Leg 204 synthesis: gas hydrate distribution and dynamics in the central Cascadia accretionary complex

Abstract

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