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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.).

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 which 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?

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

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, toghether 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

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

Drawing the future of phytoplankton in a changing ocean

Phytoplankton development is strongly linked to the dissolved iron availability in the surface waters. Iron’s behavior is sensitive to warming, stratification, acidification and de-oxygenation. In a changing ocean, these processes in addition to nutrient co-limitation interactions with iron biogeochemistry will all strongly influence phytoplankton dynamics. This paper establishes the potential future shifts in multiple facets of iron biogeochemistry, from cellular physiology to ocean circulation. Possible impacts of these multiple changes on diatoms and trichodesmium are illustrated in the figure below. This work warns us on the urgent need to improve our present knowledge of the micronutrient cycle forcing, in order to better predict their future behaviors.


17 Hutchins
Figure: Interactive influences of the changing ocean iron cycle on diatoms and nitrogen-fixing cyanobacteria.
Iron biogeochemistry will respond to global change-related warming (red), increased light (yellow), acidification (black), loss of oxygen (green), and lowered inputs of the nutrients nitrate (white), silicate (grey) and phosphate (blue). This will have direct consequences for the growth and physiology of both phytoplankton groups, as well as indirect effects on critical resource supply ratios (boxes). Important components of the marine iron cycle responding to environmental change include inputs from dust, complexation by organic ligands, redox chemistry, and biological availability (orange). Click here to view the figure larger. (adapted from Hutchins and Boyd 2016, with thanks to J. Brown for graphics)

Reference:

Hutchins, D. A., & Boyd, P. W. (2016). Marine phytoplankton and the changing ocean iron cycle. Nature Climate Change, 6(12), 1072–1079. DOI: 10.1038/nclimate3147

What constrains the hydrothermal dissolved iron isotopic signatures?

Assessing the processes leading to dissolved iron (dFe) isotope fractionation in a hydrothermal plume is a key question, because it allows a better characterization of this specific source of dFe in the deep ocean. For the first time, Fe isotope composition of dissolved and total dissolvable Fe fractions was determined and compared to the bulk chemical composition of Fe particles.

This work, conducted on the same hydrothermal vents on the East Scotia Ridge, yielded two articles simultaneously published this month (February 2017, see references below).

These complementary papers demonstrate that the dFe isotopic composition observed at the end of the plume dispersion in the deep seawater is quite different from that of the pure fluid. Changes of this signature reflect redox processes, ligand complexation, exchanges with labile particulate Fe. They more specifically reveal that the proportions of authigenic Fe-sulfide and Fe-oxyhydroxide minerals that precipitate in the buoyant plume exert opposing control on the resultant isotopic signature of dFe found in the neutrally buoyant plume.

Although the isotopic composition of stabilized hydrothermal dFe in the East Scotia Sea is distinct from background seawater and may be used to quantify the hydrothermal dFe input to the ocean interior, these studies underline the fact that the multiple processes occurring during the early stages of the plume depend on the nature of the ridge substrate, more specifically its sulfur (S) content. The potentially highly variable isotopic signature of hydrothermal dFe is an important consideration for the mass balance of dFe in the modern ocean and for using Fe isotopes to infer changes in the Fe cycle throughout past Earth history...

17 Klar lFigure: Evolution of the isotopic composition of dissolved (δ56dFe) and particulate iron (δ56pFe) during plume dispersion. During the initial stages of venting the precipitation of iron-sulfide (FeS2) leads to the removal of light isotopes from the buoyant plume, and the precipitation of iron-oxyhydroxides leads to the removal of heavy isotopes. The resultant isotopic composition of Fe exported from the buoyant plume depends on the Fe/H2S ratio in the vent fluid. During dispersal in the neutrally buoyant plume (NBP), the isotopic composition of Fe becomes heavier (positive values), most likely due to exchange of Fe between particulate and dissolved phases, and formation of iron-ligand and nano-particulate Fe.

References:

Klar, J. K., James, R. H., Gibbs, D., Lough, A., Parkinson, I., Milton, J. A., Hawkes, Jeffrey A., Connelly, D. P. (2017). Isotopic signature of dissolved iron delivered to the Southern Ocean from hydrothermal vents in the East Scotia Sea. Geology, G38432.1. http://doi.org/10.1130/G38432.1

Lough, A. J. M., Klar, J. K., Homoky, W. B., Comer-Warner, S. A., Milton, J. A., Connelly, D. P., James, R.H, Mills, R. A. (2017). Opposing authigenic controls on the isotopic signature of dissolved iron in hydrothermal plumes. Geochimica et Cosmochimica Acta, 202, 1–20. http://doi.org/10.1016/j.gca.2016.12.022

 

What is generating the benthic nepheloid layers?

