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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.).

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 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 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

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

Lithogenic influence from the Hawaiian Islands detectable up to Station ALOHA surface waters

By coupling neodymium (Nd) and radium (Ra) isotopes and Rare Earth Element (REE) signals, Henning Fröllje and co-workers show that the coastal Hawaiian waters are affected by a prominent lithogenic influence from the Hawaiian Islands. Rare earth patterns, radiogenic εNd signatures and high 228Ra levels are clearly tracing this influence. Moreover, it is perceptible as far as ALOHA station (100 km north). This influence at ALOHA is most pronounced in February, when precipitation on the islands is highest and dust input from Asia is low. In summer, however, Nd isotopes are shifted towards the Asian dust endmember (εNd = -10), indicating seasonal dust influence overlying the Hawaiian background signal.

A close study of the rare earth distribution and speciation confirm that these elements are truly dissolved in seawater and that they are following water mass advection and mixing in the intermediate and deep central North Pacific Ocean.

16 FrolijelFigure: Location of station ALOHA and sampling stations around Oahu, Hawaii, along with εNd and Nd concentration profiles. ALOHA full water column: εNd, Nd concentrations, and Radium-228 at ALOHA. Seasonal: seasonal εNd of the upper 800m, showing a shift to higher dust influence in June-August compared to February. Oahu: εNd and Nd concentration profiles at coastal sites around Oahu. Click here to view the figure larger.

Reference:

Fröllje, H., Pahnke, K., Schnetger, B., Brumsack, H.-J., Dulai, H., & Fitzsimmons, J. N. (2016). Hawaiian imprint on dissolved Nd and Ra isotopes and rare earth elements in the central North Pacific: Local survey and seasonal variability. Geochimica et Cosmochimica Acta, 189, 110–131. doi:10.1016/j.gca.2016.06.001

Inference about rates of thorium and particle cycling in the ocean water column

Insoluble thorium (Th) isotopes are particle reactive while their radioactive parents are fairly soluble. This difference of behaviour has allowed chemical oceanographers to use Th isotopes to develop understanding about the scavenging of particle reactive elements and the cycling of particles in the water column. However, many prior models rely on numerous assumptions. Among these assumptions is vertical uniformity of the rate parameters governing sorption reactions and particle processes in the mesopelagic zone and in the deeper regions of the water column (Th adsorption and desorption, particle remineralization and settling speed, etc.).

In this work, Paul Lerner and co-authors use Th and particle data collected at station GT11-22 of the GEOTRACES North Atlantic transect (section GA03) in order to test two models of Th and particle cycling: a conventional one that assumes uniform rate parameters and another one that allows the rate parameters to vary with depth. They show that the second model leads to a significantly better fit to the data, thereby challenging the assumption of parameter uniformity in the conventional model. Moreover, by combining the second model with the data, they diagnose the various terms in the depth-dependent Th isotope budgets at GT11-22, showing in particular that the behaviour of 230Th is dominated by a balance between adsorption and desorption over most of the water column.

16 Lernerl

Figure: Inversion of radiochemical and particle data for station GT11-22 of section GA03 (19º26’ N, 29º 22’ W). The data used include measurements of dissolved and particulate 228,230,234Th, 228Ra, particle concentration, and observational estimates of 234,238U. Panel (a) shows the budget of dissolved 230Th (left box) and particulate 230Th (right box) in terms of vertical averages (dpm m-3 yr-1) along the water column (below 125 m). The numbers are estimated fluxes and their estimated errors. Panels (b) and (c) show the vertical distribution of the 230Th fluxes.

