Abstract
Two gas hydrate cores from Integrated Ocean Drilling Program Expedition 311 Site U1328 (northern Cascadia margin) were analyzed for structural and compositional characterization. Gas hydrate was present mainly as nodules surrounded by sediment. The samples were recovered from 4.5 to 8.68 meters below seafloor using a conventional nonpressurized coring technique. Quenched in liquid nitrogen upon recovery, the gas hydrates were kept at cryogenic temperatures for sampling with Raman and nuclear magnetic resonance spectroscopy. Pure structure I (sI) methane gas hydrate was found in one sample and a methane-rich sI gas hydrate containing hydrogen sulfide was found in the second sample. Small local variations in the hydrogen sulfide amounts were shown. A hydration number of 6.08 was measured for both samples, in agreement with previous work with synthetic and recovered natural gas hydrates from similar locations. A method for determining the fraction of hydrate vs. ice using a Raman peak from water is given. This approach provides a possible tool to estimate sample degradation (conversion of gas hydrate to water + gas) as a result of recovery. Quantitative agreement between the methane cage occupancy ratios was shown for the nuclear magnetic resonance and Raman spectroscopic techniques for both sample sets, comprising pure methane and a mixture of methane + hydrogen sulfide.
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