High-pressure Reactive Melt Stagnation Recorded in Abyssal Pyroxenites from the Ultraslow-spreading Lena Trough, Arctic Ocean

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 center dot 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

High latitude controls on dissolved barium isotope distributions in the global ocean

The high latitude regions play a key role in regulating the marine biogeochemical cycling of barium (Ba) and the pre-formed Ba isotope compositions in the global ocean. In this study, we present 17 new depth profiles of dissolved Ba concentrations ([Ba]) and isotope compositions (δ138Ba) from the high latitude Atlantic, Pacific and Southern Oceans to trace the ventilation of deep waters in the Southern Ocean and their subsequent transport throughout the global ocean. Our data reveal how biogeochemical processes in the Southern Ocean generate distinct δ138Ba signatures of upper ocean water masses, and that large scale ocean circulation constrains the meridional gradient of δ138Ba distributions in the deep Atlantic Ocean. The significant increase in [Ba] of deep waters in the North Pacific is mainly achieved through dissolution of sinking particles which adds a δ138Ba signal comparable to the deep Pacific Ocean

Water mass transformation in the Barents Sea inferred from radiogenic neodymium isotopes, rare earth elements and stable oxygen isotopes

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

Nutrient and Silicon Isotope Dynamics in the Laptev Sea and Implications for Nutrient Availability in the Transpolar Drift

Realistic prediction of the near‐future response of Arctic Ocean primary productivity to ongoing warming and sea ice loss requires a mechanistic understanding of the processes controlling nutrient bioavailability. To evaluate continental nutrient inputs, biological utilization, and the influence of mixing and winter processes in the Laptev Sea, the major source region of the Transpolar Drift (TPD), we compare observed with preformed concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP), silicic acid (DSi), and silicon isotope compositions of DSi (δ30SiDSi) obtained for two summers (2013 and 2014) and one winter (2012). In summer, preformed nutrient concentrations persisted in the surface layer of the southeastern Laptev Sea, while diatom‐dominated utilization caused intense northward drawdown and a pronounced shift in δ30SiDSi from +0.91 to +3.82‰. The modeled Si isotope fractionation suggests that DSi in the northern Laptev Sea originated from the Lena River and was supplied during the spring freshet, while riverine DSi in the southeastern Laptev Sea was continuously supplied during the summer. Primary productivity fueled by river‐borne nutrients was enhanced by admixture of DIN‐ and DIP‐rich Atlantic‐sourced waters to the surface, either by convective mixing during the previous winter or by occasional storm‐induced stratification breakdowns in late summer. Substantial enrichments of DSi (+240%) and DIP (+90%) beneath the Lena River plume were caused by sea ice‐driven redistribution and remineralization. Predicted weaker stratification on the outer Laptev Shelf will enhance DSi utilization and removal through greater vertical DIN supply, which will limit DSi export and reduce diatom‐dominated primary productivity in the TPD.Plain Language Summary: Ongoing warming and sea ice loss in the Arctic Ocean may significantly impact biological productivity, which is mainly controlled by light and nutrient availability. To investigate nutrient inputs from land, biological utilization, and the influence of water mass mixing and winter processes on the nutrient distributions, we measured nutrient concentrations and silicon isotopes in the Laptev Sea. We found high concentrations in the southeastern Laptev Sea in agreement with nutrient inputs from the Lena River. Toward the northern Laptev Sea, nutrient concentrations decreased in the surface layer and the silicon isotope signatures shifted to heavier values, consistent with nutrient utilization by phytoplankton. In contrast to the depleted surface layer, the bottom layer beneath the Lena River plume was strongly enriched in some nutrients, which we attribute to different physical and biogeochemical processes. These observations are important for our understanding of nutrient bioavailability in the Laptev Sea and the Transpolar Drift (TPD), which is a surface current that connects the Laptev Sea with the central Arctic Ocean and the Fram Strait. The changing hydrography of the Laptev Sea will likely cause a decrease in silicic acid concentrations and thus a reduction in nutrient export and diatom‐dominated primary productivity in the TPD.Key Points: Surface dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), silicic acid (DSi), and Si isotope dynamics are controlled by marine and riverine inputs and uptake by phytoplankton. Strong DIP and DSi enrichments beneath the Lena River plume are due to sea ice‐driven nutrient redistribution and remineralization. Enhanced DSi utilization in the Laptev Sea will lead to a reduced diatom‐dominated primary productivity in the Transpolar Drift.Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347Ocean Frontier Institute http://dx.doi.org/10.13039/501100010784Canada First Research Excellence Fund http://dx.doi.org/10.13039/501100010785https://doi.pangaea.de/10.1594/PANGAEA.931257https://doi.pangaea.de/10.1594/PANGAEA.931240https://doi.pangaea.de/10.1594/PANGAEA.931209https://doi.pangaea.de/10.1594/PANGAEA.93825