Abstract:
Anoxic marine sediments are characterized by systematic decrease in concentration of dissolved sulfate in the interstitial 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 this decrease in sulfate concentration is an increase in delta (super 34) S, which is due to preferential utilization of the (super 32) S-isotope during bacterial sulfate reduction. The degree of (super 34) S-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 to diffusive replenishment of sulfate during bacterial sulfate reduction. Gradients in stable sulfur isotope ratios and concentration of dissolved sulfate in porewaters of marine sediments have been compiled from about 40 sites of the Deep Sea Drilling Project and the Ocean Drilling Program, ranging from Legs 4 through 188. Linear sedimentation rate for these sites ranges from about 5 to 1500 meters per million years. The apparent (super 34) S-enrichment factor (from Rayleigh equation) ranges from 4 to 72 per mil (alpha = 1.004 to 1.072), and is directly proportional to a system "openness" parameter (i.e., diffusive sulfate flux divided by the diffusive plus burial sulfate flux) which ranges from 0.06 to 0.87 for the compiled sites. Under conditions represented by DSDP/ODP cores, bacterial sulfate reduction occurs with a relatively large (super 34) S-isotopic enrichment (75 per mil), which is diluted by variable degrees of replenishment with unfractionated seawater sulfate.