Forty-two deployments (runs) of the PCS on Leg 164 were recovered at a pressure greater than 780 psi (5.5 MPa; Table 1; Paull, Matsumoto, Wallace, et al., 1996). These runs are of interest because significant quantities of hydrocarbon gas are probably not released from the PCS above this pressure (Paull, Matsumoto, Wallace, et al., 1996, pp. 195 and 265). The reason is that essentially all methane exists as solid hydrate or dissolved in water at pressures greater than a threshold pressure of ~5.5 MPa (and at temperatures less than 5ºC; Dickens et al., Chap. 11, this volume).
Data collection for PCS runs generally proceeded as follows (with the notable exception of two "sediment only" cores at Site 997 that were deployed to collect gas hydrate specimens; Paull, Matsumoto, Wallace, et al., 1996, pp. 295-296). The PCS was retrieved, separated from the drill string, and placed in an ice bath. A gas manifold system and sampling chamber were attached to a port on the PCS. Incremental volumes of gas then were released over time until the inside of the PCS was at atmospheric pressure (or a port or manifold became clogged with sediment). The PCS was removed from the ice bath and warmed to ambient temperature (~15ºC). Any additional volumes of gas were then collected. Aliquots of gas were taken from numerous gas volume increments for compositional analyses. After measuring gas volumes and taking gas samples, the PCS was opened, and the sediment core was examined for length and overall condition and sampled for physical properties measurements. Data collection configurations and manifold designs are described by Paull, Matsumoto, Wallace, et al. (1996, pp. 24-26). Deviations to these general procedures are noted in Table 2, Table 3, Table 4, and Table 5.
An important aspect of understanding PCS data collection concerns sampling ports on the PCS. The PCS has an inner chamber and an outer chamber with a sampling port to each. The inner chamber contains the sediment core and borehole water with a total volume of ~1.3 L. The outer chamber contains borehole water with a volume of ~2.7 L. The two chambers are connected inside the PCS. Gas release experiments on Leg 164 prior to Core 164-995A-18P exclusively used the port to the inner chamber because this was the original design for use of the PCS. The problem with this configuration, however, is that sediment at high pressure (especially shallow and unconsolidated sediment) can be extruded into the port (or worse, the manifold), clogging the system and preventing gas release. Starting with Core 164-995A-27P, the manifold was connected to the port of the outer chamber (1) after sediment clogging of the port to the inner chamber, or (2) at the start of a gas-release experiment (see notes in Table 2, Table 3, Table 4, and Table 5). Sediment clogging was not a problem with this alternative configuration; apparently, the connection between the PCS chambers or borehole water in the outer chamber serves to limit sediment extrusion. Because the path of gas flow is different for each port, there may be "port dependent" variations in how gas volumes or compositions are released from the PCS with drops in pressure over time. However, the total amount of gas and its bulk composition should be independent of its path through the PCS.
Measurements of PCS data were necessarily simple for multiple reasons, including PCS configuration and manifold-design problems, insufficient high-pressure components aboard the JOIDES Resolution, and the absence of a shipboard "Downhole Tools" scientist with appropriate time and experience. Time was recorded with a watch and rounded to the nearest half minute. Pressure was determined with a pressure transducer on the PCS. Gas volumes for PCS runs retrieved after Core 164-994C-66P were collected by: (1) attaching an inverted 1-L graduated cylinder immersed in water saturated with NaCl to the gas manifold system, (2) purging the gas-manifold system and bubbling chamber with He at 1 atm pressure, and (3) releasing incremental gas volumes from the PCS through the gas-manifold system and into the bubbling chamber (Paull, Matsumoto, Wallace, et al., 1996, p. 25; Dickens et al., 1997). The length of the sediment core inside the PCS was determined with a meter stick. Temperature inside (or outside) of the PCS was not recorded.
Uncertainties in individual pressure and volume measurements are within 10 psi and 10 mL, respectively. Uncertainties in measured sediment core length are within 1 cm. However, the error in reported core length (Table 1) likely exceeds 1 cm, and the error in total gas volume (Table 1) likely exceeds the cumulative error of incremental volumes.
Core lengths may be longer than reported in Table 1 by perhaps 3 cm. Certain cores contained "moussey" and "soupy" intervals. Although such intervals were compressed after opening the PCS, the length of these wet sections is difficult to measure accurately because a circumference similar to that of dry sections cannot be established. Small (but unknown) quantities of sediment also can be lost during gas venting when pressurized sediment passes through the manifold, or during extrusion of the sediment core from the PCS.
Total gas volumes may be greater than reported in Table 1; in some cases, by up to 300 mL. The primary source of this error is the volume of gas remaining inside of the PCS after reducing pressure to 1 atm. This unknown "residual" volume depends on temperature, initial gas volume (and composition), and the pressure, time, and volume history of the core prior to opening the PCS. Sediment cores that are kept inside of the PCS at ambient temperature (~15ºC) for greater than 7 hr can release upwards of 300 mL of gas after reaching 1 atm (e.g., Core 164-997A-55P). Cores that were not given sufficient time to equilibrate at ambient conditions may therefore contain more gas than reported in Table 1 (see Dickens et al., Chap. 11, this volume). Small volumes of gas may have escaped when the inner port became clogged, and the manifold was removed during a PCS run (see Table 2, Table 3, Table 4, and Table 5). However, this volume loss was minimal because the manifold was kept at 1 atm pressure for the entire PCS run (at least those where incremental volumes were collected). The principal effect of removing the manifold at 1 atm, therefore, was to change the composition of gas from primarily methane to air or helium.