Gas samples were obtained by two different methods. The primary method was 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. The secondary procedure was used occasionally when gas expansion pockets appeared in cores before they were cut into 1.5-m sections. A heavy-duty needle was used to puncture the butyrate core liner, and an attached syringe was employed to collect the gas.
Methane and ethane contents of gas samples were analyzed using 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). Additional gas contents were analyzed using a Hewlett Packard 5890 II Plus gas chromatograph equipped with a 60 m x 0.32 mm DB-1 capillary column connected to an FID to measure hydrocarbons from methane to hexane and a packed column connected to a thermal conductivity detector to measure nonhydrocarbon gases, such as CO2 and H2S. Both instruments were calibrated using Scotty IV gas mixtures, and gas concentrations are reported in parts per million by volume (ppmv).
The Ocean Drilling Program (ODP) gas analysis techniques are designed to provide quick determinations of the relative amounts and possible origins of the residual interstitial gases recovered during drilling. In particular, they 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 may be lost because of the decrease in pressure and increase in temperature experienced by the cores. The results of the 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 drill sites.