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

摘要

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 δ~(13)C 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 ~(13)C-enrichment of methane indicates a transition from microbially- to thermogenically-sourced methane. However, the co-existing CO_2 exhibits a similar trend of ~(13)C-enrichment along the transect with values ranging from - 22.5‰ to +25.7‰. The magnitude of the carbon isotope separation between methane and CO_2 (ε_c = 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 CO_2 reduction is the predominant methane source at all investigated sites. The magnitude of the intra-site downhole ~(13)C-enrichment of CO_2 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 ~(13)C-enrichment of the CO_2 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 ~(13)C-enrichment. The magnitude of the ~(13)C-enrichment of CO_2 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.