The pressure core sampler (PCS) is a tool designed to recover a 1385-cm3 cylindrical sediment core at in situ pressure (Pettigrew, 1992). Forty-two deployments (runs) of the PCS on Leg 164 successfully recovered core at pressures greater than 3.45 MPa (Paull, Matsumoto, Wallace, et al., 1996; Dickens et al., Chap. 43, this volume). Incremental volumes were determined for 29 of these cores at Sites 994, 995, 996, and 997 as pressure was released over time (Table 1).
Ten of the 29 cores most likely contained gas hydrate filling more than 1% porosity at in situ pressure, temperature, and gas concentration (Table 1). These 10 PCS cores (labeled "Cl" or "S" in Table 1) were recovered from sediment regions where all available information indicates the presence of in situ gas hydrate. The inference is made for PCS cores at Sites 995 and 997 on the basis of interpretations of well-log resistivity and velocity, interstitial water Cl- concentrations, and vertical seismic profiling (Holbrook et al., 1996; Paull, Matsumoto, Wallace, et al., 1996), as well as our interpretation of PCS data (see "Discussion" section and Dickens et al., 1997). The inference is made for PCS cores at Site 996 because nonpressurized sediment cores contained abundant visible gas hydrate specimens (Paull, Matsumoto, Wallace, et al., 1996). An additional six of the 29 PCS cores (labeled "Q" in Table 1) were recovered from sediment regions where certain interpretations of data suggest that in situ gas hydrate was present (cf. Holbrook et al., 1996; Paull, Matsumoto, Wallace, et al., 1996; Egeberg and Dickens, 1999).
Data collection for the 29 PCS runs of interest generally proceeded as follows (Paull, Matsumoto, Wallace, et al., 1996; Dickens et al., Chap. 43, this volume). The PCS was placed in an ice bath after core recovery. A gas manifold system (PCS-M3B or PCS-M4; Paull, Matsumoto, Wallace, et al., 1996, pp. 24-26) and sampling chamber were attached to a port on the PCS. Incremental volumes of gas were released from the PCS over time until the inside of the PCS was at atmospheric pressure. The PCS was removed from the ice bath and warmed to ambient temperature (~15ºC). Additional volumes of gas were then collected. Aliquots of gas were taken from many gas volume increments for compositional analyses. The PCS was opened, and the sediment core was examined for volume and physical properties measurements.
Errors in individual gas volume and pressure measurements were within 10 mL and 10 psi (0.07 MPa), respectively; errors in measured core lengths were within 1 cm.
It is stressed, however, that there was a lack of experimental consistency with PCS operations on Leg 164 due to a variety of technical and operational reasons. For example (Dickens et al., Chap. 43, this volume), five cores were not maintained at isothermal (0ºC) conditions (Cores 164-995A-52P*, 164-995A-70P, 164-996E-8P, 164-997A-25P, and 164-997B-10P); most cores were not given sufficient time to equilibrate after changes in pressure (and gas concentration); the manifold port connection for many cores had to be switched during the middle of gas release experiments; and individual cores had different initial pressures, temperatures, gas concentrations, and sediment volumes. In summary (and as highlighted in the "Discussion" section), each core needs to be considered separately for experimental artifacts and errors. Some of the potential problems are noted in Table 1 and Table 2 (and notes for Fig. 4); additional problems can be identified by examining records of individual PCS cores (Paull, Matsumoto, Wallace, et al., 1996; Dickens et al., Chap. 43, this volume).