Sulfur and oxygen isotopes of coeval sulfate–sulfide in pore fluids of cold seep sediments with sharp redox gradients

Leakage of crude oil and gas through fault conduits intersecting the seafloor gives rise to scores of point-source anoxic enclaves on the oxic northern Gulf of Mexico slope. A study of 13 short cores recovered with a manned submersible from these seepage-affected sediments reveals that microbial processes fueled by hydrocarbons cause extensive sulfur diagenesis. Sulfate reduction and sulfide release occurring in the pore fluids reach completion 10–25 cm below the sediment–water interface. Bacterial sulfate reduction (BSR) rates are highly variable between sites but maximum values (97 and 917 μmol SO4 cm−3 year−1) in a bacterial mat and mussel bed, respectively, are unusually high for cold, deepwater habitats. nd δ18O values of the residual sulfate range from 20.7‰ to 70.8‰ (CDT) (n=45) and from 11.1‰ to 23.6‰ (SMOW) (n=33) compared to the overlying Gulf of Mexico bottom water values of 20.3‰ and 9.7‰, respectively. δ34S values of H2S yield a mean of 12.4±5.4‰ (CDT) (n=15). δ34S (SO4) data yield an integrated fractionation factor of αS=1.015 under a closed system assumption but a substantial higher fractionation of αS=1.023 under an open system assumption. Paired SO4–H2S inventory indicates that up to 28% of sulfide is removed from the system and supports the contention that seep sediments constitute an open system. The sulfur isotope fractionations reported here compare well with experimental data for cold-adapted sulfate-reducing bacteria but are substantially smaller than the “geological” fractionation of αS=1.055 derived from coeval sulfate-sulfide in Phanerozoic sediments. Isotope enrichments in the SO4 are 2.4 times greater in δ34S than in δ18O and the relations documented with f(SO4) are indicative of mixing between two end-member sulfate sources; seawater sulfate cycled through microbial dissimilatory sulfate reduction at depth and secondary sulfate produced by oxidation of H2S at near-surface through bacterial disproportionation (BDS) processes. The evidence for superimposed metabolic reactions in the reducing and oxidative parts of the anaerobic sulfur cycle in seep sediments has important implications regarding the proposed use of pore-water sulfate profiles as proxies of upward methane fluxes resulting from dissociation of marine-based gas hydrates.