3. Site 12441

Shipboard Scientific Party2

INTRODUCTION

Site 1244 (proposed Site HR1a) is located in 895 m of water on the eastern flank of Hydrate Ridge ~3 km northeast of the southern summit (see Fig. F1C in the "Leg 204 Summary" chapter). The three-dimensional (3-D) seismic data available from a Leg 204 site survey show that the bottom-simulating reflector (BSR) is present at a depth of ~125 meters below seafloor (mbsf) at this site (see Fig. F5 in the "Leg 204 Summary" chapter). The temperature and pressure at the seafloor are well within the gas hydrate stability zone (GHSZ) (see Fig. F3 in the "Leg 204 Summary" chapter), indicating that gas hydrates can exist within the entire stratigraphic section above the BSR if hydrate-forming gases are available in concentrations that exceed their in situ solubility. The 3-D seismic data also image a zone of incoherent seismic reflections that forms the core of Hydrate Ridge. At Site 1244, the top of this incoherent zone, denoted as Horizon AC in the seismic sections, is located at a depth of ~300 mbsf. This facies has been interpreted to comprise fractured accretionary complex material. Dipping, faulted, and strongly reflective strata interpreted to be an uplifted and deformed slope basin, overlie this facies.

The primary drilling objectives at this site were the following:

  1. Determine the distribution and concentration of gas hydrate within the GHSZ;
  2. Determine the nature of a pair of strong reflections (referred to as Reflections B and B´) that underlies much of the eastern flank of Hydrate Ridge;
  3. Determine the composition, structure, and fluid regime within the seismically incoherent unit underlying the stratified sediments; and
  4. Sample the subsurface biosphere associated with these features.

Five holes were cored at Site 1244, and an additional hole was drilled (Fig. F1). Hole 1244A was abandoned when the first core overshot and did not record a mudline. Hole 1244B was abandoned at 53.1 mbsf after six cores were obtained because the bottom-hole assembly (BHA) had to be brought to the surface to retrieve a downhole instrument (Fugro piezoprobe) that had become unscrewed from the Schlumberger conductor cable. One advanced piston corer temperature (APCT) tool measurement was taken at 35.1 mbsf in this hole. Hole 1244C, which comprises 39 cores, began at the seafloor and continued to 334 mbsf. Special tools in Hole 1244C included three APCT tool (63, 82, and 110 mbsf), one Davis-Villinger Temperature Probe (DVTP) (64 mbsf), one Davis-Villinger Temperature-Pressure Probe (DVTPP) (150 mbsf), and three pressure core sampler (PCS) (120, 131, and 142 mbsf) runs. Hole 1244C was abandoned 17 m above the target depth of 350 mbsf when hole conditions suggested that a change from extended core barrel (XCB) to rotary core barrel (RCB) coring would be appropriate. We returned later in the leg to drill Hole 1244D to 380 mbsf, which was dedicated to wireline logging and seismic work. This was followed by Hole 1244E (cored to 136 mbsf), which was extensively sampled for geochemistry, hydrates, and microbiology, and Hole 1244F (cored to 24 mbsf), primarily for high-resolution microbiological sampling.

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-103

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