-3.5
SMOW, which is 3.5-4.0
heavier than the ambient interstitial water.1. Leg 164 scientists recovered a number of solid gas hydrates that were large enough to be visible to the naked eye from Sites 994 and 997 on the Blake Ridge and from Site 996 on the Blake Ridge Diapir. Blake Ridge samples are either nodular to thick massive whereas Carolina Rise samples mostly occurs as fracture-filling veins or as rectangular platy fragments.
2. Textural relations between gas hydrate and surrounding host sediment are clearly observed on an X-ray CT imagery in a resolution of 0.35 mm × 0.35 mm area and 1.0 mm in thickness. The CT value of gas hydrate is -65, whereas those of water and water ice are -100 and -250, respectively.
3. The NMR measurement has revealed that the Blake Ridge hydrate is Structure I methane hydrate in which both the small and large cages are filled by methane.
4. Low-temperature X-ray diffraction analysis has determined that Blake Ridge hydrate is Structure I; the crystal is cubic with a lattice constant of a = 11.95 ± 0.05Å at -59ºC, and the space group is Pm3n.
5. Raman spectroscopy indicates that the specimen is almost pure methane. Thermodynamic calculation based on observed Raman spectra gives the hydration number of 6.2.
6. Oxygen isotopic
composition of the water of gas hydrate is 2.67-3.5
SMOW, which is 3.5-4.0
heavier than the ambient interstitial water.
7. Gases of the Blake
Ridge gas hydrate are dominated by methane (99.9 vol%) with minor heavier
hydrocarbon gases (C2-C5 = 0.1 vol%). The 
13C
(-66 to -70)
and D
(-201 to -206)
of methane strongly indicate that the methane trapped in gas hydrate was
generated by bacterial CO2 reduction.
8. Thermal conductivity of the specimens ranges between 0.3 and 0.5 W/(m·K), which is similar to that of synthetic methane hydrate.
9. Equilibrium dissociation experiments indicate that the three-phase equilibrium condition of the Blake Ridge hydrate is 3.27 MPa at 274.7 K. This is nearly identical to that of a pure methane hydrate formed in pure water.
