HR: 1340h
AN: B13A-0199    [Abstracts]
TI: Sulfur Isotope Geochemistry of the Lost City Hydrothermal Vent Fluids
AU: * Frueh-Green, G L
EM: frueh@erdw.ethz.ch
AF: ETH-Zurich, Dept. of Earth Sciences, Zurich, CH-8092 Switzerland
AU: Bernasconi, S M
EM: stefano@erdw.ethz.ch
AF: ETH-Zurich, Dept. of Earth Sciences, Zurich, CH-8092 Switzerland
AU: Butterfield, D A
EM: David.A.Butterfield@noaa.gov
AF: Joint Institute for the Study of the Atmosphere and Oceans, Univ. of Washington, Seattle, WA 98195 United States
AU: Kelley, D S
EM: kelley@ocean.washington.edu
AF: Univ. of Washington, School of Oceanography, Seattle, WA 98195 United States
AB: At the Lost City Hydrothermal Vent Field (Mid-Atlantic Ridge, $30\deg$N), reactions between seawater and ultramafic rocks produce high alkaline (pH 9 to 11) fluids that are venting at temperatures of 40 to $90\deg$C and result in the formation of up to 60m tall carbonate-brucite structures. The fluids are enriched in hydrogen, methane and other hydrocarbons, and support dense microbial communities. We present sulfur isotope data of dissolved sulfate and coexisting sulfide in the fluids venting at Lost City, which together with C-isotope data provide constraints on the links between chemical and biological processes associated with serpentinization. The sulfur isotope composition of sulfate increases from seawater values of +21$\permil$ (VCDT) in fluids with sulfate concentrations of 28 mM to values of up to +30$\permil$ in the low sulfate-, high pH end-member hydrothermal fluids. Sulfide concentrations range between 50 and 2780 micromolar. Sulfur isotope compositions of the sulfides lie in a narrow range of +34 to +37$\permil$ (VCDT) and show no clear correlation with concentrations. The isotopic compositions of dissolved inorganic carbon vary between -0.5$\permil$ (VPDB) in the high sulfate samples and -18$\permil$ in the low sulfate samples. This covariance indicates active sulfate reduction in the vent structures and/or in the shallow serpentinite subsurface. Sulfate reduction likely contributes to the variability of carbon isotope compositions observed in both the dissolved inorganic carbon and the carbonate minerals forming the structures. These data, together with C- and O-isotope data of the vent structures, provide evidence that methane oxidation coupled with sulfate reduction during mixing of the more pristine, hydrogen and methane-rich hydrothermal end-member fluids with seawater is an important process in hydrothermal carbonate precipitation at Lost City. Our results are consistent with previous microbiological and organic geochemical studies, which indicate a close association of methane-cycling archaea with sulfate-reducing bacteria in the vent structures.
UR: http://www.lostcity.washington.edu
DE: 8135 Hydrothermal systems (8424)
DE: 4835 Inorganic marine chemistry
DE: 3035 Midocean ridge processes
DE: 1040 Isotopic composition/chemistry
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting