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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.).

Complex cobalt story in the Mediterranean Sea

Dulaquais and co-authors (2017, see reference below) propose the first comprehensive study of cobalt behaviour in the Mediterranean Sea, work conducted in the framework of MedBlack GEOTRACES cruise (GA04N). They measured the following cobalt (Co) fractions: soluble (sCo<0.02 μm), dissolved (DCo<0.2 μm), colloidal (cCo, as DCo minus sCo), and particulate (pCo>0.2 μm).

While soluble Co is the predominant form (90%) of the dissolved Co in the Mediterranean Sea, colloidal Co and particulate Co show a close distribution, yielding the authors to suspect a biogeochemical link between these two fractions.

More striking is the scavenged-like profile observed everywhere, with up to 350 nM dissolved Co concentrations in the surface waters dropping to 45 nM at depth. Such behaviour results from several mechanisms. High-surface Co inputs at Gibraltar Strait are horizontally transported by the Mediterranean circulation, surface dissolved Co is stabilized in a soluble form and biogenic particulate Co is very rapidly regenerated: all these processes concur to the accumulation of dissolved Co in the surface layers. Conversely, low particulate Co export, low remineralization of biogenic particulate Co at depth, and removal of dissolved Co by scavenging prevented its accumulation in the intermediate and deep sea.

17 Dulaquais l

Figure: Distribution and partitioning of dissolved cobalt (DCo) in the Mediterranean Sea. We measured DCo along the GA04N section (a) and observed a scavenged like profile in all the different sub-basins of the Mediterranean Sea (b). In the Med, DCo was almost entirely composed of soluble cobalt (sCo) and colloids represented only 10% of the DCo pool (c). Resulting from high recycling rate and its stabilization under a soluble form, surface DCo concentrations increased eastward with ageing of surface waters (d). Differently accumulation of DCo by remineralization in the intermediate water was not discernable (d) and surprisingly the zonal distribution of DCo in the deep sea showed homogenous concentrations (d). We related these features to scavenging rates depth dependents and of different magnitude in the two Mediterranean basins as well as to the fast Mediterranean circulation that homogenize concentrations in the deep sea. Click here to view the figure larger.

Reference:

Dulaquais, G., Planquette, H., L’Helguen, S., Rijkenberg, M. J. A., & Boye, M. (2017). The biogeochemistry of cobalt in the Mediterranean Sea. Global Biogeochemical Cycles, 31(2), 377–399. DOI: 10.1002/2016GB005478

 

 

A new method for simultaneous analysis of nickel, copper and zinc isotopes in seawater

Takano and co-workers (2017, see reference below) have developed a new method to determine nickel (Ni), copper (Cu) and zinc (Zn) isotopes in seawater. This method is very simple and rapid only using single chelating extraction and single anion exchange. First, target metals are extracted from seawater by NOBIAS Chelate PA-1 resin. Then, target metals are purified by anion exchange. Finally, isotope ratios are measured by MC-ICPMS. The analyses of GEOTRACES reference samples showed this method is precise and accurate. Vertical profiles of δ60Ni, δ65Cu, and δ66Zn in the South Pacific Ocean were revealed using this method.

This method is expected to accelerate isotopic research and contribute to our understanding of biogeochemical cycling in the ocean.

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Figure: A schematic diagram of the procedure for isotopic analysis of dissolved Ni, Cu, and Zn in seawater (upper panel). Depth profiles of dissolved δ60Ni, δ65Cu, and δ66Zn at GR15 station (30.00°S, 170.00°W) in the subtropical South Pacific Ocean. The error bars represent 2 standard errors for the MC-ICPMS measurement (lower panel). Click here to view the figure larger.

Reference:

Takano, S., Tanimizu, M., Hirata, T., Shin, K.-C., Fukami, Y., Suzuki, K., & Sohrin, Y. (2017). A simple and rapid method for isotopic analysis of nickel, copper, and zinc in seawater using chelating extraction and anion exchange. Analytica Chimica Acta, 967, 1–11. DOI: 10.1016/j.aca.2017.03.010

 

GEOTRACES intercalibration of the stable silicon isotope composition of dissolved silicic acid in seawater

Dissolved silicon (Si) is a major oceanic nutrient and variations of its stable isotope values are reflecting the intensity of surface primary production. As for other isotopes, agreement between the different laboratories is crucial. Here, the first intercalibration study of the stable silicon (Si) isotopes in seawater (δ30Si(OH)4) is presented as a contribution to the international GEOTRACES programme. 

