Geotimes
Highlights
Ocean Drilling Program
Tom Davies

While JOIDES Resolution, the drilling vessel of the Ocean Drilling Program (ODP), continued her routine probing of the ocean floor throughout 2002, the year was marked by two cruises that were anything but routine. ODP Leg 201, off the coast of Peru, was the first ODP cruise devoted entirely to sub-seafloor biosphere studies. ODP Leg 204, off the coast of Oregon, brought an array of new tools and techniques to bear on the issue of gas hydrates and their formation.
Leg 201 drilled seven sites selected to represent the general range of subsurface environments that exist in marine sediments throughout most of the world's oceans. Water depths ranged from 5,300 meters to 150 meters, with the drill penetrating as deep as 420 m into oceanic sediments and the underlying rocky crust. The sediments ranged in temperature from 1degree to 25 degrees Celsius and in age from 0 to almost 40 million years old. Sediments buried on the continental shelf of Peru show much higher microbial abundances than sediments of the open ocean. Indeed, the open Pacific sites contained some of the lowest average microbial concentrations ever observed in deep-sea sediments, whereas sediments recovered from the Peru shelf contained the highest concentrations of sub seafloor microbes ever reported.

The shipboard scientific team, composed largely of microbiologists and geochemists, performed a wide range of experiments, including the use of radioisotopes, to document metabolic activity and subsurface communities. Active microbial respiration occurs throughout the sediment column at every site. At all sites sub seafloor respiration is supported by the diffusion of sulfate from the overlying ocean, as well as by the dissolution of iron- and manganese-bearing minerals. At the open ocean sites transport of sulfate, nitrate and oxygen by fluids circulating through the underlying basaltic crust also supports respiration. At both the open Pacific sites and the Peru margin sites, iron reduction and manganese reduction often co-occur with sulfate reduction and methanogenesis, the latter processes co-occurring everywhere. Current rates of microbial activity are clearly affected by sedimentary properties and, by inference, past oceanographic conditions.

Coring and logging at nine sites on the Oregon continental margin during Leg 204 was focussed on determining the distribution and concentration of gas hydrates in an accretionary ridge (Hydrate Ridge) and adjacent slope basin, investigating the mechanisms that transport methane and other gases into the gas hydrate stability zone (GHSZ), and determining the range of physical properties of gas hydrates in situ. A 3-D seismic survey conducted in June 2000 allowed identification of potential subsurface fluid conduits and indicated the depth of the GHSZ throughout the survey region. Logging-while-drilling (LWD) data were acquired at all but one site to provide an overview of downhole physical properties prior to coring. LWD utilizes sensors in the bottom hole assembly close to the drillbit to measure downhole parameters as drilling proceeds, thus avoiding the time and risks involved in separate, post-drilling wireline logging runs. The LWD data confirmed the general position of critical seismic horizons and, through the proxy of in situ electrical resistivity, provided an initial estimate of hydrate concentration. Hydrates were subsequently sampled by conventional coring and by pressure coring devices designed to retain near in situ pressures. Hydrates decompose rapidly at normal temperatures and pressures. Infrared thermal imaging of cores immediately upon recovery allowed rapid assessment of the distribution and texture of hydrate within the cores. Geochemical analyses of interstitial waters and of headspace and void gases provided additional information on the distribution of hydrate within the stability zone, the origin and pathways of fluids into and through the GHSZ, and the rate of formation of hydrate. Biostratigraphic and lithostratigraphic descriptions of cores, and measurements of physical properties complete the data set.

Leg 204 showed that on Hydrate Ridge gas hydrates are distributed within a broad depth range within the GHSZ, with lithology strongly influencing hydrate concentration. Significant concentrations of higher-order hydrocarbons were also recorded within the gas hydrate system. Hydrate concentration is significantly greater beneath the ridge than beneath the adjacent slope basin where hydrates are concentrated just above the bottom-simulating reflector (BSR). The BSR, usually taken to be an indicator of the presence of hydrate, proved to represent a discontinuity in several chemical constituents of pore waters and gas voids. Clearly hydrate formation has profound geochemical effects. Since hydrate formation expels salts from the pore fluid, the very high chloride concentrations found extending to a depth of 20 to 30 meters below seafloor near the summit of Hydrate Ridge indicate that hydrate formation there must be very recent and rapid.

The exciting science promised by Leg 204 stimulated widespread interest, which in turn led to some novel operational and logistical challenges. At one time or another there were four other research vessels (Sonne, Ewing, Atlantis and New Horizon) and a submersible (Alvin) operating around JOIDES Resolution, either engaged in joint experiments with the drillship or conducting independent experiments. The ship's proximity to land (only 50 miles offshore) permitted an unprecedented nine rendezvous (seven by helicopter and two by boat) for the rapid transfer of frozen samples to laboratories on shore and the exchange of personnel (scientific and engineering staff, media representatives, and industry and government observers) and supplies.

Other ODP cruises during 2002 included: a transect of sites in the eastern Pacific from high southern latitudes to the equator to elucidate Southern Hemisphere and tropical climate variability and biogeochemical systems since the late Paleogene; a deep crustal hole in the equatorial east Pacific for future installation of a sub-seafloor seismic observatory; installation of CORKs with temperature and pressure sensors and osmosamplers off Costa Rica to sample formation fluids in the incoming plate and in the decollement just inboard of the deformation front; and deep crustal drilling into 15 Ma crust on the Cocos plate, formed by superfast spreading from the East Pacific Rise. The Cocos plate hole is one of the deepest ever drilled into the oceanic basement and is the first to be engineered from the start for deep crustal penetration. The goal of this multi-leg project is to penetrate into the plutonic foundation of the oceanic crust.
2003 takes JOIDES Resolution back into the Atlantic and the final four legs of ODP. The operational phase of the program will end when the ship enters St. John's, Newfoundland, in September 2003.

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Davies is the manager of the Science Services department for the Ocean Drilling Program office at Texas A&M University. Email: davies@odpemail.tamu.edu.

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