Methane sources and production in the northern Cascadia margin gas hydrate system

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Abstract

The oceanographic and tectonic conditions of accretionary margins are well-suited for several potential processes governing methane generation, storage and release. To identify the relevant methane evolution pathways in the northern Cascadia accretionary margin, a four-site transect was drilled during Integrated Ocean Drilling Program Expedition 311. The δ13C values of methane range from a minimum value of − 82.2‰ on an uplifted ridge of accreted sediment near the deformation front (Site U1326, 1829 mbsl, meters below sea level) to a maximum value of − 39.5‰ at the most landward location within an area of steep canyons near the shelf edge (Site U1329, 946 mbsl). An interpretation based solely on methane isotope values might conclude the 13C-enrichment of methane indicates a transition from microbially- to thermogenically-sourced methane. However, the co-existing CO2 exhibits a similar trend of 13C-enrichment along the transect with values ranging from − 22.5‰ to +25.7‰. The magnitude of the carbon isotope separation between methane and CO2 (εc = 63.8 ± 5.8) is consistent with isotope fractionation during microbially mediated carbonate reduction. These results, in conjunction with a transect-wide gaseous hydrocarbon content composed of > 99.8% (by volume) methane and uniform δDCH4 values (− 172‰ ± 8) that are distinct from thermogenic methane at a seep located 60 km from the Expedition 311 transect, suggest microbial CO2 reduction is the predominant methane source at all investigated sites.

The magnitude of the intra-site downhole 13C-enrichment of CO2 within the accreted ridge (Site U1326) and a slope basin nearest the deformation front (Site U1325, 2195 mbsl) is ~ 5‰. At the mid-slope site (Site U1327, 1304 mbsl) the downhole 13C-enrichment of the CO2 is ~ 25‰ and increases to ~ 40‰ at the near-shelf edge Site U1329. This isotope fractionation pattern is indicative of more extensive diagenetic alteration at sites with greater 13C-enrichment. The magnitude of the 13C-enrichment of CO2 correlates with decreasing sedimentation rates and a diminishing occurrence of stratigraphic gas hydrate. We suggest the decreasing sedimentation rates increase the exposure time of sedimentary organic matter to aerobic and anaerobic degradation, during burial, thereby reducing the availability of metabolizable organic matter available for methane production. This process is reflected in the occurrence and distribution of gas hydrate within the northern Cascadia margin accretionary prism. Our observations are relevant for evaluating methane production and the occurrence of stratigraphic gas hydrate within other convergent margins.

Introduction

Marine gas hydrate occurs along continental margins under conditions of low temperature and moderate pressure (>300–500 m water depth) where the concentration of low molecular weight gases (primarily methane) exceeds pore water solubility ( Kvenvolden, 1993, Xu and Ruppel, 1999). Enhanced primary production, efficient organic matter burial and tectonic fluid expulsion provide ideal conditions for producing methane and concentrating it within the gas hydrate stability field of convergent margins ( Hyndman and Davis, 1992, Kastner, 2001). Accordingly, accretionary convergent margins stretching along ~ 29,000 km of oceanic subduction zones ( von Huene and Scholl, 1991 ) may contain as much as two-thirds of the marine gas hydrate reservoir ( Kastner, 2001 ).

Expedition 311 of the Integrated Ocean Drilling Program (IODP) was designed to investigate the systematic evolution of the gas hydrate system across the northern Cascadia margin (NCM) accretionary prism. To that end, a 33-km long transect of five boreholes (IODP Sites U1325–U1329), including an off-transect cold seep (Site U1328), was drilled from an uplifted ridge near the deformation front to a shallow near-shelf location at the landward limit of gas hydrate occurrence ( Fig. 1 ). The margin-wide transect approach employed during Expedition 311 distinguishes it from previous Ocean Drilling Program (ODP) investigations (Legs 146 and 204) that targeted gas hydrate occurrences in this region. For example, Leg 204 focused on regional structural and stratigraphic gas hydrate accumulations around Hydrate Ridge, a large accretionary ridge offshore Oregon (USA) ( Tréhu et al., 2004 ). A spatial and mechanistic understanding of methane production in convergent margins is critical for assessing the formation, occurrence and distribution of a large component of the global gas hydrate inventory.

The NCM accretionary prism is largely comprised of sediment that has been scraped off the subducting Juan de Fuca plate since Eocene time (~ 43 Ma) ( Hyndman, 1995 ). At the deformation front, layered Pleistocene hemipelagic sediments are folded and faulted into anticlinal ridges (e.g., the frontal ridge at Site U1326; Fig. 1 B). Further tectonic compression of the accreted material results in a landward series of folds and thrusts. Sediment compaction during accretion drives upward migration of methane-charged fluids into the bottom of the gas hydrate stability zone and has been implicated as the primary cause for prominent bottom simulating reflectors (BSRs) ( Fig. 1 B) and the occurrence of gas hydrate within this accretionary prism ( Hyndman and Davis, 1992 ). A surprising result from IODP Expedition 311 is that gas hydrate was often more prevalent near the top of the gas hydrate occurrence zone (GHOZ) and the thickness of the GHOZ decreased upslope of the deformation front ( Malinverno et al., 2008 ). The GHOZ, the vertical interval where gas hydrate is present, is to be distinguished from the gas hydrate stability zone (GHSZ), where the conditions for gas hydrate stability are satisfied, but gas hydrate may not be present. The observed distribution and thickness of the GHOZ indicate that other factors, e.g., sediment grain size and production of methane within the gas hydrate stability zone, might also influence gas hydrate formation within the NCM ( Malinverno et al., 2008, Torres et al., 2008).

