Nitrogen isotopic composition of pore water ammonium, Blake Ridge, Site 997, ODP Leg 164

Ammonium (NH4 +) concentration profiles in piston-core sediments of the Carolina Rise and Blake Ridge generally have linear concentration profiles within the sulfate reduction zone (Borowski, 1998). Deep Sea Drilling Project (DSDP) Site 533, located on the Blake Ridge, also displayed a linear ammonium concentration profile through the sulfate reduction zone and the profile linearity continues into the upper methanogenic zone to a depth of ~200 meters below seafloor (mbsf), where the first methane gas hydrates probably occur (Jenden and Gieskes, 1983; Kvenvolden and Barnard, 1983). Sediments from the Ocean Drilling Program (ODP) Leg 164 deep holes (Sites 994, 995, and 997) also exhibit linear ammonium profiles above the top of the gas hydrate zone (~200 mbsf) (Paull, Matsumoto, Wallace, et al., 1996)

Marine pore-water sulfate profiles indicate in situ methane flux from underlying gas hydrate

Marine pore-water sulfate profiles measured in piston cores are used to estimate methane flux toward the sea floor and to detect anomalous methane gradients within sediments overlying a major gas hydrate deposit at the Carolina Rise and Blake Ridge (U.S. Atlantic continental margin). Here, sulfate gradients are linear, implying that sulfate depletion is driven by methane flux from below, rather than by the flux of sedimentary organic matter from above. Thus, these linear sulfate gradients can be used to quantify and assess in situ methane flux, which is a function of the methane inventory below

New technique detects gas hydrates

As exploration and development moves into deep waters, the possibility of encountering gas hydrates within seafloor sediments becomes increasingly likely. The ability to accurately detect gas hydrates is key to producing deepwater fields, allowing operators to safely design and place offshore drilling and production platforms, subsea production equipment and flow lines, as well as pipelines

Data report: Carbon isotopic composition of dissolved CO2, CO2 gas, and methane, Blake-Bahama Ridge and northeast Bermuda Rise, ODP Leg 172

Carbon isotopic data of interstitial dissolved CO2 (ΣCO2), CO2 gas, and methane show that a variety of microbial diagenetic processes produce the observed isotopic trends. Anaerobic methane oxidation (AMO) is an important process near the sulfate-methane interface (SMI) that strongly influences the isotopic composition of ΣCO2 in the sulfate reduction and upper methanogenic zones, which in turn impacts methane isotopic composition. Dissolved CO2 and methane are maximally depleted in 3C near the SMI, where C values are as light as –31.8‰ and –101‰ PDB for ΣCO2 and methane, respectively. CO2 reduction links the CO2 and methane pools in the methanogenic zone so that the carbon isotopic composition of both pools evolves in concert, generally showing increasing enrichments of C with increasing depth. These isotopic trends mirror those within other methane-rich continental rise sediments worldwide

Sulfide mineralization in deep-water marine sediments related to methane transport, methane consumption, and methane gas hydrates

Patterns of sulfide sulfur concentration and sulfur isotopic composition (d34S) are perhaps related to upward methane transport, especially in sediments underlain by methane gas hydrate deposits. Increased methane delivery augments the affect of anaerobic methane oxidation (AMO) occurring at the sulfate-methane interface (SMI). Sulfate and methane co-consumption results in production of dissolved sulfide at the interface that is eventually sequestered within sulfide minerals (elemental sulfur, iron monosulfide, pyrite). We examine the sediments of two piston cores collected over the Blake Ridge gas hydrate deposits (offshore southeastern North America) by extracting total sedimentary sulfide using chromium reduction. We use an improved titration procedure to assay for sulfide sulfur concentration that involves addition of an excess amount of potassium iodate/potassium iodide (KIO3/KI) solution in order to completely oxidize dissolved sulfide to elemental sulfur. The remaining iodine ions are then back-titrated with sodium thiosulfate solution, avoiding leakage of hydrogen sulfide gas, thus increasing measurement accuracy. Our results show that authigenic sulfide sulfur generally increases in concentration downcore from ~0.05 to peak concentrations approaching 0.4 weight per cent sulfur. These results are consistent with localized sulfide production at the SMI and rapid sulfide mineral formation there. We will further test the hypothesis by examining d34S values of authigenic sulfide minerals, expecting to see enrichments in d34S near the interface. Discrete horizons showing sulfide mineralization with 34S enrichments potentially record periods of increased methane flux, highlighting an increased role for AMO as a biogeochemical process and perhaps identifying existence of underlying gas hydrates

Sulfide mineralization in deep-water marine sediments related to methane transport, methane consumption, and methane gas hydrates