Eleven laboratories from seven countries participated in the study. Si isotope measurements were performed on three different mass spectrometer types Neptune MC-ICP-MS, Nu Plasma MC-ICP-MS and a MAT 252 IRMS.

Two seawater samples from the North Pacific subtropical gyre (Station ALOHA) collected at 300 m (9 μmol Si L-1; ALOHA300) and at 1000 m (113 μmol Si L -1; ALOHA1000) water depth were analyzed.

Agreement among laboratories is considered very good with mean values for δ30Si(OH)4:

  • ALOHA300 = +1.68 ± 0.35 (2 s.d.; Median: +1.66 ± 0.13)
  • ALOHA1000 = +1.24 ± 0.20 (2 s.d.; Median: +1.25 ± 0.06)

For future studies analyzing δ30Si(OH)4 in seawater it is recommended to analyze ALOHA300 and ALOHA1000 and report these results to facilitate and evaluate comparability of data between laboratories.

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Figure: δ30Si(OH)4 results from all groups for ALOHA300 (red circles) and ALOHA1000 (blue circles). The vertical lines indicates the mean value of all measurements for ALOHA1000 (blue) and for ALOHA300 (red). The data points represent the individual δ30Si(OH)4 values for Si isotopes measurements. Short vertical solid lines are the means obtained by individual laboratories for the two samples. Uncertainty in the mean for all measurements (2 s.d.) is indicated by the horizontal bars at the top of the figure (Modified from Grasse et al. 2017, JAAS). Click here to view the figure larger.

Reference:

Grasse, P., Brzezinski, M. A., Cardinal, D., de Souza, G. F., Andersson, P., Closset, I., et al. (2017). GEOTRACES inter-calibration of the stable silicon isotope composition of dissolved silicic acid in seawater. Journal of Analytical Atomic Spectrometry, 32, 562–578. DOI: 10.1039/C6JA00302H

Important external dissolved iron inputs, HNLC water formation and strong biological seasonality explained in the North Pacific Ocean

Subarctic Pacific is known as High Nutrient, Low Chlorophyll (HNLC) area, where phytoplankton growth is limited by dissolved iron (DFe) availability. The biological activity in the surface waters is also characterized by a marked seasonality. Nishioka and Obata (2017, see reference below) propose a detailed DFe zonal section across the North Pacific (~47°N), realized in the framework of Japan-GEOTRACES programme. Their data reveal important external Fe sources at mid-depth from the Sea of Okhotsk, and the continental margin followed by long range transport in the formation of Fe-rich intermediate water, although these enriched waters do not reach the Alaskan Gyre. This work also enlights why surface macronutrient consumption differ between the western and eastern gyre. Both HNLC water formation in the subarctic Pacific and high amplitude of seasonal variation in biogeochemical parameters in the western subarctic gyre are explained.

17 Nishioka

Figure: Dissolved Fe-rich intermediate water is transported laterally and distributed across the western subarctic gyre, over 2000 km (upper panel). The spatial pattern of Fe to nutrient stoichiometry supplied from the intermediate water to the surface (lower panel). Line P data is cited by Martin et al. 1989 and Nishioka et al. 2001. (OKTZ: Oyasio-Kuroshio transition zone, WSG: western subarctic gyre, CNP: central North Pacific, AG: Alaskan Gyre, AS: Alaskan stream). Click here to view the figure larger.

References:

Nishioka, J. and H. Obata, 2017, Dissolved iron distribution in the western and central subarctic Pacific: HNLC water formation and biogeochemical processes, Limnology and Oceanography, doi: 10.1002/lno.10548

Nishioka, J., S. Takeda, C. S. Wong, and W. K. Johnson. 2001. Size-fractionated iron concentrations in the northeast Pacific Ocean: distribution of soluble and small colloidal iron. Mar. Chem. 74: 157-179. doi:10.1016/S0304-4203(01)00013-5

Martin, J. H., R. M. Gordon, S. Fitzwater, and W.W. Broenkow. 1989. VERTEX: Phytoplankton/iron studies in the Gulf of Alaska. Deep Sea Res. Part I Oceanogr. Res. Pap. 36(5): 649–680. doi:10.1016/0198-0149(89)90144-1

 

Disentangling the paleo signals brought by neodymium isotopic composition

Which paleo information are traced by the imprint neodymium isotopic signatures (εNd) in the different archives: paleo circulation or paleo erosion? The scientific debate on this issue is opened for years now. Tachikawa and co-workers (2017, see reference below) are proposing an important step forward with a thorough review of the fate of εNd in the modern ocean combined with other tracers as hydrography parameters (temperature, salinity, nutrients, oxygen), but also dissolved carbon-14 (14C) and δ13C. In addition, they compiled archive εNd data for leachates, foraminiferal tests (left picture), deep-sea corals (middle picture) and fish teeth/debris (right picture) from the Holocene.

Analytical treatment of these important updated databases allows these authors to draw the following general schemes:

  • At water depths ≥1500 m, large-scale water mass mixing is a primary control of deep water εNd distribution.
  • At ≥200 m, basin-scale seawater temperature-salinity-εNd diagrams demonstrate the isotopic evolution of different water masses.
  • At 600–1500 m water depths, the relationships are weaker. Basin-scale seawater vertical εNd profiles demonstrate larger variability.
  • At surface and subsurface depths, suggesting local/regional detrital influence at shallower water depths.
  • Empirical equations were established to predict the main, largescale, deepwater εNd trends from hydrography parameters revealing that continental influence on seawater and archive εNd is observed mainly within 1000 km from the continents.
  • Seawater and archive εNd values present clear latitudinal trends in the Atlantic and Pacific Oceans at water depths ≥600 m: this reinforces the potentiality of Nd isotopes to distinguish between northern/southern sourced water contributions at intermediate and deep water depths in the present and past ocean.

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Figure: Major latitudinal trends of seawater (a and c) and archive (b and d, and pictures blow) εNd values in the Atlantic and Atlantic sector of the Southern Ocean (a and b) and Pacific and Pacific sector of the Southern Ocean (c and d) at ≥ 600 m. Major water masses are indicated. The grey dots on archive figures show seawater values. All the figures are created using ODV (Schlitzer, 2015). Click here to view the figure larger. 

Reference:

Tachikawa, K., Arsouze, T., Bayon, G., Bory, A., Colin, C., Dutay, J.-C., Frank, N., Giraud, X., Gourlan, A. T., Jeandel, C., Lacan, F., Meynadier, L., Montagna, P., Piotrowski, A. M., Plancherel, Y., Pucéat, E., Roy-Barman, M., Waelbroeck, C. (2017). The large-scale evolution of neodymium isotopic composition in the global modern and Holocene ocean revealed from seawater and archive data. Chemical Geology, In press. DOI: 10.1016/j.chemgeo.2017.03.018

 

Contrasting lithogenic inputs from North Atlantic to North Pacific Oceans traced by thorium isotopes

Dissolved thorium (Th) isotopes and iron (Fe) are used to document the transfer of lithogenic material to the ocean.

Two contrasting areas are compared: the Atlantic Ocean around Barbados Islands, under the influence of the Amazon plume and dust of Saharan origin, and the remote North East Pacific Ocean, far from dust inputs. 

The Amazon is a substantial source of dissolved 232Th and iron (Fe) to the low-latitude Atlantic Ocean, even as far away a 1900 km from the river’s mouth. This complicates the use of 232Th as a dust proxy in river-influenced ocean regions.

A striking feature is the similarity in Fe concentrations from the North Pacific to the North Atlantic Oceans, while 232Th reveals a dust flux six fold higher in the later. This supports the idea that dissolved Fe distribution is highly buffered in the ocean.

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Figure: The North Atlantic Ocean receives a much larger input of mineral dust blown from the continents than does the remote North Pacific. This contrast is seen clearly in the seawater concentrations of dissolved Thorium-232, the isotope of thorium that is enriched in the continental crust (left panel). The distribution of Fe, however, is much more homogeneous between these two ocean basins (right panel), despite that fact that continental dust is the major source of Fe in these areas. We think this is because Fe is highly buffered in the ocean by a combination of biological uptake, adsorption onto particles, and complexation by organic molecules, or ligands. See our paper for the colloidal nature of these dissolved metals and for evidence of a large input of metals from the Amazon River. Click here to view the figure larger.

Reference:

Hayes, C. T., Rosen, J., McGee, D., & Boyle, E. A. (2017). Thorium distributions in high- and low-dust regions and the significance for iron supply. Global Biogeochemical Cycles, 31, 1–20. DOI: 10.1002/2016GB005511

 

What controls hydrothermal plume transport of iron over 4000 km in the deep Pacific Ocean?

The striking extension of the dissolved iron and manganese plumes over more than 4000 km from their hydrothermal sources along the US GEOTRACES East Pacific Zonal Transect (EPZT) cruise (GP16) has challenged our understanding of these element cycles (Resing et al., 2015 see GEOTRACES science highlight).

Fitzsimmons and co-workers (2017, see reference below) analysed the particulate iron and manganese (Mn) in the same plume and showed that they also exceed background concentrations, even 4,000 km from the vent source, despite anticipated gravitational settling losses. Both dissolved and particulate Fe plumes deepen by more than 350 m relative to the conservative helium-3 (3He) one, while the Mn plumes do not show such descent.

Based on Fe speciation and isotope data, the authors suggest that dissolved iron fluxes and geospatial positioning may depend on the balance between stabilization in the dissolved phase by organic ligands and the reversibility of exchange onto sinking particles.

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Figure:  Interpolated concentrations and station map along the US GEOTRACES EPZT (GP16) section. a, Map of the station locations (colours corresponds to bathymetry; green hues shallower) b, Excess 3He concentrations in fmol kg−1. c, Dissolved Fe concentrations (<0.2 µm, in nM). d, Dissolved Mn concentrations (<0.2 µm, in nM). e, Particulate Fe (>0.45µm, in nM). f, Particulate Mn (>0.45µm, in pM). The black reference line at 2,500m in each panel highlights  the deepening of the Fe plumes. Ocean Data View was used to carry out the simulations. Click here to view the figure larger.

 

References

Fitzsimmons, J. N., John, S. G., Marsay, C. M., Hoffman, C. L., Nicholas, S. L., Toner, B. M., German, C. R., Sherrell, R. M. (2017). Iron persistence in a distal hydrothermal plume supported by dissolved-particulate exchange. Nature Geoscience. DOI: 10.1038/ngeo2900

Resing, J. A., Sedwick, P. N., German, C. R., Jenkins, W. J., Moffett, J. W., Sohst, B. M., & Tagliabue, A. (2015). Basin-scale transport of hydrothermal dissolved metals across the South Pacific Ocean. Nature, 523(7559), 200–203. DOI: 10.1038/nature14577

Changing the paradigm on the oceanic iron cycle

Tagliabue and co-workers (2017, see reference below) discuss an extensive review on the recent findings on iron (Fe) cycle in the ocean. They figure out clearly that:

  • Fe is a nutrient as essential as nitrogen (N) or phosphorus (P) for the phytoplankton. In other words, the full understanding of any marine ecosystem cannot neglect the analysis of micronutrients anymore.
  • Fe oceanic sources are multiple, and supply from continental margins extends far beyond the coastal zone while striking Fe inputs from hydrothermal activity along mid-ocean ridges were observed in all the oceans. This revolutionizes the preceding view of the dust inputs, although those are essential drivers of N2 fixation at low latitude.
  • The cycling of organic iron-complexing ligands has also emerged as a crucial component of the ocean iron cycle, ligand concentrations being not as uniform as considered earlier.
  • It is also recognized that phytoplankton can exhibit substantial variations in their iron stoichiometry in different environments...

Synthesizing these new insights provides a more refined picture of the ocean iron cycle, challenging the global ocean modelling for testing hypotheses and projections of change. The authors also draw exciting new frontiers for the oceanic Fe cycle...

17 TagliabuelFigure: This figure shows a revised model of the major processes in the ocean iron cycle, with focus on the Atlantic Ocean. Note that there is a broad meridional contrast between the iron-limited Southern Ocean and the major nutrient-limited low-latitude regimes. Dust remains a dominant source in the low latitudes, but continental margin and upwelled hydrothermal sources are more important in the Southern Ocean. Flexible iron uptake and biological cycling, together with the production of excess iron-binding ligands, dominate the Southern Ocean. Nitrogen fixation occurs in the low latitudes (although this process can also be restricted by lack of iron outside the North Atlantic subtropical gyre). The particulate organic iron flux is decoupled from that of phosphorus at high latitudes and the flux of lithogenic material is important at low latitudes influenced by dust. Subduction of excess organic iron-binding ligands from the Southern Ocean has a remote influence on the interior ocean at low latitudes. Click here to view the figure larger. (Modified from Tagliabue et al., 2017, Nature)

Reference:

Tagliabue, A., Bowie, A. R., Boyd, P. W., Buck, K. N., Johnson, K. S., & Saito, M. A. (2017). The integral role of iron in ocean biogeochemistry. Nature, 543(7643), 51–59. DOI: http://doi.org/10.1038/nature21058

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