Metal Release from Manganese Nodules in Anoxic Seawater and Implications for Deep-Sea Mining Dewatering Operations.

The potential mining of deep-sea polymetallic nodules has been gaining increasing attention due to their enrichment in metals essential for a low-carbon future. To date, there have been few scientific studies concerning the geochemical consequences of dewatered mining waste discharge into the pelagic water column, which can inform best practices in future mining operations. Here, we report the results of laboratory incubation experiments that simulate mining discharge into anoxic waters such as those that overlie potential mining sites in the North Pacific Ocean. We find that manganese nodules are reductively dissolved, with an apparent activation energy of 42.8 kJ mol-1, leading to the release of associated metals in the order manganese > nickel > copper > cobalt > cadmium > lead. The composition of trace metals released during the incubation allows us to estimate a likely trace metal budget from the simulated dewatering waste plume. These estimates suggest that released cobalt and copper are the most enriched trace metals within the plume, up to ∼15 times more elevated than the background seawater. High copper concentrations can be toxic to marine organisms. Future work on metal toxicity to mesopelagic communities could help us better understand the ecological effects of these fluxes of trace metals.

Isotopes illustrate vertical transport of anthropogenic Pb by reversible scavenging within Pacific Ocean particle veils.

Reversible scavenging, the oceanographic process by which dissolved metals exchange onto and off sinking particles and are thereby transported to deeper depths, has been well established for the metal thorium for decades. Reversible scavenging both deepens the elemental distribution of adsorptive elements and shortens their oceanic residence times in the ocean compared to nonadsorptive metals, and scavenging ultimately removes elements from the ocean via sedimentation. Thus, it is important to understand which metals undergo reversible scavenging and under what conditions. Recently, reversible scavenging has been invoked in global biogeochemical models of a range of metals including lead, iron, copper, and zinc to fit modeled data to observations of oceanic dissolved metal distributions. Nonetheless, the effects of reversible scavenging remain difficult to visualize in ocean sections of dissolved metals and to distinguish from other processes such as biological regeneration. Here, we show that particle-rich "veils" descending from high-productivity zones in the equatorial and North Pacific provide idealized illustrations of reversible scavenging of dissolved lead (Pb). A meridional section of dissolved Pb isotope ratios across the central Pacific shows that where particle concentrations are sufficiently high, such as within particle veils, vertical transport of anthropogenic surface-dissolved Pb isotope ratios toward the deep ocean is manifested as columnar isotope anomalies. Modeling of this effect shows that reversible scavenging within particle-rich waters allows anthropogenic Pb isotope ratios from the surface to penetrate ancient deep waters on timescales sufficiently rapid to overcome horizontal mixing of deep water Pb isotope ratios along abyssal isopycnals.

Strong Margin Influence on the Arctic Ocean Barium Cycle Revealed by Pan-Arctic Synthesis.

Early studies revealed relationships between barium (Ba), particulate organic carbon and silicate, suggesting applications for Ba as a paleoproductivity tracer and as a tracer of modern ocean circulation. But, what controls the distribution of barium (Ba) in the oceans? Here, we investigated the Arctic Ocean Ba cycle through a one-of-a-kind data set containing dissolved (dBa), particulate (pBa), and stable isotope Ba ratio (δ138Ba) data from four Arctic GEOTRACES expeditions conducted in 2015. We hypothesized that margins would be a substantial source of Ba to the Arctic Ocean water column. The dBa, pBa, and δ138Ba distributions all suggest significant modification of inflowing Pacific seawater over the shelves, and the dBa mass balance implies that ∼50% of the dBa inventory (upper 500 m of the Arctic water column) was supplied by nonconservative inputs. Calculated areal dBa fluxes are up to 10 µmol m-2 day-1 on the margin, which is comparable to fluxes described in other regions. Applying this approach to dBa data from the 1994 Arctic Ocean Survey yields similar results. The Canadian Arctic Archipelago did not appear to have a similar margin source; rather, the dBa distribution in this section is consistent with mixing of Arctic Ocean-derived waters and Baffin Bay-derived waters. Although we lack enough information to identify the specifics of the shelf sediment Ba source, we suspect that a sedimentary remineralization and terrigenous sources (e.g., submarine groundwater discharge or fluvial particles) are contributors.

Shallow particulate organic carbon regeneration in the South Pacific Ocean.

