Geochemical and iron isotopic insights into hydrothermal iron oxyhydroxide deposit formation at Loihi Seamount

Olivier Rouxel, Brandy Toner, Yoan Germain, Brian Glazer

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Low-temperature hydrothermal vents, such as those encountered at Loihi Seamount, harbor abundant microbial communities and provide ideal systems to test hypotheses on biotic versus abiotic formation of hydrous ferric oxide (FeOx) deposits at the seafloor. Hydrothermal activity at Loihi Seamount produces abundant microbial mats associated with rust-colored FeOx deposits and variably encrusted with Mn-oxyhydroxides. Here, we applied Fe isotope systematics together with major and trace element geochemistry to study the formation mechanisms and preservation of such mineralized microbial mats. Iron isotope composition of warm (<60 °C), Fe-rich and H2S-depleted hydrothermal fluids yielded δ56Fe values near +0.1‰ indistinguishable from basalt values. Suspended particles in the vent fluids and FeOx deposits recovered nearby active vents yielded systematically positive δ56Fe values. The enrichment in heavy Fe isotopes between +1.05‰ and +1.43‰ relative to Fe(II) in vent fluids suggest partial oxidation of Fe(II) during mixing of the hydrothermal fluid with seawater. By comparing the results with experimentally determined Fe isotope fractionation factors, we determined that less than 20% of Fe(II) is oxidized within active microbial mats, although this number may reach 80% in aged or less active deposits. These results are consistent with Fe(II) oxidation mediated by microbial processes considering the expected slow kinetics of abiotic Fe oxidation in low oxygen bottom water at Loihi Seamount. In contrast, FeOx deposits recovered at extinct sites have distinctly negative Fe-isotope values down to −1.77‰ together with significant enrichment in Mn and occurrence of negative Ce anomalies. These results are best explained by the near-complete oxidation of an isotopically light Fe(II) source produced during the waning stage of hydrothermal activity under more oxidizing conditions. Light Fe isotope values of FeOx are therefore generated by subsurface precipitation of isotopically heavy Fe-oxides rather than by the activity of dissimilatory Fe reduction in the subsurface. Overall, Fe-isotope compositions of microbial mats at Loihi Seamount display a remarkable range between −1.2‰ and +1.6‰ which indicate that Fe isotope compositions of hydrothermal Fe-oxide precipitates are particularly sensitive to local environmental conditions where they form, and are less sensitive to abiotic versus biotic origins. It follows that FeOx deposits at Loihi Seamount provides important modern analogues for ancient seafloor Fe-rich deposits allowing for testing hypotheses about the biogeochemical cycling of Fe isotopes on early Earth.

Original languageEnglish (US)
Pages (from-to)449-482
Number of pages34
JournalGeochimica et Cosmochimica Acta
StatePublished - Jan 1 2018

Bibliographical note

Funding Information:
This work is dedicated to the memory of Katrina Edwards. None of this work would have been possible without her support and intellectual contribution. We also thank the Microbial Observatory Project (FeMO) Principle Investigators: Katrina Edwards (University of Southern California), Dave Emerson (Bigelow Laboratory), Craig Moyer (Western Washington University), Hubert Staudigel (Scripps Institution of Oceanography), and Brad Tebo (Oregon Health & Science University) for their support and input during cruise operations. We thank the ROV Jason-II pilots and the crews of the RV Melville, RV Kilo Moana, and RV T. Thompson for assistance with deployments and sample collection during the cruises. We thank Maureen Auro for laboratory assistance at the Woods Hole Oceanographic Institution (WHOI), Lary Ball and Emmanuel Ponzevera for daily maintenance of the MC-ICPMS at WHOI and Ifremer. We thank Maxence Guillermic for help with leaching experiments, Sandrine Cheron for XRD analysis and Celine Liorzou for ICP-AES analysis. The Woods Hole Oceanographic Institution (WHOI), the European Institute for Marine Studies (IUEM), the French Research Institute for Exploitation of the Sea (IFREMER), and the School of Ocean and Earth Science and Technology (SOEST) all provided support and research facilities for this study. Support for Rouxel was provided by the Institut Carnot Ifremer EDROME , the LabexMer ANR-10-LABX-19-01 , Europole Mer and FP7 (#247837) grant. Support for Toner was provided by the National Academies of Science and NASA post-doctoral fellowship programs. A portion of this work was conducted at beamlines and 11.0.2 of the Advanced Light Source (ALS). We thank ALS beamline scientists David Kilcoyne and Tolek Tyliszczak for training, and Sirine Fakra (ALS), Beth Orcutt (Bigelow Laboratory for Ocean Sciences), and Katrina Edwards for collaborative interactions and staffing of beamtime. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We thank the AE Silke Severmann, Tim Conway and two anonymous reviewers for their constructive and helpful comments, which improved the quality of this paper.

Publisher Copyright:
© 2017 Elsevier Ltd


  • Hydrothermal Systems
  • Iron Isotopes
  • Mineral deposits
  • Seamounts


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