Sulfur geochemistry of thermogenic gas hydrate and associated sediment from the Texas-Louisiana continental slope

Abstract

Thermogenic gas hydrate and associated sediment were recovered from the northern Gulf of Mexico east of the Mississippi Canyon to investigate the interactions between gas hydrate and sedimentary sulfides. Sediment solid phase analyses included total reduced sulfide (TRS), acid volatile sulfide, and citrate-dithionate and HCl extractable iron. Pore-fluid measurements included []H₂S, chloride, sulfate, ammonia and total dissolved inorganic carbon. Gas hydrate hydrogen sulfide and carbon dioxide content were measured using a new wet chemical technique. The []³⁴S relative to Vienna Canyon Diablo troilite was determined for TRS and hydrate H₂S. Extensive (>95%) reduction of pore-fluid sulfate occurred, resulting in exceptionally high []H₂S concentrations (up to ~10 mM) and TRS concentrations (271 ± 50 []mole/g). However, the mole fraction of H₂S within the gas hydrate was too low (~0.3%) to significantly influence hydrate stability. This appears related to high reactive iron concentrations which average 256 ± 66 []mol/g (pyrite iron + HCl extractable iron). These iron-rich sediments are thus capable of sequestering much of the generated sulfide in the form of TRS minerals, thereby making it unavailable for incorporation by gas hydrate. The TRS concentrations are about an order of magnitude greater than expected for sites at similar water depths in the northern Gulf of Mexico. Steep dissolved []H₂S concentration gradients were observed both above and below the gas hydrate indicating diffusion of sulfide from the surrounding system into the gas hydrate. The gradients were used to estimate an incorporation rate of ~1 []mol H₂S/yr-cm² assuming molecular diffusion. TRS in close proximity to the hydrate was depleted in ³⁴S by ~10[0/00] relative to TRS remote to the hydrate. The precise mechanism responsible for this relative depletion in ³⁴S is not clear, but may prove an important geochemical indicator of sediments in which gas hydrate is or has been present. Studies at other sites will be necessary to confirm the generality of these observations.