Oxygen isotopes and hydrocarbon composition of gas hydrate and hydrate water from ODP Leg 165 sites

Sites 994 and 997 samples, either nodular or thick massive pieces, were subjected to laboratory analysis and measurements to determine the structure, molecular and isotopic composition, thermal conductivity, and equilibrium dissociation conditions. X-ray computed tomography (CT) imagery, X-ray diffraction, nuclear magnetic resonance (NMR), and Raman spectroscopy have revealed that the gas hydrates recovered from the Blake Ridge are nearly 100% methane gas hydrate of Structure I, cubic with a lattice constant of a = 11.95 ± 0.05 angström, and a molar ratio of water to gas (hydration number) of 6.2.

The d18O of water is 2.67 per mil to 3.51 per mil SMOW, which is 3.5-4.0 heavier than the ambient interstitial waters. The d13C and dD of methane are -66 per mil to -70 per mil and -201 per mil to -206 per mil, respectively, suggesting that the methane was generated through bacterial CO2 reduction. Thermal conductivity values of the Blake Ridge hydrates range from 0.3 to 0.5 W/(m K).