How ubiquitous, variable or persistent are nepheloid layers? What is the main process generating these "clouds at the bottom of the sea"? Gardner and co-workers (2017, see reference below) explore these two critical questions, with a focus on the western North Atlantic for which numerous time series and survey data exist. They piece together a detailed review of the mechanisms and provide important new insights into the creation, persistence, and decay of nepheloid layers, a major issue for the geochemistry of particle-reactive elements. Their main results are: Deep western boundary currents are too weak to create benthic storms and therefore to generate intense nepheloid layers; benthic storms are created primarily by deep cyclones beneath Gulf Stream meanders; benthic storms erode the seafloor and maintain the benthic nepheloid layer; and finally, benthic nepheloid layers are weak to non-existent in areas of low eddy kinetic energy.

17 GardnerFigure 1: Contours of integrated benthic particle load (red lines, in μg cm− 2) and abyssal eddy kinetic energy (EKE, dashed green lines, in cm2 s− 2). Numbers by stars and triangles are related to the mean time-series particle concentration and standard deviation of particle concentration (in parentheses). Click here to view the figure larger.

17 Gardner2
Figure 2: Map of surface EKE based on satellite observations during 2002–2006 (Dixon et al., 2011). Time-series stations are indicated. Click here to view the figure larger.


References:

Gardner, W. D., Tucholke, B. E., Richardson, M. J., & Biscaye, P. E. (2017). Benthic storms, nepheloid layers, and linkage with upper ocean dynamics in the western North Atlantic. Marine Geology. DOI:10.1016/j.margeo.2016.12.012 Open Access

K.W. Dixon, T.L. Delworth, A.J. Rosati, W. Anderson, A. Adcroft, V. Balaji, R. Benson, S.M. Griffies, H.-C. Lee, R.C. Pacanowski, G.A. Vecchi, A.T. Wittenberg, F. Zeng, R. Zhang Ocean circulation features of the GFDL CM2.6 & CM2.5 high-resolution global coupled climate models. WCRP Open Science Conference, October 2011, Denver, Colorado (2011)

The coupled zinc-silicon cycle paradox solved

The strong similarities between zinc (Zn) and silicon (Si) vertical profiles have led many studies to suggest the uptake of Zn in diatom frustules, followed by simultaneous remineralisation at depth. However, recent lab experiments have demonstrated that Zn, although essential for diatoms, is located in the organic part of the cell. These cells are characterized by particularly high Zn/P ratios in the Southern Ocean (up to 8). Such contrasting observations has raised the question as to what processes could lead to such consistent Si-Zn relationship, given that Zn and Si uptake are obviously not controlled by the same biological process. Vance and co-workers (2017, see reference below) demonstrate that the oceanic zinc distribution is the result of the interaction between the specific uptake stoichiometry in Southern Ocean surface waters and the physical circulation through the Southern Ocean hub.

Their approach couples in situ data collected in the different oceanic basins, experimental results from the literature and physical-biogeochemical coupled modelling on a global scale. This work emphasizes how the consideration of 1D cycling only can bias the understanding of (macro and micro) nutrient behaviours, and therefore their paleo-applications.

17 Vance lFigure: Depth profiles of dissolved zinc, silica and phosphate in three different ocean basins (bottom), with the locations of each profile shown on the map (top). Both zinc and silicate show deep maxima whereas phosphate has a much shallower maximum, despite the fact that the oceanic biogeochemical cycle of Zn is dominated by uptake into the organic parts of diatom cells with phosphate. Vance et al. explain these features in terms of biological and physical processes in the Southern Ocean. Modified from Nature Geoscience. Please click here to view the figure larger.

Reference:

Vance, D., Little, S. H., de Souza, G. F., Khatiwala, S., Lohan, M. C., & Middag, R. (2017). Silicon and zinc biogeochemical cycles coupled through the Southern Ocean. Nature Geoscience. DOI: 10.1038/ngeo2890

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