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USGS Scientists Discover Gas Hydrate in Southern California During Cruise to Study Offshore Landslides, Earthquake Hazards, and Pollution
A sediment-sampling cruise led by U.S. Geological Survey (USGS) scientists in late July 2003 discovered gas hydrate, an icelike crystalline solid containing trapped molecules of natural gas, in sediment off southern California. To our knowledge, this is the first discovery of gas hydrate in the area north of the Gulf of California and south of the Mendocino Triple Junction (in northern California). The cruise was part of the USGS' CABRILLO (Southern CAlifornia Bight Regional Investigations Life, Land, and Ocean) project, which addresses issues that affect southern California coastal communities. The most heavily populated urban corridor along the U.S. West Coast, the southern California coastal region hosts millions of human inhabitants onshore and countless marine organisms, including marine mammals and commercially important fish, in diverse habitats offshore. Both the human and nonhuman inhabitants are at risk from pollution and toxic waste, degradation of freshwater supplies by saltwater intrusion, and the potential for earthquakes, underwater mass-wasting events (landslides and slumps), and tsunamis. USGS scientists, in conjunction with local agencies (such as the Water Replenishment District of Southern California, the Los Angeles County Department of Public Works, the Southern California Coastal Water Research Project, the city of Los Angeles, the Orange County Sanitation District, and the Southern California Earthquake Center), are studying these potential threats. The recent cruise had multiple goals, primarily:
To achieve these goals, the scientific staff sampled sediment by using the various coring devices, and they conducted CHIRP (compressed high-intensity radar pulse) high-resolution seismic-reflection profiling. Piston cores for the earthquake-hazard and tsunami studies were taken at sites identified on high-resolution stratigraphic records compiled from Huntec and Geopulse boomer data collected on previous cruises. To prepare for statistical analysis, more than half of the box cores were placed randomly, and the rest were placed by using information from a multibeam map. Vibracoring stations were chosen on the basis of CHIRP data collected on this cruise. It was in connection with the fourth goal—study of contaminants—that the gas hydrate was discovered, at a site chosen for sampling of sea-floor discharges of methylmercury. The gas hydrate was discovered in a short piston core taken from a water depth of 813 m near the summit of a mud diapir in Santa Monica Basin. The discovery core was 2.1 m long and apparently stopped in the gas hydrate, as evidenced by chunks of gas-hydrate ice at the bottom of the core. Violent degassing of a section of the core resulted in spontaneous extrusion of the sample from the coreliner in the ship's laboratory. Fresh mussel shells recovered from the top of the core indicate the presence of a "cold seep" community supported by methane venting from the diapir. The existence of gas hydrate at such a shallow depth in the sediment—only 2.1 m below the sea floor—was unexpected. The mud diapir on which the gas hydrate was sampled has extruded through well-bedded sediment on the lower slope of the basin; it is about 300 m in diameter at its base and less than 100 m across its gently sloping summit. Subsequent sampling in the area of the diapir, using piston, gravity, and box corers, failed to sample more gas hydrate but did recover additional cold-seep fauna. In all, we collected 29 piston cores, 25 gravity cores, 43 box cores, and 19 vibracores, plus 3 hours of CHIRP high-resolution seismic-reflection data in support of the vibracoring operations. We successfully collected core samples from each planned site within the study area. More cores were collected per day on this cruise than on any of the preceding cruises supporting the CABRILLO project. Most piston cores and vibracores, as well as subsamples from box cores, were analyzed for physical properties by using the Geotek Multi-Sensor Core Logger (MSCL) at sea to avoid changes caused by pore-water loss and sediment compaction during transport or storage. Planned onshore analyses of cores collected on the cruise include strat/sed (SS), radiocarbon dating, chlorinated hydrocarbon (CH), trace metals (TM), 210Pb, texture, foraminifers, and grain size. Statistical analyses will evaluate the variation of contamination levels over the area of the Los Angeles margin. The cruise was conducted from the Auriga, a 176-ft-long crabbing boat out of Seattle, WA, which personnel from the USGS' Marine Facility in Redwood City, CA, transformed into an oceanographic research vessel by installing the Coastal and Marine Geology team's standard shipping-container vans, each outfitted for a specific scientific function. The ship departed from Redwood City on July 18, 2003, carrying 17 cruise participants (15 USGS Coastal and Marine Geology team members and two Skidaway Institute of Oceanography employees) from San Francisco Bay to southern California. About a week into the cruise, a group from the Dana Point Ocean Institute arrived onboard with a new cruise member and were given a short tour of the ship. At the end of the cruise, a representative from the University of California, Los Angeles (UCLA), joined the Auriga to lead the recovery of a mooring that had been deployed jointly by UCLA and the Monterey Bay Aquarium Research Institute (MBARI) about 2 years ago in about 500 m of water in the middle of Santa Monica Bay. The mooring, protected by a large surface buoy with numerous topside instruments, had been collecting data for a study of long-term oceanographic and atmospheric conditions at the site. The success of this cruise was due to the excellent precruise planning conducted by the cruise chief scientists and to the enthusiastic support of the cruise participants. USGS employees (in alphabetical order) included Simon Barber, Mike Boyle, Brad Carkin, Jamie Conrad, Pete Dartnell, Brian Edwards (co-chief scientist), Cathy Frazee, Eric Grossman, Lori Hibbeler, Homa Lee (co-chief scientist), Bill Normark, Walt Olson, Kevin O'Toole, Jon Warrick, and Hal Williams. Skidaway Institute of Oceanography employees onboard were Claudia Venherm and Alyson Cotton. Representing UCLA's Institute of the Environment was Levanto Schachter. We owe special thanks to the USGS Marine Facility team for their timely and professional support, and to the Auriga crew for their excellent piloting of the ship and timely transits: owner/engineer, Richard Kelly; captain, Ted Blinkers; second mate, Donna White; third mate, William White; and crew mates, Tom Lyon and John Kelly. Last, but certainly not least, we appreciate the superior cooking of the Auriga's chef, Modu Ndiaye.
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in this issue:
Gulf of Mexico Deep-Water Coral Habitats
Monterey Bay National Marine Sanctuary
Maender Receives Communicator of the Year Award |
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U. S. Department of the Interior | U.S. Geological Survey
cover story:
