Transport and reduction of sulfate and immobilization of sulfide in marine black shales

In fine-grained sediments in which the amount of reduced sulfur retained in stable phases substantially exceeds that present initially in pore waters, rates of sulfate reduction may have equaled or exceeded rates of sulfate transport, resulting in enrichment of ³⁴S in pore waters and reduction products. Abundance and isotopic compositions of reduced sulfur compounds can be used to calculate the extent of sulfide retention and improve reconstructions of carbon-sulfur oxidation-reduction (redox) budgets. The Miocene Monterey Formation and Upper Devonian New Albany Shale represent distinct types of black shales that accumulated under different conditions of sulfate reduction. Our results suggest that the rate of sulfate reduction was controlled largely by mass transport in the Monterey and by the reduction process it-self in the New Albany. Sulfide was more efficiently retained in the Monterey; thus each mole of sulfide in the New Albany represents a greater amount of sedimented organic carbon removed during sulfate reduction.