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Science Highlights


Some recent GEOTRACES science findings are reported below.  
When getting older they are compiled in the Science Highlights Archive where the "Title Filter" search box will allow you to filter them by words in title (please note that only one-word search queries are allowed e.g. iron, Atlantic, etc.).

Dissolved iron isotopes reveal that distinct processes are controlling this micronutrient distribution in the ocean

Abadie and co-workers propose new dissolved iron concentration and isotopic composition distributions (DFe and ICFe respectively) along the Bonus-Goodhope IPY section (GIPY4), in the Atlantic sector of the Southern Ocean. DFe vertical profiles display a continuous increase with depth (see figure B), classically interpreted as due to biological uptake at the surface followed by remineralization at depth. However, heterogeneous profiles of ICFe (see figure B) suggest a more complicated story driven by distinct processes, discussed here for the first time. Indeed, the authors demonstrate that in the intermediate waters, DFe primarily originates from remineralization of organic matter and the redistribution of this regenerated DFe through mixing. Moreover it is also due to horizontal advection of DFe released by reducing sediments of the nearby South African coast. The scheme changes deeper, where abiotic processes are dominating the DFe distribution as for example non-reductive release of DFe from lithogenic particles. This last process would add an additional source to the global oceanic DFe budget, which should be considered in the biogeochemical models. In addition, it suggests that the oceanic DFe budget could be more sensitive than previously thought to continental erosion, particle transport, and dissolved/particle interactions.

17 Lacan l2

Figure: (A) Position of the 5 stations sampled for iron isotopes during the Bonus-Goodhope IPY GEOTRACES cruise; (B) Examples of profiles for dissolved iron concentration (DFe) and dissolved iron isotopic composition (expressed by δ56DFe) obtained in one of the 5 stations sampled (other stations show similar patterns); Iron isotopes show a very sharp minimum in intermediate depths (i.e. between about 200 and 1500 m below the surface). The contrast between these intermediate depths and the deep ocean (3000-5000 m) demonstrates that two different processes dominate dissolved iron sources in the ocean at these two levels. (C) Dissolved iron isotopic composition (δ56DFe) along the Bonus-Goodhope section. Negative values (in cold colours, blue, green) indicate iron that is naturally enriched in light isotopes, while high values (in warm colours, red, orange) indicate heavy iron isotopes enrichment. Click here to view the figure larger.

 Reference:

Abadie, C., Lacan, F., Radic, A., Pradoux, C., Poitrasson F. (2017) Iron isotopes reveal distinct dissolved iron sources and pathways in the intermediate versus deep Southern Ocean, PNAS, DOI: 10.1073/pnas.1603107114

Up to four mercury species measured along the GEOTRACES East Pacific Zonal Transect

Total mercury (HgT), elemental mercury (Hg0), monomethyl-mercury (MMHg), and dimethyl-mercury (DMHg) were measured at an unprecedent resolution so far along the East Pacific Zonal Transect (EPZT) GEOTRACES cruise (GP16). Knowing that MMHg is a neurotoxin that accumulates in the trophic chain, establishing the full speciation of this element is required in order to better understand oceanic Hg cycle and more specifically where and how MMHg is formed and accumulated.

The other exciting feature of this article is that contrasted biogeographic oceanic provinces have been sampled during EPZT (Peru upwelling region, a suboxic OMZ, and an expansive submarine hydrothermal vent plume).

Bowman and co-authors results show that while HgT is not enriched in the hydrothermal plume, total Hg is accumulated with age and that DMHg is the dominant methylated species in deep waters. Importantly, filtered HgT is accumulated in upwelled waters near the coast of Peru where oxygen concentrations were the lowest, and MMHg represented 10–20% of the total Hg upwelling flux. This is the first identification of Hg behaviour along the Peru coast, a region that supports the largest fisheries of the world...

16 Bowman
Figures:
Images depict mercury concentrations in the water column from the coast of Peru (right), across the Eastern Pacific Rise (EPR), to Tahiti (left), measured during the 2013 U.S. GEOTRACES Eastern Pacific Zonal Section. Total mercury (HgT), gaseous elemental mercury (Hg0), methylmercury (MMHg), and dimethylmercury (DMHg) in filtered water is shown, in addition to methylmercury in suspended particles (MMHgpart.). Click here to view the figure larger.

Reference:

Bowman, K. L., Hammerschmidt, C. R., Lamborg, C. H., Swarr, G. J., & Agather, A. M. (2016). Distribution of mercury species across a zonal section of the eastern tropical South Pacific Ocean (U.S. GEOTRACES GP16). Marine Chemistry, 186, 156–166. DOI: 10.1016/j.marchem.2016.09.005

Water mass circulation and weathering inputs in the Labrador Sea based on coupled hafnium-neodymium isotope compositions and rare earth element distributions

Filippova and co-authors (2017, see reference below) show distinct water mass signatures in the Labrador Sea revealed by combined dissolved hafnium (Hf) and neodymium (Nd) isotope compositions and REE distribution patterns along the AR7W transect in May 2013.

