Ocean Drilling Program (ODP) Leg 201 was devoted to understanding the range and nature of microbial activity below the seafloor. Of importance to this objective, as well as to the goals of future legs (e.g., Leg 204), are the amounts and distributions of gas, specifically CH4, in marine sediment sequences. Traditionally, gas concentrations are determined using a headspace technique where a plug of sediment is taken from a core retrieved at depth, placed into a closed container on the ship, and degassed. Although this approach may work for cores with low in situ gas concentrations (e.g., Fossing et al., 2000; Hoehler et al., 2000), it clearly fails where targeted sediment sequences contain abundant gas (Dickens et al., 1997; Paull et al., 2000). In these cases, significant amounts of gas can escape from cores during recovery when the drop in pressure or increase in temperature lowers CH4 saturation (Dickens et al., 2000a).
The Pressure Core Sampler (PCS) is a downhole tool designed to recover a cylindrical sediment core—including gas and interstitial water—at in situ pressure (Pettigrew, 1992). When properly sealed at depth, controlled release of pressure from the PCS through a manifold (below) should permit collection of gases that would otherwise escape on the wireline trip. In late 1995, after several early attempts at coring under pressure (e.g., Kvenvolden et al., 1983), the PCS was used successfully to capture and analyze gases for their composition and volume during Leg 164 (Paull, Matsumoto, Wallace, et al., 1996; Dickens et al., 1997, 2000a, 2000b). Consequently, there was community interest in deploying the PCS during Leg 201 (1) to ensure that the tool was fully operational for future legs targeting gas-rich sediments, especially Leg 204 off the Oregon margin, and (2) to quantify gas abundance along the Peru margin, where gas concentrations at depth exceed solubility at shipboard pressures and temperatures. This paper describes basic PCS operations during Leg 201 and resultant gas yields from recovered cores. Advanced interpretations of these experiments will be presented elsewhere.