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Science Highlights

Estimation of the trace element deposition fluxes to the Atlantic Ocean using two different methods

Shelley and co-workers (2016, see reference below) established that atmospheric deposition of trace elements was low throughout May-June 2014 along the GEOVIDE (GA01) cruise track in the North Atlantic Ocean. They also demonstrate that the aerosol trace element composition could be represented as simply the mixing of two aerosol sources: mineral dust and mixed mineral dust-sea salt-anthropogenic aerosols. In other words, the aerosols were not significantly affected by the Saharan dust plume in this northern part of the Atlantic Ocean.

Converting the trace element concentrations into an atmospheric deposition flux is a known challenge. Here, the authors discuss the comparison of fluxes obtained using the "traditional" methods (i.e. summing dry and wet deposition) and the 7Be content of the upper water column as a proxy for atmospheric deposition. Excellent agreement is obtained for ca 50% of the trace elements, among them iron, silver, strontium, yttrium, and in both studied areas (see figure below). Hypotheses for observed discrepancies could be differences in the timescale of integration of processes and selection of representative deposition velocities and precipitation rates.

16 Shelley lFigures: (A) The GEOVIDE cruise transect from Lisbon (Portugal) to St. John’s (Canada) showing the locations of aerosol sample collections (black dots), precipitation samples (yellow crosses), and seawater samples (yellow boxes); (B) atmospheric deposition flux estimates for Area 1 (west of 30°W; top) and Area 2 (east of 30°W; bottom) using the traditional (black triangles) and 7Be approaches (white circles). Click here to view the image larger. (modified from Deep Sea Research, see reference below)

Reference:

Shelley, R. U., Roca-Martí, M., Castrillejo, M., Masqué, P., Landing, W. M., Planquette, H., & Sarthou, G. (2016). Quantification of trace element atmospheric deposition fluxes to the Atlantic Ocean (>40°N; GEOVIDE, GEOTRACES GA01) during spring 2014. Deep Sea Research Part I: Oceanographic Research Papers. DOI: 10.1016/j.dsr.2016.11.010

 

Solute-particle interactions and the enhanced dissolved barium flux from the Ganga River estuary

Dissolved and particulate barium (Ba) were investigated in samples that were collected in six periods of contrasting water discharge over two years (2012 and 2013) by Saumik and Dalai (2016, see reference below) in the Ganga (Hooghly) River estuary. The authors thoroughly documented anthropogenic sources and submarine groundwater discharges, which account for less than 2% and 5%, respectively, of the total dissolved Ba discharged annually by this estuary to the oceans. A dominant fraction of dissolved Ba results from desorption of Ba from clay minerals and/or iron-manganese hydroxides in the particulate matter.

The estimates of Ba flux show that annually (1.5–1.9) x 107 moles of Ba is transported by the Hooghly River. Additionally, about (3.6–4.3) x 107 moles of Ba is generated annually in the estuary through ion-exchange and desorption. This means that in the Ganga River estuary, the solute-particle interactions enhance the riverine Ba flux by >300%.

16 Samuik l

Figure: Variation of dissolved Ba, particulate magnesium (Mg) / aluminium (Al), exchangeable Mg and potassium (K) as a function of salinity in the Hooghly estuary. Similar variation patterns of particulate Mg/Al and dissolved Ba (with a few exceptions) are as a result of desorption of Ba in the low- to mid-salinity regions in response to adsorption of Mg. The distribution patterns of dissolved Ba in the estuary are inferred to be a direct consequence of adsorption of Mg and K in the particulate phases as evident from the variation of exchangeable Mg and K concentrations. Click here to view the figure larger.

Reference:

Samanta, S., & Dalai, T. K. (2016). Dissolved and particulateBarium in the Ganga (Hooghly) River estuary, India: Solute-particle interactions and the enhanceddissolved flux to the oceans. Geochimica et Cosmochimica Acta, 195, 1–28. doi: 10.1016/j.gca.2016.09.005

Testament of the efficiency of environmental policies

Human activities, such as the combustion of leaded petrol, emissions from non-ferrous metal smelting, coal combustion and waste incineration constitute major environmental lead (Pb) sources during the past century. This resulted in a considerable increase of anthropogenic Pb in the surface and deep waters of the North Atlantic, large enough to mask the natural lead signal.

Increased usage and then phasing-out of leaded-petrol since the mid-70's yielded a decrease of this contamination. By measuring lead concentrations and isotopes (excellent tracers of the different sources of lead) along the GEOTRACES sections GA02 and GA06, Bridgestock and his co-workers (2016, see reference below) reveal for the first time that natural lead can be detected again in the surface water of the North Atlantic. Indeed, significant proportions of up to 30–50% of natural Pb, derived from mineral dust, are observed in Atlantic surface waters off the Sahara. This clearly reflects the success of the global effort to reduce anthropogenic Pb emissions.

