Iron persistence in a distal hydrothermal plume supported by dissolved–particulate exchange

Journal name:
Nature Geoscience
Volume:
10,
Pages:
195–201
Year published:
DOI:
doi:10.1038/ngeo2900
Received
Accepted
Published online

Abstract

Hydrothermally sourced dissolved metals have been recorded in all ocean basins. In the oceans’ largest known hydrothermal plume, extending westwards across the Pacific from the Southern East Pacific Rise, dissolved iron and manganese were shown by the GEOTRACES program to be transported halfway across the Pacific. Here, we report that particulate iron and manganese in the same plume also exceed background concentrations, even 4,000km from the vent source. Both dissolved and particulate iron deepen by more than 350m relative to 3He—a non-reactive tracer of hydrothermal input—crossing isopycnals. Manganese shows no similar descent. Individual plume particle analyses indicate that particulate iron occurs within low-density organic matrices, consistent with its slow sinking rate of 5–10myr−1. Chemical speciation and isotopic composition analyses reveal that particulate iron consists of Fe(III) oxyhydroxides, whereas dissolved iron consists of nanoparticulate Fe(III) oxyhydroxides and an organically complexed iron phase. The descent of plume-dissolved iron is best explained by reversible exchange onto slowly sinking particles, probably mediated by organic compounds binding iron. We suggest that in ocean regimes with high particulate iron loadings, dissolved iron fluxes may depend on the balance between stabilization in the dissolved phase and the reversibility of exchange onto sinking particles.

At a glance

Figures

  1. Interpolated concentrations and station map along the US GEOTRACES GP16 Eastern Pacific Zonal Transect.
    Figure 1: Interpolated concentrations and station map along the US GEOTRACES GP16 Eastern Pacific Zonal Transect.

    a, Station locations and names in relation to the South American continent and the East Pacific Rise (colours are bathymetry; green hues shallower) b, Excess 3He concentrations in fmolkg−1. c, Dissolved Fe concentrations (<0.2μm, in nM). d, Dissolved Mn concentrations (<0.2μm, in nM). e, Particulate Fe (>0.45μm, in nM). f, Particulate Mn (>0.45μm, in pM). Note that in each panel a black reference line is indicated at 2,500m to highlight the deepening of the Fe plumes. The simulations were carried out using Ocean Data View.

  2. Illustration of Fe, Mn and 3Hexs transport and transformation along the SEPR hydrothermal plume.
    Figure 2: Illustration of Fe, Mn and 3Hexs transport and transformation along the SEPR hydrothermal plume.

    Physical plume bounds are indicated in grey, at representative nonlinear distances off-axis (labelled at bottom). Concentric circles represent relative concentrations of particulate and dissolved metals; circle sizes represent relative concentrations but are not quantitatively accurate among Fe, Mn and 3Hexs maxima. Pie diagrams show chemical speciation of dissolved Fe. Particulate Fe and Mn are removed through aggregation onto sinking particles from above (white arrows43) and through near-field self-aggregation of hydrothermally sourced particles. Note that Fe descends relative to Mn and 3Hexs, which mix along slightly deepening isopycnals.

  3. Relationship between excess 3He and metal inventories in the dissolved and particulate phases in the SEPR hydrothermal plume (2,200-3,000[thinsp]m).
    Figure 3: Relationship between excess 3He and metal inventories in the dissolved and particulate phases in the SEPR hydrothermal plume (2,200–3,000m).

    a,b, Inventories for Fe and Mn, respectively. All stations are included with the exception of Sta. 18 (directly over vent). Sta. 20 is plotted as open circles for Mn because those points fall off of the distal plume trend8. Integrating between 2,200–3,000m captures the entirety of the sinking Fe plume. Linear relationships between 3Hexs and dissolved metals suggest that dissolved metal inventories are apparently conserved (controlled by mixing/dilution), whereas the exponential relationship between particulate metals and 3Hexs indicates aggregative removal of particles to >3,000m depth.

