Hydrothermal venting associated with mid-ocean ridge volcanism is globally widespread. This venting is responsible for a dissolved iron flux to the ocean that is approximately equal to that associated with continental riverine runoff. For hydrothermal fluxes, it has long been assumed that most of the iron entering the oceans is precipitated in inorganic forms. However, the possibility of globally significant fluxes of iron escaping these mass precipitation events and entering open-ocean cycles is now being debated, and two recent studies suggest that dissolved organic ligands might influence the fate of hydrothermally vented metals. Here we present spectromicroscopic measurements of iron and carbon in hydrothermal plume particles at the East Pacific Rise mid-ocean ridge. We show that organic carbon-rich matrices, containing evenly dispersed iron(II)-rich materials, are pervasive in hydrothermal plume particles. The absence of discrete iron(II) particles suggests that the carbon and iron associate through sorption or complexation. We suggest that these carbon matrices stabilize iron(II) released from hydrothermal vents in the region, preventing its oxidation and/or precipitation as insoluble minerals. Our findings have implications for deep-sea biogeochemical cycling of iron, a widely recognized limiting nutrient in the oceans.
Bibliographical noteFunding Information:
We thank D. Adams, S. Beaulieu, S. Mills, B. Govenar and T. Shank for trap deployment/collection; J. P. Cowen, K. Von Damm, A. Thurnherr (NSF Ridge 2000), L. Mullineaux, J. Ledwell and A. Thurnherr (NSF OCE BIO and PO) for cruise berths; C. S. Chan for STXM standards; and ALS BL 10.3.2 users for Fe K-edge XAS reference spectra. Financial support: NASA Postdoctoral Program (B.M.T.), NSF OCE 0425737 (K.J.E. and J.W.M.), WHOI DOEI (L. Mullineaux, O.R., C.R.G. and K.J.E.), NSF OCE 0648287 (K.J.E., C.R.G. and O.R.) and NSF OCE 0424953 (L. Mullineaux). The Advanced Light Source is supported by the Office of Science, Basic Energy Sciences, Division of Materials Science of the US Department of Energy under contract No. DE-AC02-05CH11231.