title: Iron Transformation Pathways and Redox Micro-Environments in Seafloor Sulfide-Mineral Deposits: Spatially Resolved Fe XAS and δ(57/54)Fe Observations.
abstract: Hydrothermal sulfide chimneys located along the global system of oceanic spreading centers are habitats for microbial life during active venting. Hydrothermally extinct, or inactive, sulfide deposits also host microbial communities at globally distributed sites. The main goal of this study is to describe Fe transformation pathways, through precipitation and oxidation-reduction (redox) reactions, and examine transformation products for signatures of biological activity using Fe mineralogy and stable isotope approaches. The study includes active and inactive sulfides from the East Pacific Rise 9°50'N vent field. First, the mineralogy of Fe(III)-bearing precipitates is investigated using microprobe X-ray absorption spectroscopy (μXAS) and X-ray diffraction (μXRD). Second, laser-ablation (LA) and micro-drilling (MD) are used to obtain spatially-resolved Fe stable isotope analysis by multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS). Eight Fe-bearing minerals representing three mineralogical classes are present in the samples: oxyhydroxides, secondary phyllosilicates, and sulfides. For Fe oxyhydroxides within chimney walls and layers of Si-rich material, enrichments in both heavy and light Fe isotopes relative to pyrite are observed, yielding a range of δ(57)Fe values up to 6‰. Overall, several pathways for Fe transformation are observed. Pathway 1 is characterized by precipitation of primary sulfide minerals from Fe(II)aq-rich fluids in zones of mixing between vent fluids and seawater. Pathway 2 is also consistent with zones of mixing but involves precipitation of sulfide minerals from Fe(II)aq generated by Fe(III) reduction. Pathway 3 is direct oxidation of Fe(II) aq from hydrothermal fluids to form Fe(III) precipitates. Finally, Pathway 4 involves oxidative alteration of pre-existing sulfide minerals to form Fe(III). The Fe mineralogy and isotope data do not support or refute a unique biological role in sulfide alteration. The findings reveal a dynamic range of Fe transformation pathways consistent with a continuum of micro-environments having variable redox conditions. These micro-environments likely support redox cycling of Fe and S and are consistent with culture-dependent and -independent assessments of microbial physiology and genetic diversity of hydrothermal sulfide deposits.
authors.affiliation: Département des Ressources Physiques et Écosystèmes de Fond de Mer, Water, and Climate, University of Minnesota-Twin Cities St. Paul, MN, USA.
authors.forename: Brandy M
authors.initials: BM
authors.lastname: Toner
authors.affiliation: Department of Deep-sea Physical Resources and Ecosystems, Centre de Brest, Institut Français de Recherche pour l'Exploitation de la Mer Plouzané, France.
authors.forename: Olivier J
authors.initials: OJ
authors.lastname: Rouxel
authors.affiliation: Department of Earth Sciences, University of Minnesota-Twin Cities Minneapolis, MN, USA.
authors.forename: Cara M
authors.initials: CM
authors.lastname: Santelli
authors.affiliation: Department of Geosciences and MARUM, University of Bremen Bremen, Germany.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Bach
authors.affiliation: Department of Biological Sciences, University of Southern California Los Angeles, CA, USA.
authors.forename: Katrina J
authors.initials: KJ
authors.lastname: Edwards
keywords: East Pacific Rise; X-ray absorption spectroscopy; biosignature; hydrothermal; iron; micro-environment; mineral alteration; stable isotopes