Isotopic Composition of Interstitial Fluids and Origin of Methane in Slope Sediment of the Middle America Trench, Deep Sea Drilling Project Leg 84

Claypool, George E.; Threlkeld, Charles N.; Mankiewicz, P. N.; Arthur, Michael A.; Anderson, Thomas F.

doi:10.2973/dsdp.proc.84.124.1985

Cited by 34 publications

(33 citation statements)

References 24 publications

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“…The 613C values of methane in marine sedi ments become generally heavier with depths par allel to depth-trends of 613C values of co-existing carbon dioxide in regions where gas hydrates are found (Claypool and Kaplan, 1974;Galimov and Kvenvolden, 1983;Jeffrey et al, 1985;Claypool et al, 1985;Kvenvolden and Kastner, 1990). Similar patterns of the 613C change with depth are also observed in regions where gas hydrates are not stable, such as Cariaco Trench (Claypool and Kaplan, 1974).…”

Section: Introductionsupporting

confidence: 62%

“…Similar patterns of the 613C change with depth are also observed in regions where gas hydrates are not stable, such as Cariaco Trench (Claypool and Kaplan, 1974). These similar patterns in both hy drate-bearing and hydrate-free sediments suggest that the hydrate formation causes no 13C fraction ation in methane (Claypool et al, 1985). The S13C values of methane in gas hydrates are similar to those of interstitial gases in sediments at the depth where the gas hydrates were found, also suggest ing that hydrate formation involves no 13C frac tionation in methane (Pflaum et al, 1986;Kennicutt et al, 1993).…”

Section: Introductionsupporting

confidence: 60%

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Organic carbon content, bacterial methanogenesis, and accumulation processes of gas hydrates in marine sediments.

Waseda

1998

Geochem. J.

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Gas hydrate volume% filled in pore space of sediments by in situ bacterial methane production is cal culated as a function of total organic carbon (TOC) contents in sediments, assuming that all the excess amount of methane beyond the solubility in pore water can form the hydrate under ordinary conditions of outer continental margins. The results suggest that at least 0.5% TOC is required for the hydrate formation. Average volume of gas hydrates filled in pore space of hydrate-bearing sediments is estimated to be about 5-6% at a site on the Blake Ridge (Matsumoto et al., 1996). TOC required to fill the 5% pore volume as gas hydrate is calculated to be about 2% in the case of water depth of 3000 m and utilizable organic carbon for methanogenesis of 10%. This TOC value is comparable to the measured TOC values in the sediments at the site (0.8 to 2.3%, average 1.4%; Shipboard Scientific Party, 1996b). Therefore, hydrate formation by in situ bacterial methanogenesis can roughly explain the average amount of gas hydrates in the sediments. However, for the formation of locally concentrated massive gas hydrates, some accumulation processes are required. Accumulation of gas hydrates near the base of gas hydrate stability zone (BGHS) is possible by the recycling of methane and migration of methane from depths below the BGHS.

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Section: Introductionsupporting

confidence: 62%

Section: Introductionsupporting

confidence: 60%

Organic carbon content, bacterial methanogenesis, and accumulation processes of gas hydrates in marine sediments.

Waseda

1998

Geochem. J.

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“…A completely closed system would result in δ 13 C-CO 2 values that continually increase with depth, whereas the δ 13 C-CO 2 profiles at each site approach constant values with increasing depth. Constant δ 13 C values with increasing depth for CO 2 and methane may be explained by mixing with a deeper isotopically uniform gas reservoir (Paull et al, 2000) or contributions from organic matter fermentation that balance losses to carbonate reduction (Claypool et al, 1985).…”

