Leg 204 Synthesis: Gas Hydrate Distribution and Dynamics in the Central Cascadia Accretionary Complex

摘要: Ocean Drilling Program (ODP) Leg 204 to Hydrate Ridge, located on the continental slope offshore Oregon (USA), was first drilling expedition dedicated understanding gas hydrate processes in accretionary complexes and provided a testbed for number of different techniques estimating content sediments. It also time that (1) digital infrared scans core temperature were systematically recorded all cores from within or near stability zone, facilitating identification samples further study, (2) hydrate-bearing recovered logged at situ pressure, (3) ODP allowed acquisition logging-while-drilling data prior coring given site, providing an initial comprehensive estimate distribution used design subsequent program. Gas estimates based variety geophysical geochemical indicate heterogeneous hydrate, which results part because two distinct regimes delivery zone. In “reaction regime,” is pervasive throughout study region, average sediments relatively low (2%–8% pore space), no present upper ~30 meters below seafloor (mbsf) methane water saturation, fine-scale depends strongly lithology. Superimposed reaction regime “transport-dominated re1Trehu, A.M., Torres, M.E., Bohrmann, G., Colwell, F.S., 2006. synthesis: dynamics central Cascadia complex. Trehu, F.S. (Eds.), Proc. ODP, Sci. Results, 204: College Station, TX (Ocean Program), 1–40. doi:10.2973/odp.proc.sr.204.101.2006 2College Oceanic Atmospheric Sciences, State University, Corvallis OR 97331, USA. Correspondence author: trehu@coas.oregonstate.edu 3Research Center Margins, University Bremen, D-28334 Germany. 4Idaho National Laboratory, Idaho Falls ID 83415, Present address: Initial receipt: 22 March 2006 Acceptance: 4 September Web publication: 1 November Ms 204SR-101 A.M. TREHU ET AL. LEG SYNTHESIS: GAS HYDRATE DISTRIBUTION AND DYNAMICS 2 gime” focused into stratigraphically controlled conduit transported as free structural summit. At summit, high pressure drives through resulting shallow deposit comprises ~25% total sediment volume depth ~25 mbsf. Geochemical most forms summit has migrated greater either thermogenic altered biogenic character, modeling suggests abundant needed form these conditions. Although this contains only ~2% estimated trapped it may be particularly susceptible destabilization response oceanographic change. INTRODUCTION 204, topographic complex ~80 km west Newport, 15 east current deformation front (Fig. F1), builds Deep Sea Project (DSDP) 18 (Sites 174–176) (Kulm von Huene, 1973) 146 888–892) (Westbrook, Carson, Musgrave, et al., 1994), studied process subduction accretion margin, 164, drilled Blake Ridge (Paull, Matsumoto, Wallace, 1996), leg processes. complemented by Integrated (IODP) Expedition 311, targeted segment northern margin characterized coarse-grained trench (Expedition 311 Scientists, 2005), contrast finer-grained dominate lithologies sampled during (Trehu, Rack, 2003; Su volume, Gracia volume). Nine sites occupied (Table T1; Fig. F2A). The depths 780–1210 sea level (mbsl) southern (SHR). All × 11 region imaged subseafloor ~1 three-dimensional (3-D) seismic data. 3-D provide tectonic stratigraphic links among F2B–F2F) enable reconstruction geologic history (Chevallier volume; Bangs 2005). Logging-while-drilling (LWD) acquired beginning inferred electrical resistivity anomalies. These plan Immediately after recovery, zone (GHSZ) scanned with track-mounted camera detect “cold spots” indicative recent ongoing dissociation hydrate. sections showing strongest cold spots shipboard experiments calibrate proxies presence preserve shore-based analyses. determined measurements (see “Intercalibration Different Proxies,” p. 10). data, limited interpretation, presented Reports 2003). -000 0 000