  4. Depth of peak concentrations in the SEPR hydrothermal plume.
    Figure 4: Depth of peak concentrations in the SEPR hydrothermal plume.

    a,b, Vertical bars indicate depths where concentrations were within 2.5% of maximum. The 27.737 line is the potential density layer on which maximum 3Hexs was emplaced at Sta. 20; this is the isopycnal surface on which all dissolved species should have travelled. Notably, Fe species deepened (a), falling below this isopycnal, whereas Mn species mixed along it (b). The label ‘dFe-Resing’ indicates dFe maxima published previously8, while ‘dFe-John’ are independent, mass spectrometric dFe measurements reported here; we report both to show that the pattern of dFe descent is reproducible and unrelated to data error.

  5. Scanning transmission X-ray microscopy (STXM) images, elemental maps, and spectra for representative plume particles (>0.2[thinsp][mu]m).
    Figure 5: Scanning transmission X-ray microscopy (STXM) images, elemental maps, and spectra for representative plume particles (>0.2μm).

    a,d, Transmission images collected at 290eV. b,e, Distribution of total carbon with optical density of 1.8 (b) and 0.63 (e). c,f, Distribution of total iron with optical density of 2.6 (c) and 0.57 (f). Note that f does not cover the whole of the area imaged in d and e. g, Carbon 1s XANES spectra for particulate organic carbon from Sta. 20–21. h, Iron 2p XANES spectrum for particulate Fe(III) from Sta. 20–21, compared to standard ferrihydrite. All scale bars 2μm.

  6. Dissolved and labile particulate [delta]56Fe results for hydrothermal depths 2,200-2,800[thinsp]m.
    Figure 6: Dissolved and labile particulate δ56Fe results for hydrothermal depths 2,200–2,800m.

    a, Constant labile particulate50 δ56 Fe (−0.25 ± 0.14‰) over a wide range of pFe concentrations suggests that pFe loss is controlled by non-fractionating, physical aggregation/disaggregation processes. b, Dissolved δ56Fe increases down-plume, modelled as mixing (black line) between a hydrothermal nanoparticulate Fe(III) oxyhydroxide endmember (−0.19‰) and an isotopically heavier ligand-bound phase (+0.66‰, 0.77nM; background and hydrothermal FeL complexes). Errors in [Fe] and particulate δ56Fe are smaller than data points (5% and 0.02–0.03‰, 2σs.e.m., respectively). Errors for some Sta. 20 dissolved δ56Fe were unusually high because of an incorrect dilution (light grey).

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Author information

Affiliations

  1. Department of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey 08901, USA

    • Jessica N. Fitzsimmons &
    • Robert M. Sherrell
  2. Department of Oceanography, Texas A&M University, College Station, Texas 77843, USA

    • Jessica N. Fitzsimmons
  3. Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA

    • Seth G. John
  4. Department of Earth and Ocean Sciences, University of South Carolina, Columbia, South Carolina 29208, USA

    • Seth G. John &
    • Christopher M. Marsay
  5. Skidaway Institute of Oceanography, University of Georgia, Savannah, Georgia 31411, USA

    • Christopher M. Marsay
  6. Department of Earth Sciences, University of Minnesota—Twin Cities, Minneapolis, Minnesota 55455, USA

    • Colleen L. Hoffman &
    • Brandy M. Toner
  7. Department of Soil, Water, and Climate, University of Minnesota—Twin Cities, St Paul, Minnesota 55108, USA

    • Sarah L. Nicholas &
    • Brandy M. Toner
  8. Department of Earth and Ocean Sciences, National University of Ireland, Galway H91 TK33, Ireland

    • Sarah L. Nicholas
  9. Department of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA

    • Christopher R. German
  10. Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey 08854, USA

    • Robert M. Sherrell

Contributions

J.N.F. determined the digested particulate metal concentrations, led data interpretation, and wrote the manuscript. R.M.S., C.R.G. and B.M.T. co-proposed the particulate studies. R.M.S., S.L.N. and C.R.G. collected samples on the GP16 cruise (C.R.G. as Chief Scientist). S.G.J. and C.M.M. made the Fe isotope measurements. C.L.H. and B.M.T. made the synchrotron measurements. All authors helped to refine the interpretation and contributed to manuscript revisions.

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The authors declare no competing financial interests.

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