SEDIMENTARY SULFUR CYCLING AND OCEAN PRODUCTIVITY NEAR METHANE SEEPS

The abundant biomass of thiotrophic organisms found in cold seeps at the Brine Pool, in the Gulf of Mexico, and Blake Ridge, off the South Carolina coast, is dependent upon in situ sulfide production. Stable isotopic tracing of the origin and fate of sulfur can help determine whether sulfide produced from sulfate reduction of seawater is in a sufficient supply to sustain the chemosynthetic productivity. Intensive sulfur cycling in these anoxic sediments underlying oxic bottom waters is fueled by interstitial bacterial consumption of marine organic matter and/or light aliphatic hydrocarbons. Bacterial sulfate reduction becomes thermodynamically favorable in such organic rich sediments when free oxygen, metal oxides, and nitrate concentrations become depleted. Bacterial sulfate reduction is thus the likely precursor to sulfide in both of these locations with a high supply of organic matter. The majority of the source organic carbon at both locations is methane of either thermogenic or biogenic origin with CH₄ primarily in the Blake Ridge and a complex suite of hydrocarbons ranging from C1 to C5 in the Northern Gulf of Mexico. The sedimentary sulfur cycle was characterized for the Blake Ridge and Louisiana Slope from sulfate and sulfide precipitated from extracted porewaters. Seawater sulfate, originating at +20‰, tends to become isotopically enriched as a result of kinetic isotope effects associated with bacterial sulfate reduction. Progressive downcore isotopic enrichment in Blake Ridge porewater sulfates, ranging from +20‰ in the upper most sediment to +26‰ at depth, reflects this microbially mediated process. Likewise in the Gulf of Mexico, concomitant downcore isotopic enrichments in d ³⁴ S^2- and d ³⁴ SO₄ ^2- result from the distinct fractionation associated with dissimilatory sulfate reduction (DSR). Thus, it appaears that organic matter produced and preserved at these seep sites is significantly influenced by DSR.