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doi:10.2204/iodp.proc.314315316.132.2009

Organic geochemistry

The shipboard organic geochemistry program during Expedition 316 included analyses of volatile hydrocarbon content (C₁–C₄), measurement of inorganic carbon and carbonate content of the sediments, and elemental analyses of total nitrogen, sulfur, and carbon. Procedures used during Expedition 316 follow Pimmel and Claypool (2001) and Shipboard Scientific Party (2003).

Gas hydrates

In order to identify the distribution of gas hydrates immediately upon recovery, a handheld IR camera (FLIR Systems ThermaCAM SC640) was used to scan the surface of the core to detect thermal anomalies caused by gas hydrate dissociation. Gas hydrate–bearing sections were immediately sampled and stored in liquid nitrogen bottles for further shore-based analyses.

Gas analysis

For safety and pollution prevention, concentrations and distributions of light hydrocarbon gases, mainly methane, ethane, and propane, were monitored for each core following standard headspace sampling. A 5 cm³ sediment sample was collected with a cut-off plastic syringe from the exposed end of Section 1 in each core. The sample was extruded into a 20 mL glass vial and immediately sealed with a septum and metal crimp cap. The vial was placed in an oven at 70°C for 30 min. The monitoring procedure was complemented by additional headspace analyses following a slightly different approach with the intent to better constrain the concentrations of dissolved gases. Compared to the rapid safety-oriented protocol, the latter more time-consuming analyses led to higher yields of methane. Upon core retrieval, a 3 cm³ sediment sample was collected with a cut-off plastic syringe from a freshly exposed end of a core section and was extruded into a 20 mL glass serum vial containing 10 mL of 4% NaOH. The vial was immediately capped with a butyl septum and crimp cap. After shaking for 2 min using a tube mixer, the vials were subsequently left to stand for at least 24 h at room temperature prior to gas chromatographic analysis. When the sediments became too lithified, a cork borer was used to take the sample. Additionally, when gas pockets were observed, headspace samples were complemented by void gas samples, which were collected directly from gas voids by penetrating the core liner and using a gastight syringe. Gas chromatographic analyses of headspace samples resulting from both protocols were performed in an identical manner. The evolved C₁–C₄ gases were analyzed using an Agilent 6890N gas chromatograph (GC) equipped with a flame ionization detector (FID). Chromatographic response on the GC was calibrated against five different authentic standards with variable quantities of low molecular weight hydrocarbons. A 5 mL volume of headspace gas was extracted from the vial using a standard gas syringe and injected into the GC.

From the additional headspace analyses, the methane concentration in interstitial water was derived from the headspace concentration by the following equation where quantities of methane that remain undetected because dissolution in the aqueous phase is minimal (e.g., Duan et al., 1992) and are not accounted for:

where

• V_H = volume of the sample vial headspace,
• V_S = volume of the whole sediment sample,
• χ_M = molar fraction of methane in the headspace gas (obtained from GC analysis),
• P_atm = pressure in the vial headspace (assumed to be the measured atmospheric pressure when the vials were sealed),
• R = universal gas constant,
• T = temperature of the vial headspace in degrees Kelvin, and
• φ = sediment porosity (determined either from MAD measurements on nearby samples or from porosity estimates derived from GRA data representative of the sampled interval).

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). Gas samples collected from gas pockets were analyzed on the NGA.

Inorganic carbon

Inorganic carbon concentration was determined using a Coulometrics 5012 CO₂ coulometer. About 10–20 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 percent of calcium carbonate was calculated from inorganic carbon content, assuming that all the evolved CO₂ was derived from dissolution of calcium carbonate, by the following equation:

No correction was made for the presence of other carbonate minerals. NIST-SRM 88b (Standard Reference Material) was used to confirm accuracy. Standard deviation for the samples was less than ±0.1 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 catalyst. 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 gas chromatography 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 (GSJ) reference samples.

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