Sulfur, carbon, and oxygen isotopes of coexisting sulfides and carbonates in gas and oil seeps from the Gulf of Mexico

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Microbial processes consisting of bacterial sulfate reduction (BSR), bacterial disproportionation of sulfur (BDS), and BSR coupled with anaerobic oxidation of methane (BSR-AOM) occur in the Gulf of Mexico (GOM) at sites where hydrocarbon oil and gas seep through conduits to the seafloor. Thus far, no studies have measured the solid-phase sulfides from GOM seeps in-situ that provide a superior analogue to sulfides from the geologic record. This study employs in-situ sulfur isotope measurements by secondary ion mass spectrometry (SIMS) in authigenic sulfides associated with barites and carbonates, and isotope measurements of carbon and oxygen by isotope ratio mass spectrometry (IRMS) in coexisting carbonates from 5 sites (GC-140, GC-185, GC-272, MC-929, and GB-382) in the GOM. Pyrite grains (FeS₂) yield variable δ³⁴S values and are considerably more ³⁴S-depleted in seeps with dominant carbonate phase (GC-140 & GC-185: range of -50.5 to -9.0‰ CDT) compared to seeps with minor to dominant barite phase (GC-272 & MC-929: range -23.9 to 19.5 ‰ CDT; GB-382: range 5.3 to 25.9‰). Measurements of carbonate reveals variably ¹³C-depleted and ¹⁸O-enriched δ¹³C and δ¹⁸O values in all seeps (GC-140 & GC-185: -36.7 to -3.5‰ VPDB and 2.5 to 5.3‰ VPDB, respectively; GC-272 & MC-929: -38.0 to -1.2‰ VPDB and 1.6 to 2.5‰ VPDB, respectively; GB-382: -31.4 to -1.2‰ VPDB and -0.5 to 4.1‰ VPDB; respectively). The isotope data suggest a strong influence of: (i) BSR and BDS processes using crude oil and unlimited SO₄²⁻ supply near the sediment-water interface (GC-140 & GC-185); (ii) BSR using crude oil/ non-methane sublimated gas hydrate and variable availability of SO₄²⁻ at depth within the sediment column (GC-272 & MC-929; GB-382), or (iii) possible BSR-AOM using sublimated methane hydrate and SO₄²⁻ near the sulfate-methane transition zone (GC-272 and GB-382). Additionally, Fe-oxides reported in association with pyrites at GC-140, GC-185, and GC-272, could potentially influence fractionation of sulfur isotopes during microbial processes by promoting greater sulfur recycling and thus muting fractionation effects. The results of this study offer important insights on the high variability of sedimentary pyrites in cold seeps that has implications for the biogeochemical cycling of sulfur and carbon in marine environments.

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Geology, Geobiology, Geochemistry