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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.).

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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

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

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

Astonishing protactinium and thorium profiles in the Mediterranean Sea

In the framework of the GEOTRACES cruise along the GA04 section in the Meditterranean Sea, Gdaniec et al. (2018, see reference below) measured thorium (Th) and protactinium (Pa) isotope distributions on 8 profiles across the Mediterranean Sea. Contrasting with what is observed in the open ocean:

  • Depth profiles of these tracers are non linear, indicating that these profiles are overprinted by deep water circulation. These bended shapes allow identifying convection processes in the NW basin and occurrence of depleted Aegean and enriched Adriatic waters.
  • 99% of the 230Th and 94% of the (although more soluble) 231Pa in situ produced are scavenged and deposited within the Mediterranean Sea.
  • The fractionation factors between Th and Pa (FTh/Pa) are low, reflecting the important removal of the 231Pa compare to the open ocean.
  • The 232Th distribution mainly reflects the input of lithogenic material from rivers and/or sediment resuspension.

18 Gdniec
Figure:
 Map of (a) Pa-Th sampling sites and and section plots of (b) 231Pa, (c) 230Th, and (d) 232Th measured in unfiltered seawater sampled along the GEOTRACES GA04 section in the Mediterranean Sea. SABB: Southern Algero-Balearic Basin, CABB: Central Algero-Balearic Basin and NABB: Northern Algero-Balearic Basin.

Reference:

Gdaniec, S., Roy-Barman, M., Foliot, L., Thil, F., Dapoigny, A., Burckel, P., Garcia-Orellana, J., Masqué, P., Mörth, C-M., Andersson, P. S. (2018). Thorium and protactinium isotopes as tracers of marine particle fluxes and deep water circulation in the Mediterranean Sea. Marine Chemistry, 199, 12–23. http://doi.org/10.1016/J.MARCHEM.2017.12.002

Surface South Pacific ecosystems reflect the availability of the nutrients iron, nitrate and phosphate

Thanks to a high resolution section across the South Pacific (150°E-150°W, GEOTRACES GP13 cruise), Ellwood and co-workers (2018, see reference below) identify that the gradient of sources and fates of the 3 nutrients iron (Fe), nitrogen (N) and phosphorus (P) is explaining the observed ecosystem west-east gradient. In the west, phytoplankton able to fix atmospheric nitrogen (diazotroph species) is abundant while it is the opposite in the eastern end of the section. As shown in the figure, such drop of the diazotroph species is due to the low abundance of Fe in the most remote part of the section.

18 Ellwood figureFigure: Cartoon showing the input fluxes for iron (Fe), nitrogen (N) and phosphorus (P) into surface ocean across the GP13 zonal section. In the west, diazotrophs are abundant while it is the opposite in the eastern end of the section due to the low abundance of Fe, in the most remote part of the section. Click here to view the figure larger.

Reference:

Ellwood, M. J., Bowie, A. R., Baker, A., Gault-Ringold, M., Hassler, C., Law, C. S., Maher, W. A., Marriner, A., Nodder, S., Sander, S., Stevens, C., Townsend, A., van der Merwe, P., Woodward, E. M. S., Wuttig, K., Boyd, P. W. (2018). Insights Into the Biogeochemical Cycling of Iron, Nitrate, and Phosphate Across a 5,300 km South Pacific Zonal Section (153°E-150°W). Global Biogeochemical Cycles, 32(2), 187–207. http://doi.org/10.1002/2017GB005736

Why did the concentration of atmospheric carbon dioxide rise so much and so quickly during the last deglaciation? 

During the Last Glacial Maximum, the deep southern Pacific waters were stratified, efficiently accumulating old, CO2 rich waters. Basak and co-authors (2018, see reference below) measured neodymium isotopes in sediment cores that clearly show that when these deep waters became less stratified as the climate warmed they released their carbon which could escape to the atmosphere...what a tempting prospect and beautiful teaser for the forthcoming PAGES-GEOTRACES workshop of December 2018!

