After completing the first expedition of the Integrated Ocean Drilling Program, scientists onboard the drillship JOIDES Resolution retrieved data and samples that have been stored beneath the seafloor for several years. This expedition, conducted in conjunction with a transit through the Panama Canal, obtained samples stored in long-term sea floor observatories across the Middle America Trench, offshore Costa Rica. The information collected from these observatories will provide information on
the deep biosphere, gas hydrate formation, and earthquakes.
The observatories, known to scientists as CORK IIs, are designed to obtain long-term high-resolution samples of fluids flowing below the sea floor. The Costa Rica CORKs are part of network of underwater observatories that monitor conditions on and below the seafloor.
A team of American, German and Japanese scientists successfully recovered the original downhole instruments, which were installed in late 2002 during an Ocean Drilling Program expedition, and replaced them with new instruments designed to operate for 3.5 years.
"To recover these instruments and deploy the new ones required lowering tools into a hole less than 1 meter in diameter, located in an area of almost 4400 meters water depth. The operations required the JOIDES Resolution to keep excellent position over the wellhead for extended periods of time, and necessitated great ingenuity on the part of the IODP Engineering and Operations personnel," explained Dr. Miriam Kastner of Scripps Institution of Oceanography, the chief scientist on the expedition.
One observatory is located in the fractured oceanic rock of the plate that subducts beneath the Middle America volcanic arcs at nearly 500 meters below the sea floor. The downhole instruments monitor the pressure and temperature of circulating fluids through time, and collect a time series of fluids and gases. The second observatory is located in the fault zone that is the boundary between the subducting Pacific Plate and the upper North American Plate. Instruments here are designed to measure the rate at which fluids flow up the plate boundary fault, as well as monitor pressure and temperature and collect a time-series of fluid and gas samples.
Fluids are collected using Osmotic Samplers (OsmoSamplers), which require no batteries or moving parts. Fluids are stored in long narrow Teflon and copper coils within the observatory. Weekly samples yield 3-4 ml of formation fluid. Shipboard measurements of salinity, alkalinity and ammonium show that the OsmoSamplers performed as designed, and captured a two year record of fluid and gas chemistry and flow. Temperature variations with a resolution of 0.001 °C were recorded with a small, pencil-shape data logger over the complete time period fluids were sampled. This is an additional data set which will help to interpret the results from the fluid chemistry.
The long-term observatories are used to study the fate of water and other volatiles in the shallow parts of subduction zones. The subducting plate carries large amounts of water and other volatiles into the subduction trench. Much of the water leaves the subducting plate at shallow depths, carrying methane and heavier hydrocarbons plus fluid-soluble elements to the ocean, thus contributing to gas hydrate formation, affecting the composition of the ocean, for selected elements, and may be supporting a deep biosphere. The fluids also carry a record of water-rock reactions deeper along the plate interface, where large hazardous earthquakes nucleate. Fluids that subduct deeper in the Earth power the explosive eruptions at volcanic arcs and can contribute to ore formation.
The JOIDES Resolution has now passed through the Panama Canal and will shortly arrive in St. John's, Newfoundland for a port call. The next expedition leaves from St. John's and will obtain cores of sediment and rock that are needed to understand the mechanisms of global climate change. The cores will also yield information that will improve our knowledge of the geomagnetic field.
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IODP is an international partnership of scientists and research institutions that uses multiple drilling vessels to explore the history and structure of the Earth. The JOI Alliance, JOI's partnership with Texas A&M University and Lamont-Doherty Earth Observatory of Columbia University, manages activities associated with the riserless vessel for the program with funding from the National Science Foundation. For additional information, visit IODP at http://www.oceandrilling.org/.
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