Are 34S-enriched authigenic sulfide minerals a proxy for elevated methane flux and gas hydrates in the geologic record?

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The sulfate–methane transition (SMT) zone is a diagenetic transition within anoxic marine sediments created by the metabolic activity of a consortium of sulfate-reducing bacteria and methane-oxidizing Archaea. As interstitial dissolved sulfate is consumed by microbially mediated sulfate reduction of sedimentary organic matter (SOM) and anaerobic oxidation of methane (AOM) large enrichments of 34S occur in the interstitial sulfate pool. These isotopic enrichments are transmitted to the dissolved sulfide pool (∑HS) and subsequently into sulfide minerals (So, ∼FeS, FeS2).

We investigate the sulfur isotopic composition of pore-water sulfate and sulfide minerals at three sites underlain by gas hydrates at the Blake Ridge. The isotopic composition of sulfate-sulfur is most positive at the SMT showing maximum values of +29.1, 49.6, 51.6‰ VCDT at each of the respective sites. δ34S values of bulk sulfide minerals tend to be more enriched in 34S at and below the SMT ranging from −12.7 to +23.6‰, corresponding to enrichments of 26.7–62.4‰ relative to the mean value of −38.8‰ in the sulfate reduction zone. Both enhanced delivery of methane to the SMT, and non-steady-state sedimentation appear necessary to create large 34S enrichments in sulfide minerals. Similar associations of AOM and large δ34S enrichments (>0‰) occur in other gas hydrate terranes (Cascadia margin) but their exact origin is equivocal at present. An analysis of δ34S data from freshwater and marine sedimentary environments reveals that34S enrichments within sulfide minerals occur under a range of conditions, but are statistically associated with AOM and systems not limited by dissolved interstitial iron.

In methane-rich sediments, methane delivery to the SMT increases the role of AOM in sulfate depletion that impacts the formation and isotopic composition of authigenic sulfide minerals. We hypothesize that under certain diagenetic conditions large 34S enrichments within sulfide minerals in the geologic record potentially identify: (1) the former occurrence of AOM (2) present-day and “fossil” locations of the sulfate–methane transition zone; and (3) a diagenetic terrane, today characteristic of deep-water, methane-rich, marine sediments conducive to gas hydrate formation. Thus, 34S-enriched sulfide minerals preserved in modern and ancient continental-margin sediments may allow for the identification of AOM-related processes that occur in methane-rich sediments.