AUTHIGENIC SULFIDE MINERALIZATION INDUCED BY ANAEROBIC METHANE OXIDATION IN METHANE-RICH, GAS-HYDRATE-ASSOCIATED SEDIMENTS: STRATIGRAPHIC PATTERNS OF SULFIDE-SULFUR CONCENTRATION AND SULFIDE-SULFUR ISOTOPIC COMPOSITION

Active sulfide mineralization occurs at the sulfate-methane interface (SMI) in modern, deep-water, continental-margin sediments that overlie accumulations of methane gas hydrate at the Blake Ridge (southeastern North America). Sulfide minerals (elemental sulfur, S^o; iron monosulfides, ~FeS; and pyrite, FeS₂) at the present-day sulfate-methane interface are enriched in ³⁴S relative to sulfide minerals in the sulfate reduction zone. These enrichments of ³⁴S occur because of focused anaerobic methane oxidation (AMO) at the SMI carried out by a consortium of methanotrophs and sulfate-reducing microbes, and because of non-steady-state diagenesis. ³⁴S enrichments are a potential diagenetic indicator identifying occurrences of high methane delivery to the sulfate-methane interface and the action of the AMO consortium. A key question is: Are these enrichments of ³⁴S preserved in the sedimentary record?

We extracted sulfide sulfur from authigenic sulfide minerals contained in sediments collected at Site 995, Ocean Drilling Program (ODP) Leg 164. Chromium reduction was used to extract sulfide sulfur and we measured sulfide sulfur concentration using iodometric titration. In separate extractions, we precipitated sulfide sulfur as Ag₂S for sulfur isotopic analysis. Sulfide mineral concentration at the present-day sulfate-methane interface occurring at ~20 m is as high as 0.57 wt % sulfur. Preliminary data from the depth interval of 20 to 703.80 m (sediments >0.085 to 6.0 Ma in age) show sulfide concentration ranging from 0.04 to 1.03 wt % sulfur. Twenty-six percent of samples (8 of 30) show sulfide sulfur concentrations higher than that of the present-day SMI. This implies that the diagenetic front at the SMI was stratigraphically stable for longer periods of time, and/or that methane flux to the SMI was higher than observed today. Isotopic data show that ³⁴S enrichments within sulfide minerals persist in the geologic record, and we infer that these enrichments identify SMIs that occurred in the geologic past.