Deep-sea hydrothermal plume particles are known to sequester seawater trace elements and influence ocean-scale biogeochemical budgets. The relative importance of biotic versus abiotic oxidation-reduction and other particle-forming reaction, however, and the mechanisms of seawater trace element sequestration remain unknown. Suspended particulate material was collected from a non-buoyant hydrothermal plume by in situ filtration at 9°50'N East Pacific Rise during a 3-day, 24 sample, time-series. Twenty-three samples were digested for total elemental analysis. One representative sample was selected for particle-by-particle geochemical analyses including elemental composition by X-ray fluorescence, speciation of Fe, S, and C by 1s X-ray absorption near edge structure spectroscopy, and X-ray diffraction. Consistent with past studies, positive linear correlations were observed for P, V, As, and Cr with Fe in the bulk chemistry. Arsenic was associated with both Fe oxyhydroxides and sulfides but not uniformly distributed among either mineral type. Particle aggregation was common. Aggregates were composed of minerals embedded in an organic matrix; the minerals ranged from <20nm to >10μm in diameter. The speciation of major mineral forming elements (Fe, Mn, S) was complex. Over 20 different minerals were observed, nine of which were either unpredicted by thermodynamic modeling or had no close match in the thermodynamic database. Sulfur-bearing phases consisted of polysulfides (S 6, S 8), and metal sulfides (Fe, Cu, Zn, Mn). Four dominant species, Fe oxyhydroxide, Fe monosulfide, pyrrhotite, and pyrite, accounted for >80% of the Fe present. Particulate Mn was prevalent in both oxidized and reduced minerals. The organic matrix was: (1) always associated with minerals, (2) composed of biomolecules, and (3) rich in S. Possible sources of this S-rich organic matter include entrained near vent biomass and in situ production by S-oxidizing microorganisms. These results indicate that particle aggregation with organic material is prevalent in dispersing hydrothermal plume fluxes, as well as in sinking particulate matter at this site. Particle aggregation and organic coatings can influence the reactivity, transport, and residence time of hydrothermal particles in the water column. Thus a biogeochemical approach is critical to understanding the net effect of hydrothermal fluxes on ocean and sedimentary trace element budgets.
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We would like to thank Lauren Mullineaux and the R/V Atlantis cruise AT15-26 (LADDER 2007) crew and science party (NSF OCE-0424953) for allowing us to conduct our sampling, and Steven Manganini and Olivier Rouxel for assistance with sample processing. The Woods Hole Oceanographic Institution’s Deep Ocean Exploration Institute funded construction of the SUPR sampler. Postdoctoral support for JAB was through RIDGE 2000 (NSF OCE-0550331); Mike Purcell and John Fetterman (WHOI), and Ken Doherty, Michael Mathewson, Ivory Engstrom, and Tim Shanahan (McLane Research Laboratories Inc.) all made significant contributions during the SUPR sampler development. Part of this work was carried out at the Institute of Technology, Characterization Facility, University of Minnesota (an NSF-funded Materials Research Facilities Network), in particular we thank Maria Torija for assistance with JADE. We thank T. Tyliszczak for support at ALS BL11.0.2, Amanda Turner and Katrina Edwards for ALS BL 10.3.2 beam time assistance, Dean Hesterberg for providing S reference spectra, Clara Chan for providing a ferrihydrite specimen and Shawn French (U. Guelph) for providing a lipid specimen. BMT acknowledges support from the Office of the Vice President for Research, University of Minnesota. The Advanced Light Source (SCF, MAM) is supported by the Office of Science, Basic Energy Sciences, Division of Materials Science of the U.S. Department of Energy (DE-AC02-05CH11231). CRG acknowledges support from NSF Grant OCE-0647948. AMT acknowledges support from NSF Grant OCE-0425361.