evidence for significant release of Nd from particulate phases and Nd isotopes can thus be used to assess water mass mixing together with the salinity after correction for variations in the salinity caused by sea-ice formation and melting. High fractions of riverine contributions from the Lena River (up to 75 %) are determined for the surface layer of the eastern Laptev Sea with significant interannual variations, while the less variable advection of Yenisei and Ob freshwater (up to ~20 %) is restricted to the western Laptev Sea. Essentially all Laptev shelf waters are depleted in light (L)REEs, while the distribution of the heavy (H)REEs shows a deficiency at the surface and an excess in the bottom layer. A combination of REE removal through coagulation of nanoparticles and colloids and REE redistribution within the water column through formation and melting of sea ice and river ice is suggested to account for the distribution of all REEs. Estuarine removal of riverine REEs starts at salinities close to 10 and after a drop of all REEs by about 30 % transfers into preferential LREE removal, which for Nd reaches 75 % at salinities near 34. Although the delayed onset of dissolved REE removal contrasts with observations from most other estuarine environments, the distributions coincide remarkably well with results from recent experiments simulating estuarine mixing with organic-rich river waters. The melting and formation of sea ice and river ice lead to further REE depletion at the surface and enrichments in the bottom water layer as a function of ice melting and brine transfer, respectively. The ice-related processes contribute to the redistribution of other elements and may also affect macronutrient distribution and primary productivity in high latitude estuaries.