[MIS21-P08] Gas hydrate survey off Tokachi (the Pacific Ocean) in the framework of practical education of Kitami Institute of Technology
Keywords:gas hydrate, methane, the Pacific Ocean
Samples of sediment gas, mainly dissolved gases in pore water, were obtained by a headspace gas method. 10 mL sediment was sampled from the sediment core by a plastic syringe (volume: 5 mL) and put into a 25 mL vial. 10 mL NaCl aqueous solution was introduced into the vial by using a micropipette and sealed employing a butyl rubber septum to make a headspace (5 mL volume). To avoid any changes in the headspace, preservative (benzalkonium chloride, BKC) was added and the headspace part was flushed by helium. We roughly checked gas composition of sediment gases onboard using a portable gas chromatograph, and after that we measured the molecular and isotopic compositions of headspace gases using another gas chromatograph and CF-IRMS in our laboratory.
Concentration of C1 increased with depth, and exceeded 1 [mM] in the bottom of some sediment cores (C008-GC1403, C046-GC1702, and C061-GC1803), indicating that their depths of SMI (sulfate-methane interface) were small and active gas seep existed at the sea floor. The minimum SMI depth was 50 [cmbsf] in C008-GC1403. C1 / (C2 + C3) in these gas-rich layer was from 300 to 700. Concentration of H2S exceeds 1 [mM] in the bottom of sediment cores. These results indicate that microbial gas is supplied from lower layer and AOM (anaerobic oxidation of methane) processes is active in beneath the sea floor. C1 δ13C profiles showed minimum values (lower than -100‰) at the SMI depth in C061-GC1803. C2 δ13C distributed between -55‰ and -30‰, suggesting that C2 seems also microbial origin.
Hayashi M, Saeki T, Inamori T, Noguchi S (2010) The distribution of BSRs related to methane hydrates, offshore Japan. J Jpn Assoc Petrol Technol 75(1): 42-53