16 BridgestocklFigure: Locations of the surface seawater samples analyzed in this study (left). The brown shaded box shows the area found to contain the highest amounts of naturally sourced lead (Pb) resulting from the deposition of North African mineral dust. Significant inputs of natural Pb can be identified by higher Pb isotope ratio values (206Pb/207Pb and 208Pb/207Pb; right).

Reference

Bridgestock, L., van de Flierdt, T., Rehkämper, M., Paul, M., Middag, R., Milne, A., Lohan, M.C., Baker, A.R., Chance, R.,, Khondoker, R., Strekopytov, S., Humphreys-Williams, E., Achterberg, E.P., Rijkenberg, M.J.A., Gerringa, L. J.A., de Baar, H. J. W. (2016). Return of naturally sourced Pb to Atlantic surface waters. Nature Communications, 7, 12921. doi:10.1038/ncomms12921

Prokaryotic communities display elevated trace metal concentrations in Pacific oxygen deficient zone

Local particulate maxima in many bioactive trace metals (cadmium, Cd; cobalt, Co; nickel, Ni; vanadium, V; and zinc, Zn) are found in the upper Oxygen Deficient Zone (ODZ), coincident with particulate phosporous (P) maxima that indicate biomass enrichments. This observation was made by Ohnemus and colleagues during the US GEOTRACES Eastern Pacific Zonal Transect (GP16) cruise which crossed the Pacific ODZ and oligotrophic gyre. Their data suggest elevated biotic accumulation of trace metals by ODZ organisms, by factors of 2 to 9 over surface mixed layer communities.

These observations raise many questions regarding the metal requirements and stoichiometric flexibilities of prokaryotes that dominate the ocean interior: Are particulate trace metal (pTM) associations unique to the ODZ? Do they occur because of access to generally larger inventories of dissolved TMs in the subsurface? Which metal enrichments are associated with which organisms? How do elevated–pTM associations in prokaryotic biomass relate to local and global cycling of pTMs throughout the oceans? There is no doubt that these new results open a wide field of research!

16 Ohnemus l
Figure: Metal:P biomass ratios in bulk particulate ODZ samples (black dots) compared to local mixed layer samples (red lines) demonstrate the elevated trace metal content of ODZ prokaryotic communities. All samples have been corrected for metals in lithogenic and scavenged iron-oxide phases.

Reference:

Ohnemus, D. C., Rauschenberg, S., Cutter, G. A., Fitzsimmons, J. N., Sherrell, R. M. and Twining, B. S. (2016), Elevated trace metal content of prokaryotic communities associated with marine oxygen deficient zones. Limnol. Oceanogr. doi:10.1002/lno.10363

Dealing with the chemical speciation of the elements in the different oceanic realms

First paper of the SCOR working group on modelling chemical speciation in seawater (WG145)

The form in which a trace element or other component of seawater is present, and its tendency to react, depends on its activity which is a complex function of temperature, pressure, salinity and often pH.

The most widely used equations that are used to calculate activities of dissolved ions and molecules and, in combination with thermodynamic equilibrium constants, chemical speciation are called "Pitzer equations". Models based on the Pitzer equations are used to calculate chemical speciation of any element, providing a key to establish the reactivity of this element, as for example its ability to be assimilated by the phytoplankton.

David Turner and his colleagues, all members of the SCOR Working Group 145 propose an overview of work for the development of a quality-controlled chemical speciation model for seawater and related systems, including descriptions of the different applications that can benefit from the model (open ocean acidification; micronutrient biogeochemistry and geochemical tracers; micronutrient behavior in laboratory studies; water quality in coastal and estuarine waters; cycling of nutrients and trace metals in pore waters; chemical equilibrium in hydrothermal systems; brines and salt lakes).

16 WG145 SCORFigure: Some members of the SCOR WG145 (from left to right): David Turner, Christoph Voelker, Andrew Dickson, Arthur Chen, Eric Acherberg, Ed Urban, Alessandro Tagliabue, Mona Wells, Sylvia Sander, Stan van den Berg, Rodrigo Torres, Vanessa Hatje and Ivanka Pizeta.

Reference:

Turner, D. R., Achterberg, E. P., Chen, C.-T. A., Clegg, S. L., Hatje, V., Maldonado, M. T., Sander, G.S., van den Berg, C.M.G., Wells, M. (2016). Toward a Quality-Controlled and Accessible Pitzer Model for Seawater and Related Systems. Frontiers in Marine Science, 3, 139. doi:10.3389/fmars.2016.00139

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