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Title: Fractionation of iron and chromium isotopes in hydrothermal plumes from the northern Mid-Atlantic Ridge
Authors: Wang, Wenhao
Lough, Alastair J.M.
Goring-Harford, Heather
Flanagan, Oli
González Santana, David 
Resing, Joseph
Connelly, Douglas
Lohan, Maeve C.
Tagliabue, Alessandro
James, Rachael H.
UNESCO Clasification: 251002 Oceanografía química
Keywords: Chromium isotopes
Hydrothermal plume
Iron isotopes
Rainbow, et al
Issue Date: 2023
Journal: Earth and planetary science letters 
Abstract: Hydrothermal venting impacts the global-scale biogeochemical cycles of many trace metals and their isotopes. Processes in hydrothermal plumes regulate the dispersal of vent-derived metals and may vary in response to differences in the geologic setting of vent fields and/or the geochemistry of the overlying ocean water. Here we present results of analyses of dissolved Fe and Cr concentrations, and dissolved Fe isotope (δ56Fe) and Cr isotope (δ53Cr) distributions, in seawater samples collected from above TAG and Rainbow vent sites on the Mid-Atlantic Ridge during the GEOTRACES GA13 cruise. We show that profiles of dissolved Fe and Cr isotopes through the near-field hydrothermal plumes are the mirror image of each other. Oxidation of Fe(II) and precipitation of Fe-(oxyhydr)oxides account for the low δ56Fe values of dissolved Fe, as low as −1.83 ‰ at TAG and −6.94 ‰ at Rainbow. Plume samples with low δ56Fe values are associated with elevated δ53Cr values of dissolved Cr compared to background seawater (by up to +0.14 ‰ and +0.69 ‰ at TAG and Rainbow, respectively), while particulate Cr is characterised by relatively low δ53Cr values (−1.02 to −1.22 ‰). This striking result suggests that seawater Cr(VI) is reduced to Cr(III) and precipitates on the surface of Fe(III) particles within the hydrothermal plume. Reduction of Cr(VI) and scavenging of Cr(III) by plume Fe-(oxyhydr)oxide particles mean that high-temperature hydrothermal systems are likely a net sink for seawater Cr at Rainbow (and also at TAG). As the removal flux of Cr is related to the flux of hydrothermal Fe(II) and the rate of Fe(II) oxidation in the hydrothermal plume, it may (i) vary across vent sites at a global scale and (ii) change over glacial-interglacial cycles.
ISSN: 0012-821X
DOI: 10.1016/j.epsl.2023.118468
Source: Earth and planetary science letters [ISSN 0012-821X], v. 624
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