Sulfur isotope geochemistry of cold seep sites: Northern Gulf of Mexico and Blake Ridge Diapir

The existence of cold seep chemosynthetic communities on the Louisiana Slope of the Gulf of Mexico and the Blake Ridge offshore South Carolina, depends in part upon in situ sulfide production. Sulfur utilization below free-living microbial mats and vesicomyid clam beds was studied to resolve the influence of sulfate reduction on these organisms. Dissolved sulfide concentrations below Gulf of Mexico sulfide-oxidizing microbial mats revealed sulfide levels are greater below certain mats (c.a. 6 mM) than others (c.a. 1 mM), likely related to sulfate reduction rates (0.3 to 3 mumol SO4 cm -3 d-1). Microbial mat sediments revealed petroleum-derived carbon (delta13C = -27‰) as a carbon source despite the proximity of mats to a methane saturated brine pool and/or hydrate mounds.; The dissolved sulfur chemistry below live and dead patches of vesicomyid clams from the Blake Ridge Diapir indicate a link between sulfide availability and clam bed existence. Sulfide concentrations higher below shell beds (c.a. 14 mM) than below clam beds (c.a. 3mM) suggest sulfide uptake is greater below clam beds. Differences in the sulfur isotopic enrichment factors (&egr; shell = 16.1‰ and &egr;clam = 10.3‰) may be related to clam bed bioturbation.; Bacterial sulfate reduction is the likely precursor to sulfide in both of seep locations; additional sources of sulfur may be derived from brines vented from the dissolution of diapirs. The Gulf of Mexico Brine Pool fluid is sulfate-free (0.4 mM) and chloride-rich (1910.6 mM). Sulfur isotope mixing plots and the extent of fractionation (&egr; = 4.3 to 38.7‰) observed in dissolved sulfate extracted from sediment cores adjacent to the pool suggest available sulfide is derived from the reduction of seawater sulfate; however, chloride profiles are highly suggestive of brine inclusion ([Cl-] max = 1017.9 mM).; Fluids at the summit of three Gulf of Mexico mud volcanoes were sulfate-free and sulfidic. The dissolved sulfate delta34S is indicative of seawater sulfate reduction; however, enriched sulfides (delta 34SHS = 39.2‰) suggest additional sulfur inputs to the system. Fluid discharge from the central mounds (0.2 m yr-1 ) and potential recharge (-0.02 m yr-1) along the volcano flanks is thought related to hydrate formation dynamics.