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doi:10.2204/iodp.proc.336.102.2012

Organic geochemistry

The organic geochemistry program comprised characterization of volatiles and sedimentary inorganic geochemistry, including inorganic carbon. These analyses were carried out as part of routine shipboard safety and pollution prevention requirements, to characterize the interstitial waters and sediment geochemistry for shipboard interpretation, and to provide sampling frameworks for shore-based research.

Sediment gas sampling and analysis

The organic geochemistry program monitored the compositions and concentrations of volatile hydrocarbons (C₁–C₆) and other gases (i.e., O₂, N₂, and H₂S) in the void gas samples and in the sediments from headspace gas samples at typical intervals of one per core. The IODP gas sampling protocol for pollution prevention and safety, required by IODP safety regulations, was modified to better constrain the concentrations of dissolved hydrocarbon gases. The routine headspace procedure involved placing ~5 cm³ of sediment sample in a 20 cm³ glass serum vial that was sealed with a septum and metal crimp cap and heated at 70°C for 30 min. A 5 cm³ volume of gas from the headspace in the vial was removed with a glass syringe for analysis by GC3. Detailed procedures are those described in Kvenvolden and McDonald (1986).

The headspace gas samples were analyzed using an Agilent 6890 Series II gas chromatograph (GC) equipped with a 2.4 m × 3.2 mm stainless steel column packed with 80/100 mesh HayeSep R and a flame ionization detector (FID). This instrument quickly measures the concentrations of methane (C₁), ethane (C₂), ethene (C₂₌), propane (C₃), and propene (C₃₌). The gas syringe was directly connected to the GC via a 0.25 cm³ sample loop. Helium was used as the carrier gas, and the GC oven temperature was programmed to ramp 90°–100°C at 30°C/min, ramp 100°–110°C at 15°C/min, remain at 110°C for 4.5 min, and then ramp to 150°C at 50°C/min, with a final holding time of 1.8 min. Data were collected and evaluated with an Agilent Chemstation data-handling program. Chromatographic response was calibrated against preanalyzed standards.

Bulk sediment geochemistry: sedimentary inorganic and organic carbon

Inorganic carbon concentrations were determined using a Coulometrics 5011 carbon dioxide coulometer. One carbonate determination was performed typically for each 1.5 m section of core. Samples of ~10–15 mg of freeze-dried, ground sediment were reacted with 2N HCl. The liberated CO₂ was titrated to a colorimetric end point. Calcium carbonate content, as weight percent, was calculated from the inorganic carbon (IC) content with the assumption that all inorganic carbon was present as calcium carbonate:

%CaCO₃ = %IC × 8.33.

The coulometer was calibrated with pure CaCO₃ powder during the expedition, and the analytical precision was determined.

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