Mercury in the Black Sea:New Insights From Measurements and Numerical Modeling

Redox conditions and organic matter control marine methylmercury (MeHg) production. The Black Sea is the world's largest and deepest anoxic basin and is thus ideal to study Hg species along the extended redox gradient. Here we present new dissolved Hg and MeHg data from the 2013 GEOTRACES MEDBlack cruise (GN04_leg2) that we integrated into a numerical 1-D model, to track the fate and dynamics of Hg and MeHg. Contrary to a previous study, our new data show highest MeHg concentrations in the permanently anoxic waters. Observed MeHg/Hg percentage (range 9-57%) in the anoxic waters is comparable to other subsurface maxima in oxic open-ocean waters. With the modeling we tested for various Hg methylation and demethylation scenarios along the redox gradient. The results show that Hg methylation must occur in the anoxic waters. The model was then used to simulate the time evolution (1850-2050) of Hg species in the Black Sea. Our findings quantify (1) inputs and outputs of Hg-T (similar to 31 and similar to 28 kmol yr(-1)) and MeHgT (similar to 5 and similar to 4 kmol yr(-1)) to the basin, (2) the extent of net demethylation occurring in oxic (similar to 1 kmol yr(-1)) and suboxic water (similar to 6 kmol yr(-1)), (3) and the net Hg methylation in the anoxic waters of the Black Sea (similar to 11 kmol yr(-1)). The model was also used to estimate the amount of anthropogenic Hg (85-93%) in the Black Sea

Fe-binding organic ligands in coastal and frontal regions of the western Antarctic Peninsula

Organic ligands are a key factor determining the availability of dissolved iron (DFe) in the high-nutrient low-chlorophyll (HNLC) areas of the Southern Ocean. In this study, organic speciation of Fe is investigated along a natural gradient of the western Antarctic Peninsula, from an ice-covered shelf to the open ocean. An electrochemical approach, competitive ligand exchange – adsorptive cathodic stripping voltammetry (CLE-AdCSV), was applied. Our results indicated that organic ligands in the surface water on the shelf are associated with ice-algal exudates, possibly combined with melting of sea ice. Organic ligands in the deeper shelf water are supplied via the resuspension of slope or shelf sediments. Further offshore, organic ligands are most likely related to the development of phytoplankton blooms in open ocean waters. On the shelf, total ligand concentrations ([Lt]) were between 1.2 and 6.4 nM eq. Fe. The organic ligands offshore ranged between 1.0 and 3.0 nM eq. Fe. The southern boundary of the Antarctic Circumpolar Current (SB ACC) separated the organic ligands on the shelf from bloom-associated ligands offshore. Overall, organic ligand concentrations always exceeded DFe concentrations (excess ligand concentration, [L′] = 0.8–5.0 nM eq. Fe). The [L′] made up to 80 % of [Lt], suggesting that any additional Fe input can be stabilized in the dissolved form via organic complexation. The denser modified Circumpolar Deep Water (mCDW) on the shelf showed the highest complexation capacity of Fe (αFe'L; the product of [L′] and conditional binding strength of ligands, KFe'Lcond). Since Fe is also supplied by shelf sediments and glacial discharge, the high complexation capacity over the shelf can keep Fe dissolved and available for local primary productivity later in the season upon sea-ice melting.</p

