2:45 PM - 3:00 PM
[MIS21-10] Discovery of gas hydrate at the new point off Abashiri, the Sea of Okhotsk
Keywords:gas hydrate, Sea of Okhotsk, gas origin
In the HKS18 cruise, we used a hydrostatic corer optimally tuned for a small vessel and a transponder to know exact position of the corer. We obtained nine sediment cores, including two hydrate-bound cores. Gas hydrate crystals were preserved in liquid nitrogen, and sediment gases were obtained by a headspace gas method. Raman spectroscopic analysis was conducted to know crystallographic structure, hydration number and cage occupancies. Molecular and isotopic compositions of gas samples were measured using a gas chromatograph and an isotope ratio mass spectrometer, respectively.
Crystallographic structure of gas hydrate samples belonged to the structure I, and hydration number was estimated as 6.00±0.02. Hydrate-bound H2S molecules were also detected in the Raman spectra. Hydrate-bound gas was mainly methane, and C1 / (C2 + C3) distributed between 30,000 and 60,000. C1 δ13C and δD were -66‰ and -197‰. These molecular and isotopic compositions of hydrate-bound hydrocarbons suggested microbial origin. The depths of sulfate-methane interface (SMI) in HC1807, HC1808, and HC1810 cores were less than 30 cm, indicating high gas flux. Concentrations of H2S in the headspace gas samples were in the same order (around 1 mM), therefore process of anaerobic oxidation of methane was active. C1 / (C2 + C3) of headspace gases in the hydrate-bound cores (HC1807 and HC1808) distributed between 4,000 and 9,000, and those of other sediment cores were relatively small, indicating that they are in the oxidation layer above the SMI depth. Headspace gas in the HC1808 core contained a lot of isobutane. It is possible that thermogenic gas supplied from deeper sediment layer partly mixes with microbial gas.