Enrichments of heavy sulfur (34S) in sulfide minerals: Gas hydrates, methane delivery, and anaerobic methane oxidation

The sulfur isotopic composition of authigenic, sedimentary sulfide minerals is largely controlled by sulfate reduction and related processes within sedimentary environments. Histograms show that that d34S values of sulfide minerals forming in depositional and diagenetic environments are most often negative (d34S \u3c 0o/oo CDT) reflecting the original isotopic composition of seawater sulfate (now ~21o/oo), microbially-mediated fractionations of ~-8 to -40o/oo (a = 1.029-1.059) during sulfate reduction, and more extreme fractionations caused by sulfur disproportionation. Enrichments of heavy sulfur (d34S \u3e 0o/oo) in sulfide minerals represent about 18% of measured d34S values worldwide and reflect certain diagenetic conditions. Excluding seafloor seepage sites, most (59%) heavy sulfur enrichments are associated with anaerobic methane oxidation (AMO or AOM) occurring at the sulfate-methane interface (SMI or SMTZ). Blake Ridge (offshore southeastern USA) sediments associated with methane gas hydrates experience higher rates of upward methane diffusion than sediments in similar depositional environments not coincident with hydrate occurrences. Methane delivery to the SMI fuels AMO and results in d34S values within sulfide minerals of up to +23.6o/oo. d34S values in the sulfate reduction zone are negative (-46.6 to –8.4o/oo) but become more positive approaching the SMI where maximum enrichments of heavy sulfur in interstitial sulfate and authigenic sulfide minerals generally occur. 34S enrichments below the SMI most likely reflect positions of earlier SMIs. Heavy 34S values seen in the sedimentary record with appropriate depositional and diagenetic settings may indicate the presence of ancient gas hydrate deposits, larger amounts of upward methane flux, and AMO as an important sulfate-depletion mechanism. Such 34S enrichments are not diagnostic but should be distinguished by their depositional settings and differing diagenetic signals

Significance of anaerobic methane oxidation in methane-rich sediments overlying the Blake Ridge gas hydrates

A unique set of geochemical pore-water data, characterizing the sulfate reduction and uppermost methanogenic zones, has been collected at the Blake Ridge (offshore southeastern North America) from Ocean Drilling Program (ODP) Leg 164 cores and piston cores. The δ13 C values of dissolved CO2(Σ CO2) are as 13 C-depleted as –37.7‰ PDB (Site 995) at the sulfate-methane interface, reflecting a substantial contribution of isotopically light carbon from methane. Although the geochemical system is complex and difficult to fully quantify, we use two methods to constrain and illustrate the intensity of anaerobic methane oxidation in Blake Ridge sediments. An estimate using a two-component mixing model suggests that ~24% of the carbon residing in the Σ CO2 pool is derived from biogenic methane. Independent diagenetic modeling of a methane concentration profile (Site 995) indicates that peak methane oxidation rates approach 0.005 μmol cm–3 yr–1, and that anaerobic methane oxidation is responsible for consuming ~35% of the total sulfate flux into the sediments. Thus, anaerobic methane oxidation is a significant biogeochemical sink for sulfate, and must affect interstitial sulfate concentrations and sulfate gradients. Such high proportions of sulfate depletion because of anaerobic methane oxidation are largely undocumented in continental rise sediments with overlying oxic bottom waters. We infer that the additional amount of sulfate depleted through anaerobic methane oxidation, fueled by methane flux from below, causes steeper sulfate gradients above methane-rich sediments. Similar pore water chemistries should occur at other methane-rich, continental-rise settings associated with gas hydrates