During Leg 201, the PCS was deployed 17 times, a total surpassed previously only by operations during Leg 164. The first seven runs, at Sites 1225 to 1229, were primarily undertaken to test whether the modified tool and cutting shoes would operate in rotary mode across a range of lithologies. The 10 runs at Site 1230 were specifically targeted to construct an "in situ" gas concentration profile from shallow depths near the seafloor to deeper depths below intervals with gas hydrate. Table T1 summarizes the pertinent information for all these cores.
Two runs of the PCS were made at Site 1225. Core 201-1225A-29P recovered 1.00 m of sediment under pressure using the Christensen auger shoe and an additional 0.41 m of sediment in the extended shoe. However, the recovery pressure was not determined because the port for the internal pressure transducer leaked. After ~30 min and possible release of some internal pressure, a gauge inserted into a side port indicated ~1200 psig. Approximately 70 mL of gas escaped through the manifold when the PCS was opened to atmospheric pressure. Core 201-1225C-32P recovered 1.00 m of sediment using the RBI auger shoe, although the pivot pins on the ball valve broke. A pressure gauge inserted ~30 min after recovery and several minutes after placement on ice showed 4800 psig. Over the following hour, the pressure of this core dropped to 4010 psig (Fig. F4A). No gas volume was determined when the PCS was opened to atmospheric pressure.
The PCS was also deployed twice at Site 1226, although targeted intervals were significantly deeper and harder than at Site 1225. Using the Christensen auger shoe, Core 201-1226B-42P reached the rig floor at 6208 psig with 1.00 m of sediment inside the tool and an additional 0.66 m of sediment in the extended shoe. After placing this core on ice, the pressure decreased in a logarithmic manner to 4907 psi within 150 min (Fig. F4B). Approximately 60 mL of gas was released upon opening the tool to atmospheric pressure. Using the RBI auger shoe, Core 201-1226E-21P recovered 1.00 m of sediment, but at atmospheric pressure. A post-mortem autopsy revealed that a chert layer was present at the level of the ball valve and prevented the tool from sealing at depth. There was no damage to the cutting shoe or tool.
Numerous gas release experiments during Leg 164 demonstrated that all PCS cores retrieved at high pressure (1) decreased in pressure in a logarithmic manner to a new elevated pressure over ~150 min after placement on ice and (2) released 60 to 120 mL of air upon first opening the tool (Paull, Matsumoto, Wallace, et al., 1996; Dickens et al., 2000a). Experiments at Sites 1225 and 1226, which contain very little CH4 according to conventional headspace analyses (see "Biogeochemistry" sections in the site chapters), confirm these findings. Presumably, air becomes trapped inside the tool as headspace during deployment. Despite the lack of hydrocarbon gas, the trial PCS runs at Sites 1225 and 1226 clearly show that the tool can collect full 1.00-m cores at pressure in sediment other than the fine-grained clay of the Blake Ridge.
One run of the PCS was made at Site 1227 with the RBI auger shoe. However, the tool never closed at depth and Core 201-1227A-15P failed to recover a sediment core and return to the ship at pressure.
A single PCS run was also made at Site 1228 with the RBI auger shoe, although this cutting shoe was lost in the hole after snapping off. Core 201-1228A-23P recovered 0.07 m of sediment. However, pressure could not be determined on the rig floor because water again leaked into the port with the pressure transducer. A pressure gauge inserted into the side port of the PCS immediately before placing the tool on ice gave a reading of 35 psig. Approximately 60 mL of gas was released the core when it was opened to atmospheric pressure. This gas was air.
The PCS was deployed once at Site 1229 in an interval near where gas escape structures were described during Leg 112 (Suess, von Huene, et al., 1988). Core 201-1229A-10P recovered 0.86 m of sediment at 78 mbsf using the Christensen auger shoe. However, the release of gas from this core was not straightforward. First, the pressure transducer apparently failed again, so a pressure gauge was inserted into the side port. This gauge read 420 psi before placing the tool on ice, a pressure higher than the 338 psig expected, assuming hydrostatic loading. An alternate digital pressure recorder was then connected to the tool, but pressures oscillated between 19 and 100 psig over time, even when the tool was closed on ice (Fig. F4C). After about an hour, the tool was opened and 2880 mL of gas was incrementally released though the manifold. Essentially all of this "gas" was composed of air. We assume the air was introduced during drilling; however, we do not understand where and how such a large amount of air entered the tool or whether it relates to the anomalous pressure readings.
