Gas samples were obtained by the headspace procedure (Kvenvolden and McDonald, 1986), which involves placing a nominal 5-cm3 plug of sediment into a septum-sealed vial that is then heated at 60°C for 30 min to evolve sediment gases. Methane and ethane contents of gas samples were routinely analyzed using the shipboard GC3 gas analysis system, which consists of a Hewlett-Packard 5890 II Plus gas chromatograph equipped with a 2.4 m x 3.2 mm stainless steel column packed with HaySep S connected to a flame ionization detector (FID). When elevated gas contents were encountered, samples were also analyzed using the natural gas analyzer (NGA), which is a Hewlett-Packard 5890 II Plus gas chromatograph equipped with a multiport valve system that directs gases into two analytical pathways. The first measures hydrocarbons from methane to hexane using a 60 m x 0.32 mm DB-1 capillary column connected to an FID. The second measures nonhydrocarbon gases such as CO2 and H2S using a packed column connected to a thermal conductivity detector. Both gas chromatographs were calibrated using Scotty IV gas mixtures. Gas concentrations are reported as parts per million by volume (ppmv) relative to the standard volume (5 cm3) of the headspace sample that is injected into the gas chromatograph.
The ODP gas analysis techniques are designed to provide rapid determinations of the relative amounts and compositions of the residual interstitial gases recovered during drilling. In particular, a goal of the analyses is to help avoid situations where dangerously elevated concentrations of thermogenic gas might occur. The procedures were not designed to deliver quantitatively correct in situ concentrations of the gaseous components of sediments but rather the concentrations of gas that remain after core recovery. Although samples for gas analysis are collected almost immediately after sediment cores reach the ship, a significant amount of the gas is lost because of the decrease in pressure and increase in temperature experienced by the cores. Moreover, the gas concentrations measured by the headspace procedure represent the gas concentrations of the headspace vial. Gas concentrations relative to the volume of interstitial water in the sediment plugs can be estimated, but are not normally determined. The results of the headspace gas analysis techniques nonetheless provide useful comparisons of the relative amounts and compositions of sedimentary gases at different depths at a single drill site and between different drill sites.