Particulate organic carbon (POC) produced in the surface ocean sinks through the water column and is respired at depth, acting as a primary vector sequestering carbon in the abyssal ocean. Atmospheric carbon dioxide levels are sensitive to the length (depth) scale over which respiration converts POC back to inorganic carbon, because shallower waters exchange with the atmosphere more rapidly than deeper ones. However, estimates of this carbon regeneration length scale and its spatiotemporal variability are limited, hindering the ability to characterize its sensitivity to environmental conditions. Here, we present a zonal section of POC fluxes at high vertical and spatial resolution from the GEOTRACES GP16 transect in the eastern tropical South Pacific, based on normalization to the radiogenic thorium isotope 230Th. We find shallower carbon regeneration length scales than previous estimates for the oligotrophic South Pacific gyre, indicating less efficient carbon transfer to the deep ocean. Carbon regeneration is strongly inhibited within suboxic waters near the Peru coast. Canonical Martin curve power laws inadequately capture POC flux profiles at suboxic stations. We instead fit these profiles using an exponential function with flux preserved at depth, finding shallow regeneration but high POC sequestration below 1,000 m. Both regeneration length scales and POC flux at depth closely track the depths at which oxygen concentrations approach zero. Our findings imply that climate warming will result in reduced ocean carbon storage due to expanding oligotrophic gyres, but opposing effects on ocean carbon storage from expanding suboxic waters will require modeling and future work to disentangle.

Coastal ocean and shelf-sea biogeochemical cycling of trace elements and isotopes: lessons learned from GEOTRACES.

Continental shelves and shelf seas play a central role in the global carbon cycle. However, their importance with respect to trace element and isotope (TEI) inputs to ocean basins is less well understood. Here, we present major findings on shelf TEI biogeochemistry from the GEOTRACES programme as well as a proof of concept for a new method to estimate shelf TEI fluxes. The case studies focus on advances in our understanding of TEI cycling in the Arctic, transformations within a major river estuary (Amazon), shelf sediment micronutrient fluxes and basin-scale estimates of submarine groundwater discharge. The proposed shelf flux tracer is 228-radium (T1/2 = 5.75 yr), which is continuously supplied to the shelf from coastal aquifers, sediment porewater exchange and rivers. Model-derived shelf 228Ra fluxes are combined with TEI/ 228Ra ratios to quantify ocean TEI fluxes from the western North Atlantic margin. The results from this new approach agree well with previous estimates for shelf Co, Fe, Mn and Zn inputs and exceed published estimates of atmospheric deposition by factors of approximately 3-23. Lastly, recommendations are made for additional GEOTRACES process studies and coastal margin-focused section cruises that will help refine the model and provide better insight on the mechanisms driving shelf-derived TEI fluxes to the ocean.This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.

Insights into particle cycling from thorium and particle data.

Marine particles are a main vector by which the biological carbon pump in the ocean transfers carbon from the atmosphere to the deep ocean. Marine particles exist in a continuous spectrum of sizes, but they can be functionally grouped into a small, suspended class (which constitutes most of the total particle mass) and a large, sinking class (which contributes most of the particle flux). These two classes are connected by aggregation and disaggregation processes. The interplay of processes that create, aggregate, and destroy marine particles determines the strength and transfer efficiency of the biological pump. Measurements of radiocarbon, barium, and organic biomarkers on suspended and sinking particles have provided qualitative insights into particle dynamics, and measurements of thorium isotopes have provided quantitative estimates of rates. Here, we review what has been learned so far about particle dynamics in the ocean from chemical measurements on suspended and sinking particles. We then discuss future directions for this approach.

A global ocean inventory of anthropogenic mercury based on water column measurements.

Mercury is a toxic, bioaccumulating trace metal whose emissions to the environment have increased significantly as a result of anthropogenic activities such as mining and fossil fuel combustion. Several recent models have estimated that these emissions have increased the oceanic mercury inventory by 36-1,313 million moles since the 1500s. Such predictions have remained largely untested owing to a lack of appropriate historical data and natural archives. Here we report oceanographic measurements of total dissolved mercury and related parameters from several recent expeditions to the Atlantic, Pacific, Southern and Arctic oceans. We find that deep North Atlantic waters and most intermediate waters are anomalously enriched in mercury relative to the deep waters of the South Atlantic, Southern and Pacific oceans, probably as a result of the incorporation of anthropogenic mercury. We estimate the total amount of anthropogenic mercury present in the global ocean to be 290 ± 80 million moles, with almost two-thirds residing in water shallower than a thousand metres. Our findings suggest that anthropogenic perturbations to the global mercury cycle have led to an approximately 150 per cent increase in the amount of mercury in thermocline waters and have tripled the mercury content of surface waters compared to pre-anthropogenic conditions. This information may aid our understanding of the processes and the depths at which inorganic mercury species are converted into toxic methyl mercury and subsequently bioaccumulated in marine food webs.

Revisiting carbon flux through the ocean's twilight zone.

The oceanic biological pump drives sequestration of carbon dioxide in the deep sea via sinking particles. Rapid biological consumption and remineralization of carbon in the "twilight zone" (depths between the euphotic zone and 1000 meters) reduce the efficiency of sequestration. By using neutrally buoyant sediment traps to sample this chronically understudied realm, we measured a transfer efficiency of sinking particulate organic carbon between 150 and 500 meters of 20 and 50% at two contrasting sites. This large variability in transfer efficiency is poorly represented in biogeochemical models. If applied globally, this is equivalent to a difference in carbon sequestration of more than 3 petagrams of carbon per year.