Ventilation of marine sediments indicated by depth profiles of pore water sulfate and δ34S

Abstract

Abstract Anoxic marine sediments are usually characterized by systematic decrease in concentration of dissolved sulfate in the pore waters of marine sediments. This depletion in dissolved sulfate can take place over depth scales ranging from a few centimeters up to a few hundred meters. Accompanying the decrease in sulfate concentration is an increase in δ34S, due to preferential utilization of the 32S -isotope during bacterial sulfate reduction. The degree of 34S-enrichment for a given degree of dissolved sulfate depletion is also highly variable, and reflects the extent to which the sediment column is relatively open or closed with respect to diffusive replenishment of sulfate during bacterial sulfate reduction. Gradients in stable sulfur isotope ratios (34S/32S ) and concentration of dissolved sulfate in pore waters of marine sediments with negligible advective sulfate supply have been compiled from 42 sites of the Deep Sea Drilling Project and the Ocean Drilling Program, Legs 4 through 188.The linear sedimentation rates for the sites range from about 5 to 1500 m per million years. The apparent 34S-enrichment factors (from Rayleigh equation) vary from 4 to 72% (α = 1.004–1.072), and are directly proportional to a system “openness” parameter (i.e. the diffusive sulfate flux divided by the combined diffusive plus burial fluxes), which ranges from 0.06 to 0.87. Under conditions represented by the DSDP/ODP cores in this study, microbial sulfate reduction occurs at very slow rates with a relatively large 34S-isotopic enrichment (75%). This large fractionation is diluted by variable degrees of diffusive replenishment with relatively unfractionated seawater sulfate.