ALBERT

Description: Rare earth elements and yttrium (REY) are often used as proxies to describe past enviromental conditions or to track element sources. Calcium phosphates are commonly used as archives for REY, but our study of the upper 10 m of deep-sea sediments from the equatorial Pacific, where REY are controlled by Ca phosphates, show that the shale-normalized (SN) REY patterns are heavily impacted by early diagenesis. The Ca phosphates incorporate REY from ambient pore waters without major fractionation, and thus, their REYSN patterns are similar to the pore-water REYSN pattern [1]. Our data from the Clarion Clipperton Zone (CCZ) and from the Peru Basin reveal such incorporation of pore-water REY into the Ca phosphates over long geographical distances and over a rather wide range of oxic to suboxic pore-water conditions. The pore-water REYSN patterns from the Peru Basin show similar features as seawater (e.g., heavy REY enrichment, negative CeSN and positive YSN anomalies), whereas the pore-water REYSN patterns from the CCZ display middle REY enrichment, the development of a negative CeSN-anomaly with depth, and either no or a slightly negative YSN-anomaly [1]. The differing pore-water REYSN patterns are possibly due to different REY sources to the pore water. These results cast doubt on the approach of using marine Ca phosphates as archives for the REY distribution or the Nd isotope composition of seawater, because the REY are derived from pore water and the Ca phosphates therefore do not preserve a primary seawater REY signal.

Description: Due to its remoteness, the deep-sea floor remains an understudied ecosystem of our planet. The patchiness of existing data sets makes it difficult to draw conclusions about processes that apply to a wider area. In our study we show how different settings and processes determine sediment heterogeneity on small spatial scales. We sampled solid phase and porewater from the upper 10 m of an approximately 7.4×13 km2 area in the Peru Basin, in the southeastern equatorial Pacific Ocean, at 4100 m water depth. Samples were analyzed for trace metals, including rare earth elements and yttrium (REY), as well as for particulate organic carbon (POC), CaCO3, and nitrate. The analyses revealed the surprisingly high spatial small-scale heterogeneity of the deep-sea sediment composition. While some cores have the typical green layer from Fe(II) in the clay minerals, this layer is missing in other cores, i.e., showing a tan color associated with more Fe(III) in the clay minerals. This is due to varying organic carbon contents: nitrate is depleted at 2–3 m depth in cores with higher total organic carbon contents but is present throughout cores with lower POC contents, thus inhibiting the Fe(III)-to-Fe(II) reduction pathway in organic matter degradation. REY show shale-normalized (SN) patterns similar to seawater, with a relative enrichment of heavy REY over light REY, positive LaSN anomaly, negative CeSN anomaly, and positive YSN anomaly and correlate with the Fe-rich clay layer and, in some cores, also correlate with P. We therefore propose that Fe-rich clay minerals, such as nontronite, as well as phosphates, are the REY-controlling phases in these sediments. Variability is also seen in dissolved Mn and Co concentrations between sites and within cores, which might be due to dissolving nodules in the suboxic sediment, as well as in concentration peaks of U, Mo, As, V, and Cu in two cores, which might be related to deposition of different material at lower-lying areas or precipitation due to shifting redox boundaries.

Description: Due to its remoteness, the deep-sea floor remains an understudied ecosystem of our planet. The patchiness of existing data sets makes it difficult to draw conclusions about processes that apply to a wider area. In our study we show how different settings and processes determine sediment heterogeneity on small spatial scales. We sampled solid phase and pore water from the upper 10 m of an approximately 7.4 × 13 km2 large area in the Peru Basin, south-east equatorial Pacific Ocean, at 4100 m water depth. Samples were analyzed for trace metals including rare earth elements and yttrium (REY) as well as for particulate organic carbon (POC), CaCO3, and nitrate. The analyses revealed a surprisingly high small-scale heterogeneity of the deep-sea sediment composition. While some cores have the typical green layer from Fe(II) in the clay minerals, this layer is missing in other cores, i.e. showing a tan color associated with Fe(III) in the clay minerals. This is due to varying organic carbon contents: nitrate is depleted at 2–3 m depth in cores with higher total organic carbon contents, but is present throughout cores with lower POC contents, thus inhibiting the Fe(III)-to-Fe(II) reduction pathway in organic matter degradation. REY show shale-normalized (SN) patterns similar to seawater with a relative enrichment of heavy REY over light REY, positive LaSN anomaly, negative CeSN anomaly, as well as positive YSN anomaly and correlate with the Fe-rich clay layer and in some cores also with P. We, therefore, propose that Fe-rich clay minerals, such as nontronite, as well as phosphates are the REY-controlling phases in these sediments. Variability is also seen in dissolved Mn and Co concentrations, which might be due to dissolving nodules in the suboxic sediment, as well as in concentration peaks of U, Mo, As, V, and Cu in two cores, which might be related to deposition of different material at lower lying areas.

