IODP Proceedings    Volume contents     Search


Methods and materials

Gas composition

The HS samples were analyzed onboard by an Agilent 6890 Series II gas chromatograph (GC3) (see the “Methods” chapter [Harris et al., 2013b]). The VG samples were analyzed onshore by an Agilent 7890A gas chromatograph with a flame ionization detector and a thermal conductivity detector at Korea Institute of Geoscience and Mineral Resources (KIGAM).

Isotope analysis

A total of 43 HS samples from Sites U1380, U1412, and U1413 and 27 VG samples from Sites U1412 and U1413 were used for the gas isotopic analyses. The carbon (δ13CCH4 and δ13CCO2) and hydrogen (δDCH4) stable isotopic ratios of the gases were analyzed using an isotope ratio–monitoring gas chromatograph/mass spectrometer at Isotech, Champaign, IL. All isotopes are reported in the usual δ notation relative to Vienna Pee Dee Belemnite (V-PDB) for carbon and Vienna Standard Mean Ocean Water (V-SMOW) for hydrogen:

δ(‰) = [(RsampleRstandard)/Rstandard] × 1000,

where R represents the 13C/12C ratio and D/H of the sample and standard for each isotope. The analytical reproducibility was ±0.1‰ for δ13C and ±2‰ for δD.

Conversion ratio

The conversion ratio was calculated using the Rayleigh ratio (modified from Pohlman et al., 2009).

δ13CCO2,t = δ13CCO2,i – εcln(f)

δ13CCH4,t = δ13CCO2,i – εc[1 + ln(f)]


  • δ13CCO2,t = isotopic ratio of CO2 at time t,
  • δ13CCH4,t = isotopic ratio of CH4 at time t,
  • δ13CCO2,i = isotopic ratio of sedimentary organic matter at depth i,
  • εc = fractionation factor, and
  • ln(f) = remaining fraction of the initial substrate.

The model determines that the proportion of methane is produced by in situ methanogenesis.