Abstract:
The Blake Ridge area (offshore southeastern United States) consists of deep-water continental margin sediments overlying deposits of methane gas hydrate. Authigenic sulfide minerals (elemental sulfur, S degrees ; iron monosulfides, approximately FeS; and pyrite, FeS (sub 2) ) are actively deposited in these sediments due to sulfate reduction and focused anaerobic methane oxidation (AMO) at the sulfate-methane interface (SMI). Sulfide minerals at the present SMI are enriched in (super 34) S relative to those produced by sulfate reduction of organic matter higher in the sulfate reduction zone. (super 34) S enrichments preserved in the sediment column may identify relatively high amounts of AMO and thus record periods of high methane delivery to the SMI. We extracted sulfide sulfur from sulfide minerals in sediment samples using chromium reduction (Holes A and B, Site 995, Ocean Drilling Program Leg 164). Sediment samples spanned a depth of 0 to 703.80 meters (Miocene). We made two separate sulfide sulfur extractions. The first extraction measured sulfide sulfur concentration by precipitating zinc sulfide (ZnS) and subsequently titrating dissolved sulfide via iodimetry. The second extraction precipitated sulfide sulfur as silver sulfide (Ag (sub 2) S) for sulfur isotopic analysis. Baseline solid-phase sulfide sulfur concentrations are generally low (0.2 weight percent sulfur, dry weight) to 200 m but increase below 250 m ( approximately 0.4 wt %). Superimposed on these baseline values are peaks of sulfide sulfur concentration as high as approximately 1 wt %. Baseline sulfur isotopic data (delta (super 34) S) generally show relative enrichments of (super 34) S between 0 and 200 m ( approximately -30 per mil), and relative depletions in (super 34) S below 250 m ( approximately -40 per mil). Some of the high concentration samples are correlative with large (super 34) S enrichments (+2.6 to 44 per mil) in both depth zones. These correlative peaks may represent authigenic sulfide mineral formation at the SMI, where enrichments in (super 34) S are expected. This is especially likely in samples above 200 m. Baseline shifts indicated by the sulfide sulfur data possibly represent changes in organic matter delivery over time, affecting the geochemical importance of AMO versus sulfate reduction.