Patterns of sulfide sulfur concentration and sulfur isotopic composition (d34 S) are perhaps related to upward methane transport, especially in sediments underlain by methane gas hydrate deposits. Increased methane delivery augments the effect of anaerobic methane oxidation (AMO) occurring at the sulfate-methane interface (SMI). Sulfate and methane co-consumption results in production of dissolved sulfide at the interface that is eventually sequestered within sulfide minerals (elemental sulfur, iron monosulfide, pyrite). We examine the sediments of two piston cores collected over the Blake Ridge gas hydrate deposits (offshore southeastern United States) by extracting total sedimentary sulfide using chromium reduction. We use an improved titration procedure to assay for sulfide sulfur concentration that involves addition of an excess amount of potassium iodate/potassium iodide (KIO3/KI) solution in order to completely oxidize dissolved sulfide to elemental sulfur. The remaining iodine ions are then back-titrated with sodium thiosulfate solution, avoiding leakage of hydrogen sulfide gas, thus increasing measurement accuracy. Our results show that authigenic sulfide sulfur generally increases in concentration downcore from ~0.05 to peak concentrations approaching 0.4 weight per cent sulfur (dry weight). These results are consistent with localized sulfide production at the SMI and rapid sulfide mineral formation there. We will further test the hypothesis by examining d34 S values of authigenic sulfide minerals, expecting to see enrichments in d34 S near the interface. Discrete horizons showing sulfide mineralization with 34S enrichments potentially record periods of increased methane flux, highlighting an increased role for AMO as a biogeochemical process and perhaps identifying existence of underlying gas hydrates

Relative Concentration of Solid-phase Sulfide Species in Marine Sediments Overlying Gas Hydrate Deposits: Recognition of the Role of Anaerobic Methane Oxidation in Authigenic Sulfide Formation

Sulfide mineralization in marine sediments occurs when dissolved sulfide, produced by sulfate reduction processes, combines with dissolved iron to form iron sulfide minerals. Sulfide can be produced by oxidation of organic matter or by anaerobic methane oxidation (AMO), which involves the co-consumption of sulfate and methane. The latter process seems especially important within gas hydrate terrains like that of theBlakeRidge(offshore southeasternUnited States), where appreciable amounts of methane diffuse upward to the base of the sulfate reduction zone, or sulfate-methane interface (SMI). We examine the sediments of two piston cores collected over the Blake Ridge gas hydrate deposits by sequentially extracting the different phases of sulfide minerals: elemental sulfur (So), iron monosulfides (FeS), “young” pyrite, and “old” pyrite. So and FeS are extracted using dichloromethane and hot stannous chloride solution, respectively. Youthful pyrite is extracted using cold chromic chloride solution, whereas older pyrite is extracted with hot chromic chloride. We use an improved titration procedure to assay for sulfide-sulfur concentration that involves iodometry and back-titration with sodium thiosulfate solution. Our results show concentrations of elemental sulfur and iron monosulfides vary from ~0.02-0.07 weight percent sulfur with no systematic trends with depth. Young pyrite generally increases in concentration downcore from ~0.04 to peak concentrations approaching 0.17 weight percent sulfur at or near the SMI. Old pyrite concentrations are usually less than 0.05 weight percent sulfur, generally less than young pyrite concentration. Assuming that our procedure actually separates different phases of sulfide sulfur, these results seem consistent with localized sulfide production at the SMI where we expect to see an increased fraction of the young pyrite phase as a result of rapid sulfide mineral formation due to AMO occurring there. We can test this interpretation by determining the sulfur isotopic composition of each sulfide phase. We expect to see enrichments of 34S in the youthful pyrite fraction near the SMI

Sulfide mineralization in deep-water marine sediments related to methane transport, methane consumption, and methane gas hydrates

Patterns of sulfide sulfur concentration and sulfur isotopic composition (d34 S) are perhaps related to upward methane transport, especially in sediments underlain by methane gas hydrate deposits. Increased methane delivery augments the effect of anaerobic methane oxidation (AMO) occurring at the sulfate-methane interface (SMI). Sulfate and methane co-consumption results in production of dissolved sulfide at the interface that is eventually sequestered within sulfide minerals (elemental sulfur, iron monosulfide, pyrite). We examine the sediments of two piston cores collected over the Blake Ridge gas hydrate deposits (offshore southeastern United States) by extracting total sedimentary sulfide using chromium reduction. We use an improved titration procedure to assay for sulfide sulfur concentration that involves addition of an excess amount of potassium iodate/potassium iodide (KIO3/KI) solution in order to completely oxidize dissolved sulfide to elemental sulfur. The remaining iodine ions are then back-titrated with sodium thiosulfate solution, avoiding leakage of hydrogen sulfide gas, thus increasing measurement accuracy. Our results show that authigenic sulfide sulfur generally increases in concentration downcore from ~0.05 to peak concentrations approaching 0.4 weight per cent sulfur (dry weight). These results are consistent with localized sulfide production at the SMI and rapid sulfide mineral formation there. We will further test the hypothesis by examining d34 S values of authigenic sulfide minerals, expecting to see enrichments in d34 S near the interface. Discrete horizons showing sulfide mineralization with 34S enrichments potentially record periods of increased methane flux, highlighting an increased role for AMO as a biogeochemical process and perhaps identifying existence of underlying gas hydrates

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