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doi:10.2204/iodp.proc.318.201.2014 MethodsA total of 276 geochemical samples were collected from Hole U1357A at 20 cm intervals. The depth of the sampled cores ranges from 0 to 54.35 m. After being freeze-dried for 48 h, all samples were powdered and homogenized using a ball mill. Rock-Eval pyrolysis was performed on powdered bulk samples using the Rock-Eval VI instrument to obtain several valuable indicators, including S1, S2, HI, OI, and Tmax. The Rock-Eval pyrolysis method consists of a programmed heating cycle of a small sample (~100 mg) in an inert atmosphere (helium). The pyrolysis oven temperature program was as follows. For 3 min, the oven is kept isothermally at 300°C and the free hydrocarbons are volatilized and measured as the S1 peak (detected by flame ionization detector [FID]). The temperature is then increased from 300° to 550°C (at 25°C/min). This is the phase of volatilization of very heavy hydrocarbons compounds (>C40) and cracking of nonvolatile organic matter. The hydrocarbons released from this thermal cracking are measured as the S2 peak (by FID). The temperature at which S2 reaches its maximum depends on the nature and maturity of the kerogen and is called Tmax. The CO2 emitted from the cracking of kerogen is trapped in the temperature range 300°–390°C. The trap is heated, and CO2 is released and detected by a thermal conductivity detector (TCD) during the cooling of the pyrolysis oven, giving the S3 peak (Pimmel and Claypool, 2001). When these components are normalized to the TOC content, the S2 peak gives the HI (S2 × 100/TOC) and S3 gives the OI (S3 × 100/TOC) (Tissot and Welte, 1984; Peters, 1986). Residual powdered bulk samples were subsampled to make carbonate-free samples. They were treated with 3 N HCl, diluted to pH 7, dried, and powdered again to remove carbonates. TOC, total nitrogen (TN), and stable isotopes of carbon (δ13Corg) and nitrogen (δ15Norg) in the organic matter were determined using a continuous-flow stable isotope ratio mass spectrometer (IRMS; IsoPrime-EA, Micromass, UK) linked with a CN analyzer (NA Series 2, CE Instruments, Italy) at Seoul National University (Korea). Carbon and nitrogen isotopic compositions (δ13Corg and δ15Norg) were calculated using the relation
where R is the ratio of 13C/12C or 15N/14N, with a Peedee belemnite (PDB) standard for carbon and an atmospheric N2 standard for nitrogen. Precision of δ13Corg and δ15Norg measurements was checked against reference materials procured from the International Atomic Energy Agency. For δ13Corg (RM 8542: Glucose ANU, –10.47‰ ± 0.13‰) and δ15Norg (RM 8548: IAEA-N2 +20.3‰ ± 0.2‰), uncertainties in the measurements were less than 0.1‰ and 0.2‰, respectively. |
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