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doi:10.2204/iodp.proc.314315316.209.2013

Materials and methods

Six void gas samples were taken from 156 to 240 meters below seafloor (mbsf) in Hole C0001F. The sections for sampling void gas were restricted because the release of void gas from the core liner was preferred in order to avoid explosion caused by gas expansion. Void gas samples were collected directly through the core liner with a plastic syringe and were transferred into 65 mL vials that were vacuumed beforehand, sealed with rubber stoppers, and crimp-sealed with aluminum caps. Void gas samples were analyzed for volatile hydrocarbon contents on board following the method of Pimmel and Claypool (2001). After analyzing the samples, they were preserved at 4°C for later measurement in the onshore laboratory at Hokkaido University, Japan.

In the shore laboratory, concentrations and carbon isotope compositions of methane in void gas samples were measured by gas chromatograph/combustion/isotope ratio mass spectrometer (HP6850GC/Finnigan MAT 252) with the original purge-and-trap system following the method of Hirota et al. (2010). The analytical precision in the determination of the δ¹³C ([{(¹³C/¹²C_sample)/(¹³C/¹²C_PDB)} – 1] × 1000) of methane through a single analysis is estimated to be better than 0.07‰ when more than 5.5 nM of methane is introduced.

In the onshore laboratory, we also measured concentration and carbon isotope compositions of methane in headspace gas samples, but these samples were apparently affected by leakage of methane from sample vials or microbial oxidation of methane in the sample and are not discussed further.

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