Description: Increasing interest in deep-sea mineral resources, such as polymetallic nodules, calls for environmental research about possible impacts of mineral exploitation on the deep-sea ecosystem. So far, little geochemical comparisons of deep-sea sediments before and after mining induced disturbances have been made, and thus long-term environmental effects of deep-sea mining are unknown. Here we present geochemical data from sediment cores from an experimental disturbance area at 4,100 m water depth in the Peru Basin. The site was revisited in 2015, 26 years after a disturbance experiment mimicking nodule mining was carried out and compared to sites outside the experimental zone which served as a pre-disturbance reference. We investigated if signs of the disturbance are still visible in the solid phase and the pore water after 26 years or if pre-disturbance conditions have been re-established. Additionally, a new disturbance was created during the cruise and sampled 5 weeks later to compare short- and longer-term impacts. The particulate fraction and pore water were analyzed for major and trace elements to study element distribution and processes in the surface sediment. Pore water and bottom water samples were also analyzed for oxygen, nitrate, dissolved organic carbon, and dissolved amino acids, to examine organic matter degradation processes. The study area of about 11 km2 was found to be naturally more heterogeneous than expected, requiring an analysis of spatial variability before the disturbed and undisturbed sites can be compared. The disturbed sites exhibit various disturbance features: some surface sediments were mixed through, others had the top layer removed and some had additional material deposited on top. Pore water constituents have largely regained pre-disturbance gradients after 26 years. The solid phase, however, shows clear differences between disturbed and undisturbed sites in the top 20 cm so that the impact is still visible in the plowed tracks after 26 years. Especially the upper layer, usually rich in manganese-oxide and associated metals, such as Mo, Ni, Co, and Cu, shows substantial differences in metal distribution. Hence, it can be expected that disturbances from polymetallic nodule mining will have manifold and long-lasting impacts on the geochemistry of the underlying sediment.

Description: Due to its remoteness, the deep-sea floor remains an understudied ecosystem of our planet. The patchiness of existing data sets makes it difficult to draw conclusions about processes that apply to a wider area. In our study we show how different settings and processes determine sediment heterogeneity on small spatial scales. We sampled solid phase and porewater from the upper 10 m of an approximately 7.4×13 km2 area in the Peru Basin, in the southeastern equatorial Pacific Ocean, at 4100 m water depth. Samples were analyzed for trace metals, including rare earth elements and yttrium (REY), as well as for particulate organic carbon (POC), CaCO3, and nitrate. The analyses revealed the surprisingly high spatial small-scale heterogeneity of the deep-sea sediment composition. While some cores have the typical green layer from Fe(II) in the clay minerals, this layer is missing in other cores, i.e., showing a tan color associated with more Fe(III) in the clay minerals. This is due to varying organic carbon contents: nitrate is depleted at 2–3 m depth in cores with higher total organic carbon contents but is present throughout cores with lower POC contents, thus inhibiting the Fe(III)-to-Fe(II) reduction pathway in organic matter degradation. REY show shale-normalized (SN) patterns similar to seawater, with a relative enrichment of heavy REY over light REY, positive LaSN anomaly, negative CeSN anomaly, and positive YSN anomaly and correlate with the Fe-rich clay layer and, in some cores, also correlate with P. We therefore propose that Fe-rich clay minerals, such as nontronite, as well as phosphates, are the REY-controlling phases in these sediments. Variability is also seen in dissolved Mn and Co concentrations between sites and within cores, which might be due to dissolving nodules in the suboxic sediment, as well as in concentration peaks of U, Mo, As, V, and Cu in two cores, which might be related to deposition of different material at lower-lying areas or precipitation due to shifting redox boundaries.

Description: Highlights • First successful in situ DGT application in the deep ocean. • DGT-lability of dissolved (〈0.2 μm) Cu, Ni, Cd, Mn, As, V, REY differs depending on chemical speciation. • REY in deep ocean water can be almost quantitatively assessed with DGT. • Low Cu availability reflects dominating organic speciation. Abstract Geochemical behaviour and bio-availability of trace metals are closely related to their physical fractionation and chemical speciation. The DGT speciation technique allows the challenging assessment of labile concentrations of Mn, Cd, Cu, Ni, V, As, and REY in ocean waters. In this first deep-water in situ study of DGT-lability, we demonstrate the approach in bottom waters of the Clarion-Clipperton Zone in the central NE Pacific. In the dissolved fraction (〈0.2 μm), 70% to 100% of Cd, Ni, V, and REY, but only 25% of Cu and less than 50% of As were determined, reflecting their prevailing dominance of organic vs. inorganic complexation. This study demonstrates the applicability and sensitivity of DGT-passive samplers for trace metals as a suitable technique in monitoring of anthropogenic activities, such as deep seabed mining, as well as for natural process studies in abyssal environments.

Description: The release of potentially toxic metals, such as copper (Cu), into the water column is of concern during polymetallic nodule mining. The bioavailability and thus toxicity of Cu is strongly influenced by its speciation which is dominated by organic ligand (L) complexation in seawater, with L-complexes being considered less bioavailable than free Cu 2+ . The presence of CuL-complexes in deep-sea sediments has, however, not been systematically studied in the context of deep-sea mining. We thus analyzed the Cu-binding L concentration ([L]) in deep-sea pore waters of two polymetallic nodule provinces in the Pacific Ocean, the Peru Basin and the Clarion-Clipperton-Zone, using competitive ligand equilibration–adsorptive stripping voltammetry. The pore-water dissolved Cu concentration ([dCu]) ranged from 3 to 96 nM, generally exceeding bottom water concentrations (4–44 nM). Based on fitting results from ProMCC and Excel, Cu was predominantly complexed by L (3–313 nM) in bottom waters and undisturbed pore waters. We conclude that processes like deep-sea mining are unlikely to cause a release of toxic Cu 2+ concentrations ([Cu 2+ ]) to the seawater as 〉 99% Cu was organically complexed in pore waters and the [Cu 2+ ] was 〈 6 pM for 8 of 9 samples. Moreover, the excess of L found especially in shallow pore waters implied that even with a Cu release through mining activities, Cu 2+ likely remains beneath toxic thresholds.