Volume, pressure, and time information collected during step-wise gas release from each PCS core are presented by Dickens et al. (Chap. 43, this volume). Using this data, we have constructed volume-pressure (VP) plots for 20 PCS cores (Fig. 4). Seven of the 29 cores recovered at high pressure (Cores 164-995A-0P*, 164-995A-52P*, 164-997B-29P*, 164-997B-32P, 164-997B-36P, 164-997B-40P, and 164-997B-44P) released only a small volume of air with a decrease in pressure to atmospheric conditions. These seven cores did not contain sediment or CH4. An additional two cores (Cores 164-995A-70P and 164-996E-8P) were not placed in an ice bath, and thus degassing of these cores was not isothermal.
Volumes used to make the volume-pressure plots include gas volumes released at all temperatures (0º-15ºC); pressures used to make the plots are corrected for gauge offset (see Dickens et al., Chap. 43, this volume). Pressure axes on the plots are linear and range from 0 to 5 MPa. Although this scaling precludes placement of small incremental volumes at high pressure, the low-pressure region of interest at 0ºC is emphasized (Fig. 3). Note that Kvenvolden et al. (1983) used logarithmic pressure axes for time-pressure plots of gas release experiments with the pressure core barrel (PCB).
All volume-pressure plots have two features in common (Fig. 4): a near-vertical line at low volume that represents a pronounced drop in pressure with minimal gas release, and a region where significant volumes of gas are released with decreasing pressure.
These two features of the volume-pressure plots can be represented by three parameters (Table 2). The "high-pressure volume" is the small quantity of gas released at high pressure. The "threshold pressure" is the pressure where significant volumes of gas are suddenly released from the PCS. The "total gas volume" is the total quantity of gas (including the high-pressure volume) collected from the PCS.
Gas analyses indicate that the high-pressure volume is composed of air. This air is presumably trapped inside of PCS mandrels and port connections as well as dissolved in borehole water before core recovery. This is why the seven PCS cores without sediment or methane still released a small quantity of gas as pressure was released over time. Values of high-pressure air listed in Table 2 range from 40 to 175 mL. The reason for the wide range in high-pressure, air-volume estimates is unclear. However, accurate measurements of high-pressure air volumes with the current PCS manifold system are difficult because rates of gas release at high pressure cannot always be controlled.
Threshold pressures for the 22 PCS cores are listed in Table 2. These pressures are considered rough estimates for the same reason; high-pressure air volumes are poorly constrained. For some cores (e.g., Core 164-995A-45P), two threshold pressures are presented. The first threshold pressure is the pressure after the first aliquot of gas containing significant concentrations of CH4 was collected; the second threshold pressure is the pressure at which the first significant volume of gas was released from the PCS. The two pressures should be the same under ideal experimental conditions (see "Discussion" section). The existence of multiple threshold pressures may be related to different CH4 concentrations in the two chambers inside of the PCS at initial conditions (discussed below). All threshold pressures were below 4.5 MPa. Threshold pressures are somewhat proportional to total gas volumes (Table 2).
Total gas volumes reported in Table 2 are amounts of gas released from PCS cores after equilibration at 1 atm (0.1 MPa) and 15ºC. These volumes range between 255 mL for Core 164-997A-33P and 8100 mL for Core 164-996D-7P. Total gas volumes for 14 of the 19 cores with volume-pressure plots are within 5% uncertainty. Volumes reported for Cores 164-994C-70P, 164-995A-18P, 164-995A-36P, 164-995A-48P and 164-995B-7P are minimum quantities (Table 2). The gas manifold system became clogged with sediment before atmospheric pressure could be achieved inside of the PCS for Cores 164-994C-70P and 164-995A-18P; the PCS was not warmed to 15ºC after reaching 0.1 MPa for Cores 164-995A-36P and 164-995B-7P; and a small but unknown quantity of gas escaped the bubbling chamber during gas-release experiments for Core 164-995A-48P.
The composition of gas released below the threshold pressure is greater than 98.5% CH4 (Paull, Matsumoto, Wallace, et al., 1996). Thus, the total gas volume is approximately equivalent to the total amount of CH4 released from the PCS less the high-pressure air volume (Table 2).
Amounts of total gas and CH4 released over the duration of an experiment exceed amounts released at 0ºC while the PCS is in an ice bath. Warming of the PCS at 0.1 MPa to 15ºC releases additional quantities of gas because gas saturation at these pressures decreases with increasing temperature under isobaric conditions. The "0ºC total volume" and "0ºC CH4 volume" columns in Table 2 list observed gas amounts released at 0ºC and under isothermal conditions. However, almost all of these values are less than one would expect given total gas volumes (Table 2) and CH4 saturation curves (e.g., Duan et al., 1992). In other words, warming of the PCS from 0º to 15ºC releases more CH4 than the difference between the amount of CH4 that can be stored in solutions saturated with CH4 at 0º and 15ºC (~80 mL). Note that Cores 164-994C-70P, 164-995A-0P*, 164-995B-7P and 164-997B-32P were not warmed.
Average rates of gas release between the first opening of the PCS at 0ºC and high pressure and the last release of gas at 0ºC and 0.1 MPa vary by over an order of magnitude from 1 mL/min to 70 mL/min (Table 2). Gas release typically was slower for cores at Sites 996 and 997 than for cores at Sites 994 and 995.