HR: 1340h
AN: PP43B-0678    [Abstracts]
TI: Oxygen and Sulfur Isotope Composition of Dissolved Sulfate in Interstitial Waters of the Great Australian Bight, ODP Leg 182.
AU: * Bernasconi, S M
EM: stefano@erdw.ethz.ch
AF: Geological Institute, ETH-Zurich, ETH Zentrum, Zurich, 8092 Switzerland
AU: Böttcher, M E
EM: mboettch@mpi-bremen.de
AF: Deptartment of Biogeochemistry, Max Plank institute of Marine Biogeochemistry, Bremen, 28359 Germany
AU: Wormann, U G
EM: uli.wortmann@utoronto.ca
AF: Dept. of Geology, University of Toronto, 22 Russel Street, Toronto, ON M5S 3B1 Canada
AB: We measured the sulfur and oxygen isotope composition of dissolved sulfides and sulfate at ODP Sites 1129, 1130, 1131 and 1132 in the Great Australian Bight (GAB). At all Sites, a saline brine is present in the subsurface as indicated by increasing chloride concentrations with depth to reach contents up to 3 times seawater. Sulfate also increases with depth but the concentrations are reduced by intense microbial sulfate reduction. The sulfur isotope fractionation between coexisting dissolved sulfate and sulfide is very large and reaches up to 70 ‰ at all studied Sites. Due to the high sulfide concentrations and the lack of a significant source of oxidants we consider that the large sulfur isotope fractionations are induced by sulfate reducing bacteria alone without a significant contribution of elemental sulfur disproportionation and sulfide oxidation processes. The oxygen isotope composition of dissolved sulfate reaches maximum values of approximately +27 ‰ vs. VSMOW at all sites, close to the equilibrium isotope fractionation between sulfate and water. The oxygen isotope composition of dissolved sulfate positively correlates with the sulfur isotope fractionation between sulfate and sulfide. These oxygen isotope data thus support the hypothesis that that the high sulfur isotope fractionation are related to a single step fractionation by sulfate reducing bacteria and do not involve significant sulfide oxidation reactions and/or elemental sulfur disproportionation. Sulfide oxidation processes would lead to a lowering of the oxygen isotope composition of residual sulfate. Elemental sulfur disproportionation has been shown to increase the oxygen isotope composition of sulfate but to a smaller extent than that that observed in the GAB. The patterns of the oxygen isotope increase with progressive sulfate reduction indicate a predominant influence of isotope exchange rather than a kinetic isotope fractionation controlling the oxygen isotope composition of sulfate during sulfate reduction.
DE: 0400 BIOGEOSCIENCES
DE: 0404 Anoxic and hypoxic environments (4802, 4834)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1051 Sedimentary geochemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005