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Science Highlights

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.

17 Tachikawa l

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.

17 Hayes l
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

 

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

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.

17 Fitzsimmons l

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

Enlighten why macro and micronutrients display different remineralization length scales

Boyd and co-workers (2017, see reference below) explore the abiotic and biotic mechanisms that underpin internal metal cycling. Although they are focusing on iron (Fe) as the best-characterized metal, they are also discussing zinc (Zn), nickel (Ni) and copper (Cu) behaviors. Based on synchrotron X-ray fluorescence (SXRF) mapping and case studies in different biogeochemical areas of the ocean studied in the framework of GEOTRACES (productive Kerguelen plateau, seasonally oligotrophic subtropical waters, oligotrophic Bermuda and Hawaii waters), they reveal contrasting recycling patterns between trace- and macronutrients, explaining why remineralization length scales differ between elements. They also underline that external supply mechanisms of metals are required to complete their biogeochemical cycles.

17 Boyd lFigure: Processes that set the vertical length scales for the remineralization of elements within sinking particles. a, Hypothetical remineralization mechanisms for trace and major elements associated with a sinking diatom (based on SXRF element mapping). Preferential subsurface regeneration of elements is linked to their association with structural/biochemical cellular components (for example, membranes) and elemental requirements of microbes (circles). b,c, Idealized processes acting on sinking heterogeneous particles (lithogenic/biogenic components with different labilities). Particle transformations drive both remineralization (b, highlighted terms are metal specific) and depth-dependent changes in particle aggregate surface area (c, bio-optical profiling float data, courtesy of George Jackson), which influences local chemistry and microbial processes. Click here to view the figure larger. (Modified from Boyd et al., 2017, Nature Geoscience)

Reference :

Boyd, P. W., Ellwood, M. J., Tagliabue, A., & Twining, B. S. (2017). Biotic and abiotic retention, recycling and remineralization of metals in the ocean. Nature Geoscience, 10(3), 167–173. DOI: 10.1038/ngeo2876

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