Reference:

Lerner, P., Marchal, O., Lam, P. J., Anderson, R. F., Buesseler, K., Charette, M. A., Edwards, R. L., Hayes, C. T., Huang, K-F., Lu, Ya., Robinson, L F., Solow, A. (2016). Testing models of thorium and particle cycling in the ocean using data from station GT11-22 of the U.S. GEOTRACES North Atlantic section. Deep Sea Research Part I: Oceanographic Research Papers, 113, 57–79. doi:10.1016/j.dsr.2016.03.008

 

 

Hydrothermalism: a major contributor to the oceanic inventory of dissolved zinc

High-quality dataset of dissolved zinc (Zn) from the US GEOTRACES EPZT cruise (GP16 section) allows Roshan and co-workers to revisit the oceanic budget of dissolved Zn. These investigators have observed a basin-scale transport of mantle-derived dissolved Zn from the hydrothermal vents of the East Pacific Rise to the farfield regions (>4000 km away) across the South Pacific at depths 2300–2800 m (see Figure). The strong correlation (R2 = 0.9) between hydrothermal-controlled component of dissolved Zn (i.e. Excess Zn) and hydrothermal activity tracer, 3He leads to a global hydrothermal zinc flux of 1.75 ± 0.35 G mol yr− 1 that is many-fold higher than the input fluxes estimated by other studies. These results suggest that mantle-derived dissolved Zn dominates the oceanic Zn inventory and that dissolved Zn residence time is much shorter (3000 ± 600 years) than previous input-based estimates (11,000 and 50,000 years) and more consistent with previous removal-based estimate (3000–6000 years)...

16 Roshan lFigure: The top panel shows the dispersion of hydrothermal-controlled component of dissolved Zn (Excess Zn) from the East Pacific Rise (elevated topography at the east of the transect and shown in the bottom left panel) to the west of the transect (>4000 km). Excess Zn shows a strong correlation with hydrothermal tracer, 3He which allows for the accurate estimation of hydrothermal dissolved Zn input rate of 1.75 G mole/year.

Reference:

Roshan, S., Wu, J., & Jenkins, W. J. (2016). Long-range transport of hydrothermal dissolved Zn in the tropical South Pacific. Marine Chemistry, 183, 25–32. doi:10.1016/j.marchem.2016.05.005

Decreasing of the industrial lead contamination in the Amundsen Sea area

Since the phasing-out of leaded automobile gasoline, the imprint of industrial lead is decreasing everywhere. Antarctica snow and ice are no exception to this general rule, as shown by measurements of lead (Pb) concentration and isotopic composition in these environments as well as in the Amundsen Sea Polynya. Up to 95% of the Pb at the time of sampling is natural in source, allowing tracing weathering from Antarctic rocks....

16 Ndungu lPhoto: Silke Severmann, Rutgers University.

Read more: Decreasing of the industrial lead contamination in the Amundsen Sea area

More Articles ...

  1. Scandium: a new oceanic tracer with surprising properties
  2. Gadolimium, a Rare Earth Element becoming a human contaminant and tracer of wastewater discharge in the ocean
  3. 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
  4. Are the dissolved iron distributions well represented by the global ocean biogeochemistry models?
  5. An example of a fruitful international intercomparison
  6. All mercury species measured along the GEOTRACES-UK section, South Atlantic Ocean
  7. Amazingly detailed compilation of the silicon cycle, with an emphasis on the oceanic silicon isotope budget
  8. Water masses traced by neodymium isotopic compositions at an unprecedented level in the North Atlantic Ocean
  9. Important warning about the uncertainties affecting results of dissolved iron concentration measurements in seawater using flow-injection with chemiluminescence detection
  10. When a multi-parameter end-member mixing model allows a quantitative deconvolution of the dissolved rare earth elements behaviour
  11. What do the first 236-Uranium data reveal in the Arctic Ocean?
  12. Using chromium isotopes to reconstruct the oxygenation history of the oceans is challenged by modern data
  13. Impressive set of data reveal new features on the modern cadmium–phosphate relationship
  14. Lead isotopes tracks leakage of Indian Ocean seawater into the Atlantic Ocean
  15. Helium isotopes help to constrain high-resolution model dynamics in the Mediterranean Sea

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