Our system for measuring the gas and water content of dissociating gas hydrate consisted of a sample holder, a gauge block, a pressure gauge, and a manifold (Fig. 2). The manifold had a interchangeable gas-sampling port with septum or a quick connection to vacuum, a steel cylinder for collection of gas, and a pressure gauge. The device was first used on DSDP Leg 76 and a more complete description of it can be found in Kvenvolden et al., 1984. For each experiment, gas hydrate that was temporarily stored in liquid nitrogen was placed on aluminum foil and broken up. Pieces with as little sediment as possible were placed into the sample device previously cooled by liquid nitrogen. The system was sealed and then the lower portion of the device was placed into a water bath. As the gas hydrate dissociated, pressure inside the device increased, then stabilized. After about 10 min. of stable pressure, the pressure and the temperature of the water bath were recorded. Dissociated gas was allowed to expand into the sample manifold and the pre-evacuated cylinder. Gas was sampled from the manifold and analyzed by both gas chromatographs on board, as detailed in the Explanatory Notes of the Leg 164 Initial Reports (Shipboard Scientific Party, 1996a). After gas sampling, the residual water and any sediment was weighed and their respective volumes calculated and subtracted from the volume of the reaction chamber. Gas volumes were calculated according to the ideal gas law at standard temperature and pressure (STP). Residual water was decanted after centrifugation, then sealed and refrigerated pending chlorinity measurements and shore-based isotopic analyses of the water. Sediment was dried and selected samples were analyzed for mineral content by X-ray diffraction.
Unpublished gas to water volumetric ratios of a synthetic gas hydrate made from granular ice (Stern et al., 1996) give an evaluation of the accuracy of this device. For five samples of a fully gas-filled hydrate (gas to water ratio of 216) our average measured ratio was 210.6 with a standard deviation of ±16.2.
Gas composition was determined shipboard by gas chromatography. Isotopic composition of methane carbon, hydrogen, and carbon from carbon dioxide are given in Paull et al., Chapter 7, this volume. Hydrogen isotopic composition of gas hydrate water were determined by Egeberg (Chap. 22, this volume) and Bouma, Coleman, Meyer, et al. (1986).
Sediment gas was collected by three different methods: the standard ODP headspace technique; exsolved gases from sealed, whole 1.5-m-long cores; and from free gas that had expanded into the core liner during recovery (Shipboard Scientific Party, 1996a). Free-gas sample analyses are primarily used in this study. Free gas primarily represents exsolved gases once dissolved in pore water, and/or present in situ as gas bubbles or possibly dissociated gas hydrate. Free gas and analyses were preferred since sediment gas is trapped inside the core liner and contact with the atmosphere is minimal. This technique works well for insoluble gases such as the hydrocarbons; however, all gas sampling methods employed lack finesse for soluble gases such as CO2 and H2S.