We report dissolved sulfide sulfur concentrations and the sulfur isotopic composition of dissolved sulfate and sulfide in pore waters from sediments collected during Ocean Drilling Program Leg 204. Porewater sulfate is depleted rapidly as the depth to the sulfate/methane interface (SMI) occurs between 4.5 and 11 meters below seafloor at flank and basin locations. Dissolved sulfide concentration reaches values as high as 11.3 mM in Hole 1251E. Otherwise, peak sulfide concentrations lie between 3.2 and 6.1 mM and occur immediately above the SMI. The sulfur isotopic composition of interstitial sulfate generally becomes enriched in 34 S with increasing sediment depth. Peak δ34 S-SO4 values occur just above the SMI and reach up to 53.1‰ Vienna Canyon Diablo Troilite (VCDT) in Hole 1247B. δ34 S-Σ HS values generally parallel the trend of δ34 S-SO4 values but are more depleted in 34S relative to sulfate, with values from –12.7‰ to 19.3 ‰ VCDT. Curvilinear sulfate profiles and carbon isotopic composition of total dissolved carbon dioxide at flank and basin sites strongly suggest that sulfate depletion is controlled by oxidation of sedimentary organic matter, despite the presence of methane gas hydrates in underlying sediments. Preliminary data from sulfur species are consistent with this interpretation for Leg 204 sediments at sites not located on or near the crest of Hydrate Ridge.
Borowski, W.S. 2006. Data Report: Dissolved sulfide concentration and sulfur isotopic composition of sulfide and sulfate in pore waters, ODP Leg 204, Hydrate Ridge and vicinity, Cascadia margin, offshore Oregon. In Tréhu, A.M., Bohrmann, G., Torres, M.E., and Colwell, F.S. (Eds.), Proceedings ODP, Scientific Results, 204. http://www-odp.tamu.edu/publications/204_SR/122/122.htm
Proceedings ODP, Scientific Results, 204