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Science Highlights

Estuary solid loads and solid-solution exchanges yield considerable dissolved trace metal enrichments

Based on a thorough investigation of water and suspended sediment samples collected over two years and a six seasons, Samanta and Dalai (2018, see reference below) show that the annual dissolved fluxes of metals from the Ganga (Hooghly) River are enhanced by up to 230–1770% when compared to the conservative mixing. They clearly demonstrate that this enrichment results from exchange processes between the large solid load (suspended particles) and the waters of the middle and lower estuary. Groundwater and direct anthropogenic flux are negligible in these estuary segments.

On a broader scale, their work suggests that solute-particle interaction is a globally significant process in the estuarine production of dissolved metals. The authors estimate that although South Asian Rivers account for only ~ 9% of the global riverwater flux, their high sediment loads results in contributing a far higher proportion of the global supply of the dissolved metals from the rivers: 40 ± 2% of nickel (Ni) and 15 ± 1% of copper (Cu).

18 SamantaFigure: A plot of dissolved flux of nickel (Ni) and copper (Cu) vs. the sediment flux, after normalizing with the corresponding water flux, for the estuaries of some of the major rivers around the world where production of the metals is documented. The strong positive correlation is suggestive of the direct link between the solute-particle interaction and the estuarine production of the metals. The data of the river Scheldt is excluded from regression analysis. Click here to view the figure larger.

Reference:

Samanta, S., & Dalai, T. K. (2018). Massive production of heavymetals in the Ganga (Hooghly) River estuary, India: Global importance of solute-particle interaction and enhancedmetal fluxes to the oceans. Geochimica et Cosmochimica Acta, 228, 243–258. http://doi.org/10.1016/J.GCA.2018.03.002

High mesopelagic carbon remineralization traced by particulate biogenic barium in the North Atlantic Ocean

The high resolution section of particulate “excess Ba” (Baxs) measured by Lemaitre and co-authors (2018, see reference below) along the GEOVIDE GA1 section (R/V Pourquoi Pas? spring 2014) confirmed the ability of this parameter as proxy of the particulate organic carbon (POC) remineralization. Despite their importance for the biological pump quantification, POC remineralization data are still very scarce in the world ocean (see figure B below). Lemaitre’s work is a major contribution: besides validating the relationship between Baxs and oxygen consumption, it revealed significant remineralization rates at the time and location of the cruise, allowing establishing a biological pump scheme along the section (see figure C below). The link between the estimated POC export fluxes and the surface ecosystems is also discussed.

18 Lemaitre lFigure: (A) Relationship of the mesopelagic Baxs concentration versus the O2 consumption rate using the Southern Ocean transfer function (Dehairs et al., 1997) and the transfer function obtained in this study for the North Atlantic. (B) Summary of published POC remineralisation fluxes in the world ocean. (C) General schematic of the biological carbon pump in different provinces of the North Atlantic during GEOVIDE. Click here to view the figure larger.

Reference:

Lemaitre, N., Planquette, H., Planchon, F., Sarthou, G., Jacquet, S., García-Ibáñez, M. I., Gourain, A., Cheize, M., Monin, L., André, L., Laha, P., Terryn, H., Dehairs, F. (2018). Particulate barium tracing of significant mesopelagic carbon remineralisation in the North Atlantic. Biogeosciences, 15(8), 2289–2307. http://doi.org/10.5194/bg-15-2289-2018

Using ICPMS/MS to determine manganese, iron, nickel, copper, zinc, cadmium and lead concentrations on less than 40ml of seawater

Jackson and co-workers (2018, see reference below) first did a classical offline preconcentration of small seawater aliquots using a SeaFast system. More innovative is the use of a state of the art inductively coupled plasma - tandem mass spectrometry (ICPMS/MS) to analyse the eluate. Such tool combines two mass-selecting quadrupoles separated by an octopole collision/reaction cell. The collision/reaction cell was pressurized with O2 gas for the analysis of manganese (Mn), nickel (Ni), copper (Cu), cadmium (Cd) and lead (Pb) and H2 gas for the analysis of iron (Fe) and zinc (Zn), which removed common interferences (e.g. ArO+ on 56Fe and MoO+ on Cd)... and the detection limits were less than 0.050 nmol/l, which is extremely low!

18 Jackson
Figure:
A schematic diagram of the preconcentration and analysis of Mn, Fe, Ni, Cu, Zn, Cd and Pb in seawater samples. Seawater samples are preconcentrated using the seaFAST preconcentration system, and analysed on an ICP-MS/MS pressurized with either O2 gas (Mn, Ni, Cu, Cd and Pb) or H2 gas (Fe, Zn). Click here to view the figure larger.

Reference:

Jackson, S. L., Spence, J., Janssen, D. J., Ross, A. R. S., & Cullen, J. T. (2018). Determination of Mn, Fe, Ni, Cu, Zn, Cd and Pb in seawater using offline extraction and triple quadrupole ICP-MS/MS. Journal of Analytical Atomic Spectrometry, 33(2), 304–313. http://doi.org/10.1039/C7JA00237H

High production of methylmercury in the anoxic waters of the Black Sea

As part of the GEOTRACES MedBlack cruise, the research vessel Pelagia occupied 12 full-depth stations in the Black Sea along an East-West transect between July 13th and 25th, 2013. In the permanently anoxic waters of the Black Sea, a high fraction (up to 57%) of total mercury (HgT) was found to be methylmercury (MeHg). These levels are comparable to oxic open-ocean subsurface maxima. Using a 1D numerical model, the authors demonstrated that MeHg inputs from rivers, the Mediterranean Sea and sediments are negligible and that MeHg is produced in situ in the anoxic waters. The authors also reported an increasing trend of HgT and MeHg concentrations in the anoxic waters. The numerical modeling suggests that more drastic reductions of Hg emissions are required to reach decreasing Hg and MeHg levels in the Black Sea.

18 RosatiFigure: Concentrations of Hg species in the water column (OL = oxic layer, SOL = suboxic layer, AOL = anoxic layer) and sediments of the Black Sea. a) observed methylmercury (MeHg) distribution across the sampling stations of the GEOTRACES cruise; b) profile of dissolved Hg (HgD) observed (circles = layer means, bars = standard deviations) and modeled (triangles = model mean, coloured area = range of modeled concentrations) in the water; c) concentrations of total Hg (HgT) observed and modeled in the sediments. Click here to view the figure larger.

Reference:

Rosati, G., Heimbürger, L. E., Melaku Canu, D., Lagane, C., Laffont, L., Rijkenberg, M. J. A., Gerringa, L. J. A., Solidoro, C., Gencarelli, C. N., Hedgecock, I. M., De Baar, H. J. W., Sonke, J. E. (2018). Mercury in the Black Sea: new insights from measurements and numerical modeling. Global Biogeochemical Cycles. http://doi.org/10.1002/2017GB005700

Arctic rivers are discharging organic matter enriched in mercury to the Labrador Sea

In the framework of GEOVIDE-GEOTRACES GA01 cruise (spring 2014), Cossa and co-workers (see reference below) measured the first high-resolution mercury (Hg) distribution pattern along a transect from Greenland to Labrador coasts. An interesting feature is the observation of Hg enrichment originating from fluvial sources in the Canadian Arctic Archipelago waters. This excess Hg is transferred southward, in surface waters with the Labrador Current, and at depth with the lower limb of the Atlantic Meridional Overturning Circulation via the Deep Western Boundary Current. The authors underline that global warming could accelerate permafrost thawing in a near future, increasing the Hg discharge by the Arctic rivers.

18 CossaFigure: (a) Total mercury (HgT) concentratons in Labrador Sea waters ranged from 0.25 to 0.67 pmol L-1; (b) Anthropogenic Hg concentrations (Hganth) represents 36 % of the HgT present with the highest fraction (> 80%) in sub-surface and lowest fraction (< 15 %) near the bottom; (c) Hg enriched waters were identified in desalted waters originating from the Canadian Arctic Archipelago. Please click here to view the image larger.

Reference:

Cossa, D., Heimbürger, L. E., Sonke, J. E., Planquette, H., Lherminier, P., García-Ibáñez, M. I. Pérez, F.F., Sarthou, G. (2018). Sources, cycling and transfer of mercury in the Labrador Sea (Geotraces-Geovide cruise). Marine Chemistry, 198, 64–69. http://doi.org/10.1016/J.MARCHEM.2017.11.006

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