During Leg 201, a suite of downhole tools were employed to better characterize the environment hosting the subsurface microbial populations. Temperature is a key parameter because it affects the rates of microbial activities, and different groups of microorganisms are known to be active over different temperature ranges. Pore pressure is also important because if the pressure gradient differs from hydrostatic then fluid flow may affect the supply of dissolved substrates to the microbial populations. Finally, the concentrations of dissolved gases and methane hydrates at in situ conditions must be known to determine rates of certain microbial processes, particularly the formation and consumption of methane. In situ sediment thermal measurements were made during Leg 201 using the Adara APC temperature tool and the Davis-Villinger temperature probe (DVTP) (Davis et al., 1997). Formation pore pressures were measured using a DVTP modified to include a pressure port and sensor that was previously used during Leg 190. Samples of sediment, water, and gas at in situ pressure were collected with the pressure coring sampler (PCS) (Pettigrew, 1992). The instruments and procedures are summarized below. Additional information regarding the PCS is presented in Dickens et al. (this volume).
The Adara temperature tool fits directly into the cutting shoe on the APC and can therefore be used to measure sediment temperatures during regular piston coring. The tool consists of electronic components, including battery packs, a data logger, and a platinum resistance-temperature device calibrated over a temperature range of 0°-30°C. A photograph of the components can be found in Fisher and Becker (1993). The thermal time constant of the cutting shoe assembly into which the Adara tool is inserted is ~2-3 min. The only modification to normal APC procedures required to obtain temperature measurements is to hold the corer in place for ~10 min after cutting the core. During this time, the Adara tool equilibrates toward the in situ temperature of the sediments. The Adara tool logs data on a microprocessor contained in the instrument. Following deployment, the data are downloaded for processing. The tool can be preprogrammed to record temperatures at a range of sampling rates. Sampling with 10-s intervals was used during Leg 201. A typical APC measurement consists of a mudline temperature record lasting 10 min. This is followed by a pulse of frictional heating when the piston is fired, a period of thermal decay that is monitored for 10 min, and a frictional pulse upon removal of the corer. Before reduction and drift corrections, nominal accuracy of Adara temperature data is estimated at 0.1°C. A second stop of 10 min at the mudline is made before raising the core to the surface.
The temperature measurement aspects of the DVTP are described in detail by Davis et al. (1997). The probe is conical and has two thermistors, one 1 cm from the tip of the probe and the other 12 cm above the tip. A third thermistor, referred to as the internal thermistor, is in the electronics package. Thermistor sensitivity is 1 mK in an operating range of -5° to 20°C, and the total operating range is -5° to 100°C. The thermistors were calibrated at the factory and on the laboratory bench before installation in the probe. In addition to the thermistors, the probe contains an accelerometer sensitive to 0.98 m/s2. Both peak and mean acceleration are recorded by the logger. The accelerometer data are used to track disturbances to the instrument package during the equilibration interval. Temperature data were recorded at 10-s intervals. In a DVTP deployment, mudline temperatures (within the drill pipe) are measured for 10 min before descent into the hole for a 10-min equilibration interval in the bottom. The time constants for the sensors are ~1 min for the probe tip thermistor and ~2 min for the thermistor at 12 cm from the tip. Unless stated otherwise, data from the probe tip thermistor were used for estimation of in situ temperatures. Upon retrieval, a second stop of 10 min is made at the mudline.
Similar data reduction procedures were used for the two temperature tools. The transient thermal decay curves for sediment thermal probes are known to be a function of the geometry of the probes and the thermal properties of the probe and the sediments (Bullard, 1954; Horai and Von Herzen, 1985). Analysis of data requires fitting the measurements to analytical or synthetic decay curves calculated based on tool geometry, sampling interval, and tool and sediment thermal properties. For the DVTP tool, thermal decay data are analyzed by comparison to computed type curves using the software program CONEFIT, developed by Davis et al. (1997). However, it is generally not possible to obtain a perfect match between the synthetic curves and the data because (1) the probe does not reach thermal equilibrium during the penetration period; (2) contrary to ideal theory, the frictional pulse upon insertion is not instantaneous; and (3) temperature data are sampled at discrete intervals, so that the exact time of penetration is uncertain. Thus, both the effective penetration time and equilibrium temperature must be estimated by applying a fitting procedure, which involves shifting the synthetic curves in time to obtain a match with the recorded data. The data collected more than 20-50 s beyond penetration usually provide a reliable estimate of equilibrium temperature. Thermal conductivities measured shipboard were used for estimation of in situ temperatures and for calculation of heat flow. Laboratory thermal conductivity measurements were not corrected for in situ conditions because the correction would be small at the shallow depths drilled during Leg 201.