Dissolved Cd, Co, Cu, Fe, Mn, Ni and Zn in the Arctic Ocean

During the Polarstern (PS94) expedition, summer 2015, part of the international GEOTRACES program, sources and sinks of dissolved (D) Cd, Co, Cu, Fe, Mn, Ni and Zn were studied in the central Arctic Ocean. In the Polar Surface Water in which the TransPolar Drift (TPD) is situated, salinity and δ18O derived fractions indicated a distinct riverine source for silicate DCo, DCu, DFe, DMn and DNi. Linear relationships between DMn and the meteoric fraction depended on source distance, likely due to Mn-precipitation during transport. In the upper 50 m of the Makarov Basin, outside the TPD core, DCo, DMn, DNi, DCd and DCu were enriched by Pacific waters, whereas DFe seemed diluted. DCo, DFe, DMn and DZn were relatively high in the Barents Sea and led to enrichment of Atlantic water flowing into the Nansen Basin. Deep concentrations of all metals were significantly lower in the Makarov Basin compared to the Nansen and Amundsen, the Eurasian, Basins. The Gakkel ridge hydrothermal input and higher continental slope convection are explanations for higher metal concentrations in the Eurasian Basins. Although scavenging rates are lower in the Makarov Basin compared to the Eurasian Basins, the residence time is longer and therefore scavenging can decrease the dissolved concentrations with time. This study provides a baseline to assess future change, and additionally identifies processes driving trace metal distributions. Our results underline the importance of fluvial input as well as shelf sources and internal cycling, notably scavenging, for the distribution of bio-active metals in the Arctic Ocean

Climatically sensitive transfer of iron to maritime Antarctic ecosystems by surface runoff

Iron supplied by glacial weathering results in pronounced hotspots of biological production in an otherwise iron-limited Southern Ocean Ecosystem. However, glacial iron inputs are thought to be dominated by icebergs. Here we show that surface runoff from three island groups of the maritime Antarctic exports more filterable (<0.45 μm) iron (6–81 kg km−2 a−1) than icebergs (0.0–1.2 kg km−2 a−1). Glacier-fed streams also export more acid-soluble iron (27.0–18,500 kg km−2 a−1) associated with suspended sediment than icebergs (0–241 kg km−2 a−1). Significant fluxes of filterable and sediment-derived iron (1–10 Gg a−1 and 100–1,000 Gg a−1, respectively) are therefore likely to be delivered by runoff from the Antarctic continent. Although estuarine removal processes will greatly reduce their availability to coastal ecosystems, our results clearly indicate that riverine iron fluxes need to be accounted for as the volume of Antarctic melt increases in response to 21st century climate change

Electrochemical methods for speciation of trace elements in marine waters. Dynamic aspects

The contribution of electrochemical methods to the knowledge of dynamic speciation of toxic trace elements in marine waters is critically reviewed. Due to the importance of dynamic considerations in the interpretation of the electrochemical signal, the principles and recent developments of kinetic features in the interconversion of metal complex species will be presented. As dynamic electrochemical methods, only stripping techniques (anodic stripping voltammetry and stripping chronopotentiometry) will be used because they are the most important for the determination of trace elements. Competitive ligand ex- change-adsorptive cathodic stripping voltammetry, which should be considered an equilibrium technique rather than a dynamic method, will be also discussed because the complexing parameters may be affected by some kinetic limitations if equilibrium before analysis is not attained and/or the flux of the adsorbed complex is in fluenced by the lability of the natural complexes in the water sample. For a correct data interpretation and system characterization the comparison of results obtained from different techniques seems essential in the articulation of a serious discussion of their meaning

The GEOTRACES Intermediate Data Product 2014

The GEOTRACES Intermediate Data Product 2014 (IDP2014) is the first publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2013. It consists of two parts: (1) a compilation of digital data for more than 200 trace elements and isotopes (TEIs) as well as classical hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing a strongly inter-linked on-line atlas including more than 300 section plots and 90 animated 3D scenes. The IDP2014 covers the Atlantic, Arctic, and Indian oceans, exhibiting highest data density in the Atlantic. The TEI data in the IDP2014 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at cross-over stations. The digital data are provided in several formats, including ASCII spreadsheet, Excel spreadsheet, netCDF, and Ocean Data View collection. In addition to the actual data values the IDP2014 also contains data quality flags and 1-? data error values where available. Quality flags and error values are useful for data filtering. Metadata about data originators, analytical methods and original publications related to the data are linked to the data in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2014 data providing section plots and a new kind of animated 3D scenes. The basin-wide 3D scenes allow for viewing of data from many cruises at the same time, thereby providing quick overviews of large-scale tracer distributions. In addition, the 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of observed tracer plumes, as well as for making inferences about controlling processes

