SAMPLE COLLECTION AND ANALYSIS

All samples analyzed were collected at ODP Site 1230 (D'Hondt, Jørgensen, Miller, et al., 2003), located on the lower slope of the Peru Trench in water of 5086 m depth. This site was the single methane hydrate–bearing site selected for drilling during Leg 201. Sediments of this area are part of the accretionary wedge, located just landward of the Peru Trench (Suess, von Huene, et al., 1988). The upper 200 m of Pleistocene to Holocene sediment is a clay-rich diatomaceous mud. Gas void samples for this study were collected and analyzed from 23 depths between 35 and 170 meters below seafloor (mbsf).

Samples were collected on the ship's catwalk prior to cutting the core into sections. The gas phase was collected directly into a gas-tight syringe fitted with a Luer lock three-way valve. A small hole was drilled into the core liner in the location of a gas void, which allowed a Luer lock tip to fit snugly. The valve and dead space in the syringe were filled with water (preferably helium bubbled or vacuum degassed) to minimize air contamination. The Luer tip was inserted into the hole, and the gas phase was allowed to flush through the side port of the three-way valve. The gas was then directed into the syringe. The gas was allowed to nearly fill the syringe.

Mole fractions of CH4, N2 and O2 were determined; Ar and total vapor pressure were not measured. The gas was analyzed by gas chromatography with the shipboard natural gas analyzer, which separates these gases on porous polymer and molecular sieve columns in series utilizing thermal conductivity detection. N2, O2, and CH4 were quantified utilizing a standard containing 10% N2 and 20% CH4 and air for N2 and O2. The assumed value for [N2]0 used in our calculations, 617 mM/kg, is the equilibrium concentration for salinity 35 and 0°C, the potential temperature of bottom water at Site 1230 (Pilson, 1998).

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