Section: Methane Production From Co 2 Reduction and Dissolved Inorganmentioning

confidence: 99%

Expedition 311 Synthesis: scientific findings

Riedel¹,

Collett²,

Malone³

2010

Proceedings of the IODP

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Integrated Ocean Drilling Program Expedition 311 was conducted to study gas hydrate occurrences and their evolution along a transect spanning the entire northern Cascadia accretionary margin. A transect of four research sites (U1325, U1326, U1327, and U1329) was established over a distance of 32 km, extending from Site U1326 near the deformation front to Site U1329 at the eastern limit of the inferred gas hydrate occurrence zone. In addition to the transect, a fifth site (U1328) was established at a cold vent setting with active fluid and gas expulsion, which provided an opportunity to compare regional pervasive fluid-flow regimes to a site of focused fluid advection. In this synthesis, a revised gas hydrate formation model is proposed based on a combination of geophysical, geochemical, and sedimentological data acquired during and after Expedition 311 and from previous studies. The main elements of this revised model are as follows:1. Fluid expulsion by tectonic compression of accreted sediments at nonuniform expulsion rates along the transect results in the evolution of variable pore water regimes across the margin. Sites closer to the deformation front are characterized by pore fluids enriched in dissolved salts at depth, where zeolite formation from ash diagenesis is dominant. In contrast, the landward portion of the margin shows a freshening of pore fluids with depth as a result of the progressive overprinting of diagenetic salt generation with freshwater generation from the smectite-to-illite transition at greater depth. 2. In situ methane produced by microbial CO 2 reduction within the gas hydrate stability zone is the prevalent gas source for gas hydrate formation. 3. Some minor methane advection from depth is required overall to explain the occurrence of gas hydrate (and the associated downhole isotopic signatures of CH 4 and CO 2 ) within the sediments of the accretionary prism and the absence of gas hydrate within the abyssal plain sediments. In contrast, methane migrating from depth is a dominant source for gas hydrate formation at the cold vent Site U1328 (Bullseye vent). 4. Gas hydrate preferentially forms in coarser grained sandy/silt turbidites, resulting in very high local gas hydrate concentrations. Typically, gas hydrate occupies <5% of the pore space throughout the gas hydrate stability zone. Higher gas hydrate saturations were observed in intervals with abundant coarse-

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“…Methane migrated from depth will certainly be fossil. Furthermore, the G 13 C of microbial methane in continental margin settings is known to become more 13 Cenriched with increasing depth as a result of a methanogenic kinetic isotope effect during sediment burial (Galimov and Kvenvolden, 1983;Claypool et al, 1985;Paull et al, 2000;Pohlman et al, in revision). We speculate the disparity between the 14 C and 13 C content of the hydrate-bound and core gas is the result of in situ production of non-fossil,…”

Section: Application Of Methane Radiocarbon Signatures For Spatial Anmentioning

confidence: 99%

“…Although exceptions to these general characteristics have been documented (e.g., Claypool et al, 1985;Milkov and Dzou, 2007;Pohlman et al, in revision), they have proven effective for identifying the sources of methane in HGF regimes where hydrocarbon gases are hypothesized to migrate rapidly along structural features to form near-seafloor gas hydrate deposits (Milkov, 2005).…”

Section: Introductionmentioning

confidence: 99%

Methane sources in gas hydrate-bearing cold seeps: Evidence from radiocarbon and stable isotopes

Pohlman

Bauer²,

Canuel³

et al. 2009

Marine Chemistry

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ABSTRACT. Fossil methane from the large and dynamic marine gas hydrate reservoir has the potential to influence oceanic and atmospheric carbon pools. However, natural radiocarbon ( 14 C) measurements of gas hydrate methane have been extremely limited,and their use as a source and process indicator has not yet been systematically established. In this study, gas hydrate-bound and dissolved methane recovered from six geologically and geographically distinct high-gas-flux cold seeps was found to be 98 to 100% fossil based on its 14 C content. Given this prevalence of fossil methane and the small contribution of gas hydrate (<1%) to the present-day atmospheric methane flux, non-fossil contributions of gas hydrate methane to the atmosphere are not likely to be quantitatively significant. This conclusion is consistent with contemporary atmospheric methane budget calculations.In combination with G 13 C-and GD-methane measurements, we also determine the extent to which the low, but detectable, amounts of 14 C (~1-2 percent modern carbon, pMC) in methane from two cold seeps might reflect in situ production from near-seafloor sediment organic carbon (SOC). A 14 C mass balance approach using fossil methane and 14 C-enriched SOC suggests that as much as 8 to 29% of hydrate-associated methane carbon may originate from SOC contained within the upper 6 meters of sediment. These findings validate the assumption of a predominantly fossil carbon source for marine gas hydrate, but also indicate that structural gas hydrate from at least certain cold seeps contains a component of methane produced during decomposition of non-fossil organic matter in near-surface sediment.

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Isotopic Composition of Interstitial Fluids and Origin of Methane in Slope Sediment of the Middle America Trench, Deep Sea Drilling Project Leg 84

Cited by 34 publications

References 24 publications

Organic carbon content, bacterial methanogenesis, and accumulation processes of gas hydrates in marine sediments.

Organic carbon content, bacterial methanogenesis, and accumulation processes of gas hydrates in marine sediments.

Expedition 311 Synthesis: scientific findings

Methane sources in gas hydrate-bearing cold seeps: Evidence from radiocarbon and stable isotopes

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