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doi:10.2204/iodp.proc.314315316.122.2009

Organic geochemistry

Shipboard organic geochemical analyses included volatile hydrocarbon contents (C₁–C₄); inorganic carbon and carbonate contents; and elemental analyses of total carbon, nitrogen, and sulfur. Procedures used during Expedition 315 follow Pimmel and Claypool (2001).

Gas analysis

A 5 cm³ sediment sample was collected with a cut-off plastic syringe, usually from the exposed end of Section 1, and was extruded into a 20 mL glass vial. The vial was placed in an oven at 70°C for 30 min. The evolved gases were analyzed using an Agilent 6890N gas chromatograph (GC) equipped with a flame ionization detector (FID). This system determined the concentration of C₁–C₄ hydrocarbons with an FID. Chromatographic response on the GC was calibrated against five different authentic standards with variable quantities of low molecular weight hydrocarbons. When heavier molecular weight hydrocarbons (C₃ and higher) were detected, gas samples were analyzed on the natural gas analyzer (NGA). The NGA system consists of an Agilent 6890N GC equipped with four different columns, two detectors, both an FID and a thermal conductivity detector (TCD), and WASSON·ECE instrumentation. The NGA was used to measure C₁–C₁₃ hydrocarbons and nonhydrocarbons (CO, CO₂, O₂, and N₂). Compositions of gases in sediments were analyzed in at least one horizon per core.

Inorganic carbon

Inorganic carbon concentrations were determined using a Coulometrics 5012 CO₂ coulometer. About 10–12 mg of freeze-dried ground sediment was weighed and reacted with 2M HCl. The liberated CO₂ was titrated, and the change in light transmittance was monitored with a photodetection cell. The weight percentage of calcium carbonate was calculated from the inorganic carbon content, assuming that all evolved CO₂ was derived from dissolution of calcium carbonate, by the following equation based on molecular weight ratio:

CaCO₃ (wt%) = 8.33 × IC (wt%),

where IC = inorganic carbon. All carbonate minerals were treated as CaCO₃. Standard deviation for the samples is less than ±0.12 wt%.

Elemental analysis

Total carbon, nitrogen, and sulfur concentrations were determined using a Thermo Finnigan Flash EA 1112 CHNS analyzer with calibration using the synthetic standard sulfanilamide, which contains C (41.81 wt%), N (16.27 wt%), and S (18.62 wt%). About 10–20 mg freeze-dried ground sediment was weighed and placed in a tin container for carbon and nitrogen analyses. For sulfur analysis, the same amount of freeze-dried sediment was weighed and put in a tin container with the same amount of V₂O₅. Sediment samples were combusted at 1000°C in a stream of oxygen. Nitrogen oxides were reduced to N₂, and the mixture of CO₂, N₂, and SO₂ was separated by GC and detected by TCD. Total organic carbon content was calculated by subtraction of inorganic carbon from total carbon. Standard deviation of carbon, nitrogen, and sulfur for the samples is less than ±0.1%. Accuracy for carbon and sulfur analysis was confirmed using two Geological Society of Japan reference samples. Analytical accuracy for JMS-1 (C = 1.69 wt% and S = 1.32 wt%) is 1.66 ± 0.2 wt% (N = 12) for carbon and 1.28 ± 0.06 wt% (N = 11) for sulfur. Analytical accuracy for JMS-2 (C = 0.39 wt% and S = 0.29 wt%) is 0.34 ± 0.02 wt% (N = 9) for carbon and 0.29 ± 0.04 wt% (N = 7) for sulfur. This accuracy includes weighing errors.

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