The PCS was deployed 10 times at Site 1230 using the Christensen auger or RBI PDC cutting shoes (Table T1). Pressures measured on the rig floor for these deployments ranged from 254 to 8044 psi, or from 4% to 105% of hydrostatic pressure (Table T1). The range in observed pressures, including values higher than hydrostatic, is similar to that obtained at sites on the Blake Ridge (Paull, Matsumoto, Wallace, et al., 1996). However, these PCS pressures should not be used to accurately assess downcore variations in pressure. All PCS deployments trap a small volume of headspace air, as noted previously. Consequently, pressures inside the PCS change as this headspace volume warms and cools between the subsurface and the first measurement on the rig floor (Dickens et al., 2000b).
The length of core recovered by the PCS at Site 1230 varied from 0.18 to 1.00 m (Table T1), with six of the deployments retrieving the maximum length. This overall core recovery is much better than that at Sites 994, 995, and 997, where many PCS runs retrieved cores of <0.50 m (Paull, Matsumoto, Wallace, et al., 1996). Core 201-1230A-25P was extruded as a series of incoherent sediment masses that totaled 0.18 m, a length that probably represents a maximum. All sediment cores recovered by the PCS are lithologically similar to surrounding cores recovered by APC or XCB at adjacent depths.
Total gas volumes released from the PCS ranged from 200 to 6330 mL (Table T1). These volumes are primarily mixtures of helium, air (nitrogen and oxygen), methane, and carbon dioxide (Table T2), although not all incremental volumes were analyzed (see "Appendix" ). In general, helium and air dominate the first 150 mL of gas because helium was used to purge the small manifold volume (~30 mL) prior to gas release, and a small volume of air is trapped in the tool during deployment. Methane comprises most of the remaining gas after release of air, although carbon dioxide and sometimes air increasingly constitute minor components at low pressure. These results are the same as those found during Leg 164.
Using information in the "Appendix" we have constructed time-pressure (Fig. F5) and volume-pressure (Fig. F6) plots for each of the 10 PCS cores at Site 1230. In constructing these plots, pressures have not been corrected for gauge offset and volumes include all gas released at all temperatures from 0° to 21°C. Pressure axes on the volume-pressure plots are linear and range from 0 to 1000 psig to emphasize gas release at low pressure (Dickens et al., 2000a).
Gas escapes the PCS in a predictable manner, as observed by common features of the time-pressure and volume-pressure plots (Figs. F5, F6). After recording an initial pressure on the rig floor (generally >6000 psig at Site 1230), the pressure rises until the core is surrounded with ice and cooled. Pressure then decreases almost exponentially to reach a baseline value in ~100 min. Upon first opening the PCS to the manifold, a small volume of gas (air) escapes and pressure plummets to <500 psig. With each successive opening of the PCS, an incremental loss of gas (mostly methane) and drop in pressure occurs. The change in volume and pressure during these openings decrease with time until warmed to ambient laboratory conditions, when an additional volume of gas exits the tool. These time-pressure-volume relationships are entirely consistent with gas release experiments at Sites 995 and 997 on the Blake Ridge (Paull, Matsumoto, Wallace, et al., 1996; Dickens et al., 2000).
In situ methane concentrations can be calculated from gas volumes, core volumes, and porosities after correction for air contamination (Dickens et al., 1997; 2000). Preliminary estimates suggest that in situ methane concentrations at Site 1230 range from 13 mM (Core 201-1230B-4P) to 400 mM (Core 201-1230A-20P). The latter value greatly exceeds methane solubility with respect to the dissolved CH4-CH4 hydrate partial saturation curve (Handa, 1990); it is consistent with the presence of several percent gas hydrate in pore space around 150 mbsf.