The PCS was deployed at six of the locations drilled during Leg 201: Sites 1225 and 1226 at 3771 and 3308 mbsf, respectively, in the eastern equatorial Pacific; Sites 1227, 1228, and 1229 at 439, 274, and 151 mbsf, respectively, along the Peruvian shelf; and Site 1230 at 5086 mbsf on the lower slope of the Peru Trench. All six of these locations had been drilled previously during ODP Legs 112 or 138 (Suess, von Huene, et al., 1988; Mayer, Pisias, Janecek, et al., 1991).
Sediments at the six locations vary considerably (Suess, von Huene, et al., 1988; Mayer, Pisias, Janecek, et al., 1991; also see "Lithology" sections in the site chapters). The sequences at Sites 1225 and 1226 consist mostly of stiff, fine-grained nannofossil ooze. By contrast, the sediment records at Sites 1227, 1228, and 1229 are composed of alternating diatomaceous and siliciclastic packages with occasional hardgrounds and coarse-grained units. The sequence at Site 1230 consists mostly of clay and diatom ooze.
Observations made from conventional cores during Leg 201 and the previous legs suggest that significant gas loss on the wireline trip undoubtedly occurs at one site and possibly at two additional sites. Visible gas escape structures appeared in cores below 30 mbsf at Site 685/1230. Structures potentially representing gas release also were documented between 58 and 62 mbsf at Site 681/1229. High headspace methane concentrations (>1000 µL/L), which may signify gas concentrations approaching or exceeding saturation at depth, were present at these two sites as well as at ODP Site 684/1227. Gas hydrate also exists in sediment at Site 685/1230 (Kvenvolden and Kastner, 1990; also see "Gas Hydrate" in the "Site 1230" chapter).
The PCS cores were degassed in the rock polishing room at the top of the laboratory stack (Fig. F3). Other than the precruise cutting shoe modifications, the most significant change between Leg 164 and Leg 201 PCS operations was the location of gas venting. The PCS has connected inner and outer chambers with a sampling port to each (Fig. F1). The inner chamber contains the sediment core (and excess borehole water in the case of a short core) of ~1465 mL, whereas the outer chamber contains ~ 2700 mL of borehole water. For many of the PCS cores on Leg 164, gas was released from the port to the inner chamber. With this configuration, however, unconsolidated sediment often extruded into the port and manifold at high pressure, clogging the system and preventing gas release. To rectify this problem, gas was released through the port of the outer chamber on PCS cores retrieved toward the end of Leg 164 (Dickens et al., 2000b). This configuration was used for all PCS cores during Leg 201.
Measurements of PCS data were kept simple during Leg 201. Time was recorded to the nearest half minute with a clock. Discrete gauge pressures were obtained in pounds per square inch gauge pressure (psig) using a pressure transducer inside of the PCS. When possible, these pressures were then corrected to account for the expected 15-psi reading at atmospheric pressure. Incremental gas volumes were recorded to the nearest 5 mL. The length of the sediment core was determined to the nearest 1 cm. Unlike during Leg 164, most of the extruded cores were in sufficiently good condition to accurately measure length. A thermometer showed that temperature inside the laboratory stayed at 21° ± 2°C.
The "Appendix" presents shipboard data collected during the PCS gas release experiments in a set of nine columns: Shipboard Local Time, Elapsed Run Time, Gauge Pressure, Corrected Pressure, PCS Opening Number, Gas Release Volume, Gas Sample Number, Gas Split Volume, and Cumulative Gas Volume. These columns follow those used and discussed in previous PCS data reporting (Dickens et al., 2000b). For several reasons, not all PCS cores recovered at pressure have values in all nine columns. In particular, problems with the pressure transducer prevented collection of pressure measurements for some cores.