Measurement of formation pressure was achieved using a modified DVTP. The probe has a tip that incorporates both a single thermistor in an oil-filled needle and ports to allow hydraulic transmission of formation fluid pressures to a precision Paroscientific pressure gauge inside. A standard data logger was modified to accept the pressure signal instead of the second thermistor signal in the normal DVTP described above. Thermistor sensitivity of the modified tool is reduced to 0.02 K in an operating range of -5° to 20°C. Deployment of the tool consists of lowering by wireline to the mudline, where there is a 10-min pause. Subsequently, the tool is lowered to the base of the hole and latched in at the bottom of the drill string, with the end of the tool extending 1.1 m below the drill bit. The extended probe is pushed into the sediment below the bottom of the hole, and pressure is recorded for 30 min or as long as deemed operationally safe. In later deployments a 2- to 5-min stop at the base of the hole was made after the in situ measurement to provide a reference for hydrostatic pressure. If smooth pressure decay curves are recorded after penetration, then extrapolations to in situ pore pressures are possible. This pressure response is qualitatively similar to but slower than the thermal response. The model for the characteristic response of pressure to the displacement and sediment deformation associated with penetration is more complex than the model used to estimate in situ temperatures from the decay of the frictional heating pulse. Construction of a complete analytical or numerical model of the pressure response had not been completed by the time of Leg 201, so shipboard extrapolations to estimated formation pressures must be considered preliminary until thoroughly processed postcruise.
The APC-Methane tool under development is designed to continuously record temperature, pressure, and conductivity at the face of the APC piston assembly during core ascent. Its purpose is to provide a continuous record of sediment gas temperatures, internal pressure, and timing of gas headspace formation during core recovery.
Large quantities of gas can escape sediment cores when a drop in pressure or increase in temperature during recovery lowers methane saturation (Paull et al., 2000; Wallace et al., 2000). Based on previous drilling during Legs 112 and 138, significant gas loss was expected to occur at two or more of the proposed sites (Table T20). Visible gas escape structures appeared in cores between 58 and 62 mbsf and below 30 mbsf at Sites 681 and 685, respectively. High headspace methane concentrations (>1000 µL/L), which may signify gas concentrations approaching or exceeding saturation at depth, also were present at these two sites and at Site 684.
The PCS is a downhole tool designed to recover a 1-m-long, 1385-cm3 cylindrical sediment core—including gas and interstitial water—at in situ pressure (Pettigrew, 1992). When its valves seal properly, controlled release of pressure from the PCS through a manifold (below) permits collection of gases that would otherwise escape during the wireline trip. At the time of Leg 201, the PCS provided the only proven means to determine in situ gas abundance in deep-sea sediments where gas concentrations at depth exceed saturation on the ship (Dickens et al., 1997). However, the PCS had only been successfully used to capture and analyze in situ gases during Leg 164 (Paull, Matsumoto, Wallace, et al., 1996; Dickens et al., 1997). Consequently, the PCS was deployed 17 times during Leg 201 to achieve two objectives: (1) to quantify gas abundance at Site 1230 along the Peru margin and (2) to ensure that the tool was fully operational for upcoming legs targeting gas-rich sediments. The basic operations and results of these deployments are presented in Dickens et al. (this volume).
Samples of bottom water were collected at each of the Peru margin sites (1227-1231) using the Water Sampling Temperature Probe (WSTP). The WSTP is a passive sampler that is deployed in the bottom-hole assembly (BHA). Before deployment, the fluid path is filled with deionized water and an overflow chamber is filled with air. A timer is set to open the valve at a fixed time, exposing the sampling line and chamber to ambient pressure. The time also closes the chamber after a prearranged time interval has passed. In operation, the WSTP is mounted inside a core barrel and lowered down the drill pipe by wireline. Before beginning to drill, the tool is lowered to 10 m above the seafloor with the probe tip extending 1.1 m ahead of the bit. The tool is then held in position with the pumps off for a total of 15 min to measure the bottom-water temperature. Approximately halfway into this interval, the timer-operated valve is opened. During Leg 201, the interval that the valve was left open varied from 3 to 5 min. When the valve is open, bottom water is drawn under negative relative pressure through the filter and into the sample chamber, displacing the deionized water. Upon retrieval, the water sample recovered from the sample chamber is analyzed and the amount of sample dilution by deionized water is assessed by comparing the chemistry to IAPSO standard seawater.
As part of ODP's engineering development program, operations time was allocated to deployment of a third-party pressure coring tool. The Fugro Percussion Corer (FPC) uses a water hammer driven by drilling fluid circulation to push a core barrel into the sediment ~1 m ahead of the bit. After coring, the 58-mm-diameter core is retracted into an autoclave and sealed by a flapper valve to return a core under pressure. Six deployments were in our initial operation plan, spread over three sites. Ultimately, seven deployments of the FPC were completed at Sites 1227, 1228, and 1229.