HR: 1340h
AN: T33B-1372    [Abstracts]
TI: Biological Sulfate Reduction Rates in Hydrothermal Recharge Zones
AU: * Crowell, B
EM: bwcrowel@ucsd.edu
AF: Scripps Institution of Oceanography, Univeristy of California, San Diego, La Jolla, CA 92037, United States
AU: Lowell, R P
EM: rlowell@vt.edu
AF: Virginia Tech, Department of Geosciences, Blacksburg, VA 24061, United States
AB: We develop a model to determine the rate of removal of seawater sulfate in the recharge regions of deep-sea hydrothermal systems as a result of biogenic sulfate reduction. The rate of sulfate reduction as a function of temperature derived from laboratory measurements on cores from the Guaymas Basin in Mexico [Jorgensen et al., 1992] is incorporated into a steady state 1-D advection-diffusion temperature equation, and a 1-D, steady- state, advection dominated conservation of solute equation. The diffusivity of sulfate in seawater is on the order of ~ 10^-10 m²/s, and unless the flow speeds are < 10^-12 m/s, the effects of diffusion are negligible, except within thin diffusive boundary layers. This model is then compared with a model that utilizes Gibbs free energy to quantify biogenic sulfate reduction [Bach and Edwards, 2003] in the upper oceanic crust of aging lithosphere. Using the high rates determined by Jorgensen et al. [1992], our model indicates that biological activity would reduce all seawater sulfate transported into the system within the upper 10 meters or less of the crust, which is inconsistent with the estimates of Bach and Edwards [2003]. Sulfate concentrations from ODP borehole Legs 64 and 168, at the sedimented Guaymas Basin and Juan de Fuca Ridge, respectively, show that most of the seawater sulfate is removed in the upper 100 meters. If the sulfate is assumed to all be reduced biogenically, the sulfate reduction rates at the ODP sites are at least 2 orders of magnitude less than the laboratory estimates of Jorgenson et al. [1992]. Finally, we compare the rate of seawater sulfate removal as a result of the precipitation of anhydrite, with the rate of biogenic sulfate reduction. We find that if hydrothermal recharge occurs rapidly through highly permeable faults, that biogenic sulfate reduction is negligible and that anhydrite precipitation would rapidly clog the recharge zone [Lowell and Yao, 2002]. If recharge occurs through broad zones of slow downwelling (u <ƒn 10^-9 m/s); however, anhydrite precipitation would seal pore on the order of thousands of years even in the absence of biogenic sulfate reduction. At these slower flow speeds, the biogenic sulfate reduction may provide an important mechanism for the removal of seawater sulfate from the deeper parts of the reaction zone. Bach, W. and K.J. Edwards (2003), Iron and sulfide oxidation within the basaltic ocean crust: Implications for chemolithoautotrophic microbial biomass production, Geochim.Cosmochim. Acta, 67, 3871-3887. Jorgensen, B.B., M.F. Isaksen and H.W. Jannasch (1992), Bacterial sulfate reduction above 100 degrees C in deep-sea hydrothermal vent sediments, Science, 258, 1756-1757. Lowell, R.P. and Y. Yao (2002), Anhydrite precipitation and the extent of hydrothermal recharge zones at ocean ridge crests, J. Geophys. Res., 107(B9), 2183, doi:10.1029/2001JB001289.
DE: 3017 Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424)
DE: 8135 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8424)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting