Isotopic composition of CH4, CO2 species, and sedimentary organic matter within samples from the Blake Ridge: gas source implications

Paull, C. K.; Lorenson, T. D.; Borowski, Walter S; Ussler, William; Olsen, Karyn C.; Rodriguez, N. M.

doi:10.2973/odp.proc.sr.164.207.2000

Cited by 70 publications

(68 citation statements)

References 49 publications

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“…The increase in δ 13 C with sediment depth at each site is a closedsystem KIE from preferential consumption of 12 CO 2 during methanogenesis, which drives the residual CO 2 and accumulated methane toward more enriched 13 C values. Similar observations were made at Blake Ridge (Leg 164; Paull et al, 2000) and SHR (Leg 204;Claypool et al, 2006). The trend of 13 C enrichment in DIC (Torres and Kastner) is similar to that of CO 2 , with the difference being related to equilibrium isotope effects that occur during the degassing of CO 2 from the dissolved phase (Emrich et al, 1970).…”

Section: Methane Production From Co 2 Reduction and Dissolved Inorgansupporting

confidence: 75%

“…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%

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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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Section: Methane Production From Co 2 Reduction and Dissolved Inorgansupporting

confidence: 75%

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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“…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%

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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“…3). Because CH 4 is greatly depleted in 13 C, due to isotope fractionation during methanogenesis at depth (Whiticar, 1999;Paull et al, 2000), the conversion of CH 4 to HCO − 3 (Eq. 1) decreases the δ 13 C of DIC across the SMT (Torres et al, 2007;Holler et al, 2009;Chatterjee et al, 2011;Yoshinaga et al, 2014).…”

Section: Pore Water Chemistry Above Methane-charged Sedimentmentioning

confidence: 99%

Pore water geochemistry along continental slopes north of the East Siberian Sea: inference of low methane concentrations

et al. 2017

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Abstract. Continental slopes north of the East Siberian Sea potentially hold large amounts of methane (CH 4 ) in sediments as gas hydrate and free gas. Although release of this CH 4 to the ocean and atmosphere has become a topic of discussion, the region remains sparingly explored. Here we present pore water chemistry results from 32 sediment cores taken during Leg 2 of the 2014 joint Swedish-Russian-US Arctic Ocean Investigation of Climate-Cryosphere-Carbon Interactions (SWERUS-C3) expedition. The cores come from depth transects across the slope and rise extending between the Mendeleev and the Lomonosov ridges, north of Wrangel Island and the New Siberian Islands, respectively. Upward CH 4 flux towards the seafloor, as inferred from profiles of dissolved sulfate (SO 2− 4 ), alkalinity, and the δ 13 C of dissolved inorganic carbon (DIC), is negligible at all stations east of 143 • E longitude. In the upper 8 m of these cores, downward SO 2− 4 flux never exceeds 6.2 mol m −2 kyr −1 , the upward alkalinity flux never exceeds 6.8 mol m −2 kyr −1 , and δ 13 C composition of DIC (δ 13 C-DIC) only moderately decreases with depth (−3.6 ‰ m −1 on average). Moreover, upon addition of Zn acetate to pore water samples, ZnS did not precipitate, indicating a lack of dissolved H 2 S. Phosphate, ammonium, and metal profiles reveal that metal oxide reduction by organic carbon dominates the geochemical environment and supports very low organic carbon turnover rates. A single core on the Lomonosov Ridge differs, as diffusive fluxes for SO 2− 4 and alkalinity were 13.9 and 11.3 mol m −2 kyr −1 , respectively, the δ 13 C-DIC gradient was 5.6 ‰ m −1 , and Mn 2+ reduction terminated within 1.3 m of the seafloor. These are among the first pore water results generated from this vast climatically sensitive region, and they imply that abundant CH 4 , including gas hydrates, do not characterize the East Siberian Sea slope or rise along the investigated depth transects. This contradicts previous modeling and discussions, which due to the lack of data are almost entirely based on assumption.

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Isotopic composition of CH4, CO2 species, and sedimentary organic matter within samples from the Blake Ridge: gas source implications

Cited by 70 publications

References 49 publications

Expedition 311 Synthesis: scientific findings

Expedition 311 Synthesis: scientific findings

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

Pore water geochemistry along continental slopes north of the East Siberian Sea: inference of low methane concentrations

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