 18 Pahnke l
Figure: View from RV Polarstern while collecting sediment samples used in the study by Basak et al.
Read more at: https://phys.org/news/2018-02-scientists-theory-role-south-pacific.html#jCp
Credit: Dr. Katharina Pahnke


Reference:

Basak, C., Fröllje, H., Lamy, F., Gersonde, R., Benz, V., Anderson, R. F., Molina-Kescher, M., Pahnke, K. (2018). Breakup of last glacial deep stratification in the South Pacific. Science, 359(6378), 900–904. http://doi.org/10.1126/science.aao2473

Read more also at: https://phys.org/news/2018-02-scientists-theory-role-south-pacific.html#jCp

 

Particle distribution in repeated ocean sections and sediment resuspension

This is the first compilation of an expansive data base of transmissometer data on a decadal period of time. More than 7376 stations have been analyzed for “cp” value, a proxy for particle concentrations, by Gardner and co-workers (2018, see reference below). Full water-column sections confirm that particle concentrations are higher in surface waters, decrease rapidly below 200 m, most often down to the seafloor. However, cloudy near-bottom waters, known as “benthic nepheloid layers”, are generated by sediment erosion and resuspension at specific geographic areas. These locations are directly linked to energetic surface dynamics that produce regions of high Eddy Kinetic Energy (EKE). More fascinating, however, is the decadal persistence of this close surface-to-deep connection. We invite you to read this very interesting article!

18 Gardner2Figures: (A) Map of log of surface eddy kinetic energy (modified from Dixon et al., 2011) with cp transects indicated. (B) Section of cp (proxy for particle concentration) along 53° W in spring, 2012 in the Western North Atlantic. Black contours are dissolved oxygen (µmol kg-1). Click here to view the figure larger.

Reference:

Gardner, W. D., Mishonov, A. V., & Richardson, M. J. (2018). Decadal Comparisons of Particulate Matter in Repeat Transects in the Atlantic, Pacific, and Indian Ocean Basins. Geophysical Research Letters. http://doi.org/10.1002/2017GL076571

Where, how and which trace elements are released from dust at the sea surface?

Alex Baker and Tim Jickells (2017, see reference below) propose to answer to this question thanks to analysis of aerosols collected in the framework of the Atlantic Meridional Transect (AMT). They established the soluble concentrations of a range of trace metals (iron, aluminium, manganese, titanium, zinc, vanadium, nickel and copper) and major ions. They reveal much higher inputs to the North Atlantic Ocean compared to the South Atlantic Ocean, reflecting stronger land based emission sources in the Northern Hemisphere. Comparison of these inputs with the surface water contents of the same trace metals compiled in the GEOTRACES intermediate data product show surprising features that you will discover if you read this paper…

18 Baker lowFigures: (A) Approximate tracks of the AMT cruises (dots and triangles) and general flow directions of the seven major atmospheric transport routes encountered during the cruises (arrows). Abbreviations for the air transport regimes are: continental Europe (EUR), North Africa including the Sahara and Sahel: (SAH), Southern Africa impacted by biomass burning emissions (SAB), Southern Africa not impacted by biomass burning (SAF), South America (SAM), remote North or South Atlantic i.e. not crossing land for at least 5 days prior to collection (RNA and RSA respectively). (B) Box and whisker plots showing the variations in the concentrations of iron, aluminium, manganese, titanium, zinc, vanadium, nickel and copper with air transport/source type for the AMT transect. They reveal much higher inputs to the North Atlantic Ocean, reflecting stronger land based emission sources in the Northern Hemisphere. Please click here to view the figure larger. (Figures modified from Progress in Oceanography)

Reference:

Baker, A. R., & Jickells, T. D. (2017). Atmospheric deposition of soluble trace elements along the Atlantic Meridional Transect (AMT). Progress in Oceanography, 158, 41–51. http://doi.org/10.1016/j.pocean.2016.10.002

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