Four types of gas samples were collected during Leg 204: void gas, headspace gas, gas hydrate, and pressure core. Gas voids formed in the core liner were sampled by means of a core liner penetration tool with a 60-cm3 plastic syringe attached by a three-way stopcock. After sectioning the core, nominal 5-cm3 plugs of sediment collected using cut-off syringes or a cork borer were outgassed in vials and the headspace gases analyzed according to standard ODP procedures (Pimmel and Claypool, 2001). Small pieces of gas hydrate recovered from cores were quickly cleaned, placed in 60-cm3 syringes and allowed to decompose. Aliquots of gas samples collected during prolonged degassing of PCS cores were also collected in syringes. After shipboard gas chromatographic analysis, aliquots of all except headspace gas were saved for shore-based analysis by injecting into septum-sealed evacuated containers. Headspace methane results were used to estimate dissolved methane content of shallow samples. Procedures for sampling and analysis of pore waters are detailed in the "Explanatory Notes" chapter of the Leg 204 Initial Reports volume (Shipboard Scientific Party, 2003a).
Isotopic analyses were performed at Woods Hole Oceanographic Institution (USA), and the detailed analytical procedures are given in Milkov et al. (2005). The carbon isotopic composition of the dissolved inorganic carbon from interstitial water samples was analyzed at Oregon State University (USA). Water samples were acidified with phosphoric acid in an online preparation system and the evolved CO2 measured on a mass spectrometer.