Ocean Drilling Program, United States

During Ocean Drilling Program (ODP) Leg 204, we will drill a transect of sites through the gas hydrate stability zone on the southern part of Hydrate Ridge on the Cascadia accretionary margin offshore Oregon. Massive hydrates are present at the seafloor there, and high-resolution three-dimensional seismic data indicate a complicated subsurface plumbing system. The transect will cover two distinctly different sedimentary and tectonic environments-the older sediments of the uplifted accretionary complex and the younger, well-stratified sediments of an adjacent rapidly filling slope basin. Leg objectives include (1) comparing the source region for gas and the physical and chemical mechanisms of hydrate formation between accretionary ridge and slope basin settings; (2) calibrating estimates of hydrate and underlying free gas concentrations determined with geophysical remote sensing techniques; (3) testing, using geochemical tracers, physical properties measurements, and microstructural analysis, whether variations in bottom-simulating reflector (BSR) and sub-BSR reflectivity observed in seismic data result from tectonically induced hydrate destabilization; (4) developing an understanding of the geochemical effects of hydrate formation in order to identify paleoproxies for methane release that can be used to integrate the geologic data into climate models; (5) determining the porosity and shear strength of hydrate-bearing and underlying sediments in order to evaluate the relationship between hydrates, fluid flow, and slope stability; and (6) quantifying the distribution of methanogenic and methanotropic bacteria in the sediments in order to evaluate their contribution to hydrate formation and destruction and related sediment diagenesis. To achieve these objectives, the first part of Leg 204 will be dedicated to logging while drilling (LWD) to identify regions of rapid change in physical properties prior to coring. This should permit us to optimize use of time-consuming special tools to measure in situ temperature and pressure and to retrieve cores at in situ pressure. Cores will be immediately scanned with an infrared device to detect cold spots indicative of dissociating hydrate prior to the standard sampling and analysis protocols commonly used in ODP operations. The leg also includes a two-ship seismic program with the Ewing to acquire offset vertical seismic profiles and other seismic data.