The primary source for methane thus far recovered from gas hydrate-bearing convergent margins is microbial methane produced by CO2 reduction (also referred to as carbonate reduction or hydrogenotrophic methanogenesis) ( Claypool et al., 1985, Kvenvolden and Kastner, 1990, Whiticar et al., 1995, Milkov et al., 2005). Variable, though generally limited, thermogenic contributions have also been identified in gas hydrate-bearing convergent margins ( Claypool et al., 1985, Kvenvolden et al., 1990, Berner and Faber, 1993, Whiticar et al., 1995, Milkov et al., 2005, Pohlman et al., 2005). Whiticar et al. (1995) inferred the presence of thermogenic gases below the gas hydrate stability zone at ODP Leg 146 Site 889 (located 250 m from Site U1327) from the presence of higher order (C2+) hydrocarbons and the stable carbon isotope signatures of methane and CO2. At a high gas flux region on Hydrate Ridge (ODP Leg 204 Sites 1248–1250), Milkov et al. (2005) determined that thermogenic methane transported along a high permeability subvertical horizon from a sediment depth of 2 to 2.5 km supplied as much as 20% of the methane in shallow gas hydrate accumulations. The presence of a gas hydrate-bearing hydrocarbon seep in Barkley Canyon ( Pohlman et al., 2005 ), located 60 km from and at a similar water depth as Site U1329, is evidence for a potential thermogenic gas contribution along the Expedition 311 transect.

The prevalent empirical framework for characterizing gas origins is that microbial gas is dominated by methane with δ13C signatures ranging from − 90‰ to − 60‰ and thermogenic gas contains higher concentrations of C2+ hydrocarbons and methane with δ13C signatures ranging from − 50‰ to − 30‰ ( Whiticar, 1999 ). In some cases, however, preferential consumption of 12CO2 during CO2 reduction, i.e., the kinetic isotope effect (KIE), has been shown to enrich, or fractionate, the residual CO2 pool with 13CO2 to the extent that methane generated from it acquires a δ13C signature characteristic for thermogenic methane ( Claypool et al., 1985, Kvenvolden and Kastner, 1990, Whiticar et al., 1995). Likewise, acetoclastic methanogenesis (i.e., methane production by acetate fermentation) may also produce 13C-enriched methane that could be interpreted as an admixture of microbial and thermogenic methane based on δ13C-values alone ( Whiticar, 1999 ). Finally, it has been argued that non-thermogenic contributions of ethane, propane and, perhaps, butane are significant in convergent margins ( Kvenvolden et al., 1990, Hinrichs et al., 2006).

In this study, we utilize the stable isotope content of methane (δ13C and δD) and CO2 (δ13C) as well as the hydrocarbon composition from dissolved gas samples to constrain methane origins along the IODP Expedition 311 transect. Furthermore, we evaluate factors that control organic matter preservation and discuss how the quality of the organic matter may influence the carbon isotope mass balance and occurrence of gas hydrate in the northern Cascadia margin accretionary prism.

Section snippets

Sampling

Sediment cores recovered with the advanced piston corer (APC) and extended core barrel (XCB) were transferred to the catwalk for gas sampling. Gas expansion voids formed from porewater dissolved gases during core depressurization and temperature rise were extracted on the catwalk by piercing the clear core liner with a steel penetration tool connected via a 3-way valve to a 60 ml plastic syringe (Becton Dickinson). A total of 160 gas voids (also referred to as dissolved gas samples to more

Methane origins in the northern Cascadia margin

The δ13CCH4 profiles of void gas along the transect exhibit distinct patterns of 13C-enrichment with increasing sediment depth and distance from the deformation front ( Fig. 2 , Table S1 ). At all sites, near-surface methane is 13C-depleted (δ13CCH4range =  84.0‰ to − 72.8‰), and deeper methane becomes progressively 13C-enriched down each borehole. At the downslope Sites U1326 and U1325 (located ~ 5 and 11 km, respectively, from the deformation front, Fig. 1 B) the δ13CCH4 gradually increases to

Conclusions

The prevalence of gas hydrate concentrated near the top of the gas hydrate occurrence zone in the NCM is not consistent with the fluid expulsion model, which states that gas hydrate in accretionary prisms is concentrated where upwardly migrating methane-charged fluids enter and penetrate the gas hydrate stability field. Alternative explanations that explain the recent observation of a more widespread gas hydrate occurrence within the gas hydrate stability zone are that the expelled fluids

Acknowledgements

Samples and data were provided by the Integrated Ocean Drilling Program (IODP), which is funded by the U.S. National Science Foundation and participating countries under management of the Joint Oceanographic Institutions (JOI), Inc. Funding for this research was provided by the U.S. Science Support Program (USSP), and Natural Science and Engineering Research Council (MJW). We thank the Captain and the crew of the Joides Resolution and the technical staff for their support at sea; in particular,

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