Site 1249 (proposed Site HR4b) was drilled in ~775 m of water on the summit of southern Hydrate Ridge (see Fig. F1 in the "Leg 204 Summary" chapter). This area is characterized by massive gas hydrate deposits at the seafloor (Suess et al., 2001). Vigorous streams of bubbles are known to emanate from the seafloor, as documented by submersible observations and high-frequency echo sounding surveys, which have repeatedly imaged "bubble plumes" in the water column (Tréhu and Bangs, 2001; Heeschen et al., 2003). These observations are interpreted to indicate that some of the methane rising through the sediment column is trapped as hydrate near the seafloor and that some escapes into the water column (Suess et al., 2001).
The seafloor in this area is anomalously reflective (Johnson et al., in press), and the seafloor reflectivity is spatially correlated with subsurface seismic reflectivity that extends to ~30 meters below seafloor (mbsf) (see Fig. F7 in the "Leg 204 Summary" chapter). These geophysical observations have been interpreted to indicate the spatial extent of lenses of massive hydrate intercalated with sediment (Tréhu et al., 2002). Seismic data also indicate that the bottom-simulating reflector (BSR) at this site is at ~115 mbsf. However, coring was only permitted to 90 mbsf because of the possibility of trapped gas beneath the BSR at this structural high.
The primary objective at Site 1249 was to determine the distribution and concentration of gas hydrate with depth at the southern summit of Hydrate Ridge and to investigate processes that allow methane gas bubbles to coexist with gas hydrate and pore water within the hydrate stability field. A second objective was to test whether the pattern of chaotic reflectivity accurately predicts the spatial extent of massive hydrate lenses.
Twelve holes were drilled at Site 1249 (Fig. F1). Logging-while-drilling (LWD) measurements were made during drilling in Hole 1249A. Hole 1249B was drilled using the new resistivity-at-the-bit (RAB)-8 LWD and coring system, which permits simultaneous acquisition of core and logging data. The 90-m sediment sequence was then sampled with the advanced piston corer (APC) in Holes 1249C-1249F, with core recoveries of <30% in the uppermost 20 mbsf and increased core recovery (up to 70%) deeper in the holes. Six holes (Holes 1249G-1249L) were cored with the APC or extended core barrel (XCB) for a special shore-based "geriatrics" study, in which several means of preserving gas hydrates for future study will be compared. During this effort, 244 m of gas hydrate-bearing sediments were cored with 35% core recovery. The samples were either stored in liquid nitrogen or steel pressure vessels, which were repressurized using methane gas and water.
1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.
Ms 204IR-108