The relation between salinity and copper complexing capacity of natural estuarine waters and the uptake of dissolved 64Cu by Macoma balthica

The radiotracer 64Cu was used to assess the influence of natural organic ligands on the bioavailability of copper. Biological availability of the 64Cu-complexes was measured by accumulation in the bivalve Macoma balthica. The experiments were carried out in April as well as in February with water from the relatively clean Oosterschelde Sea arm and the relatively polluted Westerschelde estuary. Adsorption onto shells, as well as uptake in tissues was assessed at salinities of 10‰ and 30‰. Simultaneously with the exposure experiments, ligand characteristics of the natural waters were assessed. High ligand concentrations, as occurring in the Westerschelde around February, reduced 64Cu (320 nM) uptake by more than 50%, in spite of the much lower salinity in the Westerschelde water. At the low salinity, uptake was increased slightly in Westerschelde water, but considerably in Oosterschelde water. This implies that at low ambient ligand concentrations (during the whole year in Oosterschelde water and in the summer period also in Westerschelde water) the influence of salinity on 64Cu uptake is more pronounced

The influence of sediment, food and organic ligands on the uptake of copper by sediment-dwelling bivalves

The sediment-dwelling bivalve Macoma balthica was exposed to dissolved copper ina flow-through system in long-term experiments. Unlike another sediment-dwelling bivalve, the suspension feeder Cerastoderma edule (cockle), M. balthica accumulated copper from the sediment, while the cockles did not. When dwelling in silty, organic-rich sediment, M. balthica accumulated less from the water, whereas accumulation in the cockle was not influenced by the sediment type. In water from the Oosterschelde sea-arm, total organic ligand concentrations were around 100 nanoequivalent copper, with a free Cu2+ concentration of 6.3 x 10(-14) M (pCu 13.20). Addition of 400 nM Cu (25 mu g l(-1)) resulted in a free Cu2+ concentration of 2.19 x 10(-8) M (pCu 7.66); Addition of low concentrations of EDTA caused a reduction of Cu uptake, which was confirmed with the calculated cupric ion activity. In water with 400 nM Cu, M. balthica accumulated more copper when fed on copper-enriched algae than without the addition of the algae. This accumulation occurred despite a decrease in filtration rates and indicates an important contribution of food-associated copper to the overall accumulation by Macoma balthica. [KEYWORDS: copper; bivalves; speciation; bioavailability San-francisco bay; mussel mytilus-edulis; macoma-balthica; paratya-australiensis; cadmium uptake; brine shrimp; sea-water; toxicity; complexation; accumulation]

Organic Fe speciation in the Eurasian Basins of the Arctic Ocean and its relation to terrestrial DOM

The bio-essential trace metal iron (Fe) has poor inorganic solubility in seawater, and therefore dissolution is dependent on organic complexation. The Arctic Ocean is subject to strong terrestrial influences which contribute to organic solubility of Fe, particularly in the surface. These influences are subject to rapid changes in the catchments of the main contributing rivers. Here we report concentrations and binding strengths of Fe-binding organic ligands in relation to spectral properties of Dissolved Organic Matter (DOM) and concentrations of humic substances. Full-depth profiles of Fe and Fe-binding organic ligands were measured for 11 stations, good agreement to previous studies was found with ligand concentrations between 0.9 and 2.2 equivalent nM of Fe (Eq. nM Fe) at depths > 200 m. We found nutrient-like profiles of Fe in the Atlantic-influenced Nansen basin, surface enrichment in the surface over the Amundsen and Makarov basins and scavenging effects in the deep Makarov basin. A highly detailed surface transect consisting of two sections crossing the surface flow from the Siberian continental shelf to the Fram Strait, the TransPolar Drift (TPD), clearly indicates the flow path of the riverine contribution to Fe and Fe-binding organic ligands with concentrations of 0.7 to 4.4 nM and 1.6 to 4.1 Eq. nM Fe, respectively. This is on average 4.5 times higher in DFe and 1.7 times higher in Fe-binding organic ligands than outside the TPD flow path. Conditional binding strengths of ligands in the entire dataset were remarkably similar at 11.4