Sulfur geochemistry and diagenesis in a gas hydrate terrane, Cascadia margin, offshore Oregon: Role of anaerobic methane oxidation

We present sulfide mineral data from south Hydrate Ridge located in a gas hydrate terrane, offshore Oregon. Sulfide sulfur concentration and the isotopic composition of sulfur (d34S) in authigenic sulfide minerals are analyzed from sediment samples collected on Ocean Drilling Project (ODP) Leg 204. Shallow sediment samples (\u3c25 mbsf) assess the relative importance of anaerobic methane oxidation (AMO) as a biogeochemical process, both here and at the Blake Ridge, another well known gas hydrate terrane offshore southeastern United States. Deep samples (\u3e25 mbsf) are used to evaluate sulfur diagenesis and its controls from early Pleistocene to the present. AMO, a microbially-mediated, sulfate-depleting process, creates an environment conducive to interstitial, authigenic sulfide mineral formation. When AMO is an important process, sulfide minerals are likely to be focused near the sulfate-methane interface (SMI) and become more enriched in heavy sulfur (34S). Preliminary data from two of three shallow sites show high authigenic sulfide sulfur levels (0.27 and 0.7 weight percent sulfur) immediately above the SMI compared to lower concentrations (0.12 and 0.41 weight percent sulfur) just below the SMI. The remaining site has no discernable pattern to the vertical distribution of sulfide sulfur concentration, but shows peak amounts of 0.52 weight percent sulfur above the SMI. Based on results from other sites in the region, we hypothesize that peak amounts of sulfide sulfur are likely precipitated due to production via AMO, but that that sulfate reduction of sedimentary organic matter is also responsible for sulfide mineralization within the sediments. The identification and timing of heavy sulfur enrichments (34S) in deep samples may have implications to the recognition of past gas hydrate occurrences and identify periods of significant methane transport

A preliminary study of sulfide mineral formation in methane-rich, marine sediments associated with anaerobic methane oxidation, Cascadia continental margin, offshore Oregon

Within gas hydrate settings, sulfide mineralization in marine sediments is likely controlled by two microbially-mediated, sulfate-depleting processes: anaerobic methane oxidation (AMO) and sulfate reduction. If large amounts of methane are delivered to the sulfate-methane interface (SMI), predominantly by diffusion, larger amounts of solid sulfide sulfur should occur there as dissolved sulfide combines with iron, forming an authigenic precipitate. We measure the amount of diagenetic sulfide sulfur at three locations in the Hydrate Ridge vicinity by extracting the bulk sedimentary sulfide-phase minerals (So, FeS, and FeS2) through chromium reduction, precipitating sulfide sulfur as silver sulfide, and gravimetrically determining concentration. Two of three sites show authigenic sulfide sulfur levels of approximately 0.27 and 0.7 weight percent (wt %) sulfur, occurring immediately above the SMI. Lower concentrations of 0.12 and 0.41 wt % sulfur, respectively, occur below the SMI. The remaining site has no discernable pattern to the vertical distribution of sulfide sulfur concentration, but shows peak amounts of 0.52 wt % sulfur above the SMI. At the first two sites, we infer peak amounts of sulfide sulfur are precipitated due to the production of sulfide sulfur via AMO. We can test this interpretation by determining the sulfur isotopic composition (d34S) of the bulk sulfide minerals. Sulfide sulfur forming at the SMI should also be enriched in heavy sulfur relative to sulfide minerals forming in the sediments above. If these sulfur isotopic enrichments are unique to methane-rich settings associated with gas hydrates, then these diagenetic fingerprints can be recognized in the rock record

Dynamics of an Eutrophic Lake (Wilgreen Lake, Madison County, Kentucky): A First Step in Cleansing a Lake System Impaired by Nutrient Loading

Wilgreen Lake (Madison County, Kentucky) is an eutrophic lake formed by damming Taylor Fork, part of the Silver Creek watershed. Two principal tributaries drain urban areas of the city of Richmond, agricultural land typified by cattle grazing, and a highdensity residential area using septic systems. The lake is listed “nutrient impaired” by the Commonwealth and EPA, so it is likely that nutrient input from human activities is affecting water quality. Our study aims first to characterize the physical characteristics and water quality of the lake (2006), and then to determine the specific proportion of nutrient inputs (2007) to the lake with the aim of remediating its water quality. We will use nitrogen isotopes and microbial DNA templates to identify specific nutrient sources. Research started in May 2006 with work occurring throughout the 2006 field season with the intent of establishing a baseline for key lake parameters. We used an YSI probe to measure temperature, conductivity, oxygen concentration, and pH and assayed for total ammonia nitrogen using the sodium hypochlorite, colorimetric method. The lake was already strongly stratified in May with disoxic and anoxic water below about 4 meters. Stratification strengthened in the summer with the disoxic-oxic boundary moving upward to about 3 meters, showing a sharper gradient oxygen gradient. Ammonium concentrations are typically zero in the oxic zone, and increase in concentration with increasing water depth in anoxic waters to about 5 ppm. We anticipate that phosphate and nitrate concentrations will mirror ammonium concentration profiles in character