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Data report: clues about carbon cycling in methane-bearing sediments using stable isotopes of the dissolved inorganic carbon, IODP Expedition 311

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Proceedings of the IODP

Integrated Ocean Drilling Program

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      Abstract

      The isotopic characterization of carbon in the dissolved inorganic carbon (DIC) pool is fundamental for a wide array of scientific studies directly related to gas hydrate research. Here we present the DIC data from pore fluid samples recovered from the northern Cascadia accretionary margin during Integrated Ocean Drilling Program (IODP) Expedition 311. Comparison of these results with data obtained from offshore central Cascadia during Ocean Drilling Program Leg 204 provides clues on carbon cycling processes that control methane inventories and fluxes. Microbial methane production preferentially incorporates the light carbon isotope. As sediment ages, more of the original CO2 is converted to methane, leaving behind an isotopically heavier residual DIC. This DIC is progressively enriched in 13C below 200 meters below seafloor with increasing distance from the deformation front. In the shallower sections, minima in downcore δ13CDIC profiles coincide with the sulfate-methane transition (SMT) zone. Here the δ13CDIC values provide information on the metabolic pathways that consume sulfate and reveal that anaerobic oxidation of methane (AOM) is not the dominant reaction at all sites drilled. There appears to be no simple correlation between the extent of AOM, depth of the SMT, and thickness of the gas hydrate occurrence zone along the transect drilled in northern Cascadia.

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      Estimates of biogenic methane production rates in deep marine sediments at Hydrate Ridge, Cascadia margin.

      Methane hydrate found in marine sediments is thought to contain gigaton quantities of methane and is considered an important potential fuel source and climate-forcing agent. Much of the methane in hydrates is biogenic, so models that predict the presence and distribution of hydrates require accurate rates of in situ methanogenesis. We estimated the in situ methanogenesis rates in Hydrate Ridge (HR) sediments by coupling experimentally derived minimal rates of methanogenesis to methanogen biomass determinations for discrete locations in the sediment column. When starved in a biomass recycle reactor, Methanoculleus submarinus produced ca. 0.017 fmol methane/cell/day. Quantitative PCR (QPCR) directed at the methyl coenzyme M reductase subunit A gene (mcrA) indicated that 75% of the HR sediments analyzed contained <1,000 methanogens/g. The highest numbers of methanogens were found mostly from sediments <10 m below seafloor. By considering methanogenesis rates for starved methanogens (adjusted to account for in situ temperatures) and the numbers of methanogens at selected depths, we derived an upper estimate of <4.25 fmol methane produced/g sediment/day for the samples with fewer methanogens than the QPCR method could detect. The actual rates could vary depending on the real number of methanogens and various seafloor parameters that influence microbial activity. However, our calculated rate is lower than rates previously reported for such sediments and close to the rate derived using geochemical modeling of the sediments. These data will help to improve models that predict microbial gas generation in marine sediments and determine the potential influence of this source of methane on the global carbon cycle.
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        Gas hydrate occurrence from pore water chlorinity and downhole logs in a transect across the northern Cascadia margin (Integrated Ocean Drilling Program Expedition 311)

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          Methane hydrate formation in turbidite sediments of northern Cascadia, IODP Expedition 311

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            Author and article information

            Journal
            10.2204/iodp.proc.311.2006
            Proceedings of the IODP
            Integrated Ocean Drilling Program
            1930-1014
            30 April 2009
            10.2204/iodp.proc.311.206.2009

            This work is licensed under a Creative Commons Attribution 4.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

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            Self URI (journal page): http://publications.iodp.org/

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