The new data show that in a semi-enclosed basin such as the Labrador Sea, the radiogenic Hf isotope signatures can serve as a highly sensitive tracer of water mass mixing processes given that they allow distinction of particular water masses that do not differ in their Nd isotope compositions. Based on the new data, the authors suggest that the residence time of Hf in the Labrador Sea can only be on the order of decades in order to sustain the observed variability. The high sensitivity of Hf isotopes to decadal ocean circulation changes in the Labrador Sea suggests a potential prospect for their application in other restricted basins with similar geological settings and pronounced short-term hydrographic variability.

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 17 Filipovafig2Figures: (top) Schematic map of the study area. Blue arrows represent cold deep currents and red arrows denote warm surface currents. Red dots indicate the positions of the stations occupied during CCGS Hudson Cruise 2013. A schematic representation of the geology of the surrounding landmasses is shown and includes average ɛHf and ɛNd values of the rocks. Please click here to view the figure larger. (bottom) Water masses distribution versus depth in the Labrador Sea based on their ɛNd (A) and ɛHf (B) signatures. Please click here to view the figure larger.

Reference:

Filippova, A., Frank, M., Kienast, M., Rickli, J., Hathorne, E., Yashayaev, I.M., and Böning, P. (2017): Water mass circulation and weathering inputs in the Labrador Sea based on coupled Hf-Nd isotope compositions and rare earth element distributions.- Geochimica et Cosmochimica Acta 199, 164-184. DOI: 10.1016/j.gca.2016.11.024

Oxygen biogeochemistry exerts a strong influence on cobalt cycling

This is an important result of the US GEOTRACES East Pacific Zonal Transect (EPZT) cruise (GP16) discussed by Hawco and his co-workers (2016, see reference below). The distribution of dissolved cobalt and labile cobalt along this section is closely tied to the oxygen minimum zone. This work also shows that (1) elevated concentrations of labile cobalt are generated by input from coastal sources and reduced scavenging at low oxygen; (2) atmospheric deposition and hydrothermal vents along the East Pacific Rise are contrastingly minor sources of cobalt; (3) high cobalt waters are further upwelled and advected offshore and; (4) phytoplankton export returns cobalt to low-oxygen water masses underneath. These processes result in covariation of dissolved cobalt with oxygen and phosphates, schematically represented in the Figure below.

16 Hawco lFigure: In the South Pacific Ocean, high levels of cobalt are harbored in waters that are devoid of dissolved oxygen (upper panel, warm colors). This plume of cobalt stems from the Peru coast and is enhanced by degradation of cobalt-bearing phytoplankton in these waters, and by the absence of removal processes (scavenging) when oxygen is low (lower panel). Please click here to view the figure larger.

Reference:

Hawco, N. J., Ohnemus, D. C., Resing, J. A., Twining, B. S., & Saito, M. A. (2016). A dissolved cobalt plume in the oxygen minimum zone of the eastern tropical South Pacific. Biogeosciences, 13(20), 5697–5717. DOI: 10.5194/bg-13-5697-2016

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

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

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

More Articles ...

  1. Dealing with the chemical speciation of the elements in the different oceanic realms
  2. Lithogenic influence from the Hawaiian Islands detectable up to Station ALOHA surface waters
  3. Inference about rates of thorium and particle cycling in the ocean water column
  4. Hydrothermalism: a major contributor to the oceanic inventory of dissolved zinc
  5. Decreasing of the industrial lead contamination in the Amundsen Sea area
  6. Scandium: a new oceanic tracer with surprising properties
  7. Gadolimium, a Rare Earth Element becoming a human contaminant and tracer of wastewater discharge in the ocean
  8. Onboard analysis of dissolved zinc everywhere in the open ocean with a Lab on Valve (LOV) system of the size of a bottle of wine is becoming possible
  9. Are the dissolved iron distributions well represented by the global ocean biogeochemistry models?
  10. An example of a fruitful international intercomparison
  11. All mercury species measured along the GEOTRACES-UK section, South Atlantic Ocean
  12. Amazingly detailed compilation of the silicon cycle, with an emphasis on the oceanic silicon isotope budget
  13. Water masses traced by neodymium isotopic compositions at an unprecedented level in the North Atlantic Ocean
  14. Important warning about the uncertainties affecting results of dissolved iron concentration measurements in seawater using flow-injection with chemiluminescence detection
  15. When a multi-parameter end-member mixing model allows a quantitative deconvolution of the dissolved rare earth elements behaviour

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