A suite of physical property measurements were made to support the main scientific objectives of Leg 201. Physical characterization of the subsurface environment, particularly including density, porosity, and matrix composition, is necessary for specification of the hydrodynamic environment that is expected to strongly affect the microbial community. In addition, physical properties can be used to define geochemical and, hence, population and community boundaries. In many cases, we used nonstandard downcore spacings and instrument precision to better physically define known zones of special biochemical or geochemical interest.
Selected cores from Sites 1225, 1226, 1230, and 1231 were thermally imaged on the catwalk prior to sectioning. All other physical property measurements were conducted after the cores had equilibrated to near ambient room temperature (i.e., 22°-24°C), a settling period of typically 2-4 hr, except for cores sampled for microbiology in the cold room. Physical properties measured on the MST and thermal conductivity measurements were normally made on whole-round core sections during the same time interval. Discrete moisture and density (MAD) parameters, P-wave velocities, and electrical resistivity were subsequently measured on each split-core section. A summary of each of the physical property measurement procedures for Leg 201 is outlined below; more detailed descriptions are provided in Blum (1997).
Tables of physical properties in ASCII are provided on the "Log and Core Data" CD-ROM included with this publication. These data tables include GRA density, magnetic susceptibility, natural gamma radiation (NGR), P-wave velocity, MAD, thermal conductivity, Hamilton frame velocity (PWS), resistivity, and paleomagnetism.
Infrared thermal imaging was introduced during this leg for technique development prior to expected critical use during Leg 204. IR imaging was shown to successfully identify thermal anomalies in sediment cores attributed to the location of gas hydrate (cold anomalies) and voids (warm anomalies). The primary benefits of using IR (in preference to estimating temperature differences by touch) include more precise identification of thermal anomalies and the possible estimation of hydrate volume from processed images. It is quicker, simpler, and more compact than the system of thermistors used during Leg 164 (Paull, Matsumoto, Wallace, et al., 1996). Small-scale hydrate nodules and disseminated gas hydrate were the primary forms identified, suggesting the camera can detect small quantities. Volumetric analysis will require further study.
Another proposed use for the camera is for the lithologic characterization of ambient-temperature cores because of slight variations in their thermal emission properties attributable to sediment composition or water content. Data were collected during Leg 201 to examine this possibility. Processing and analysis will be completed postcruise.
A third use for the camera during Leg 201 was to monitor the rate of warming of cores to determine the maximum radial temperature distribution reached before of microbiological sampling in the cold room. The method and results for this monitoring effort are described in "Infrared Scanner" in "Physical Properties" in the "Site 1225" chapter.
A ThermaCam SC 2000 camera (FLIR Systems) was used. This camera images temperatures from -40° to +1500°C. For onboard application, it was set to record a range of temperatures from -40° to +120°C (range 1).
By experimentation, we determined that a 10-cm field of view on the core was obtained with the camera lens located 34 cm above the highest point on the core. In order to minimize the effect of external IR radiation reflecting from the core liner, the camera and the space between the lens and the core was enclosed within a cardboard sheath covered on the outside with crumpled aluminum foil (Fig. F12) in order to disperse ambient IR energy.
To record data for each core, the mounted camera was placed on top of the core liner and the camera was focused on the edges of the core using computer controls. During focusing, camera span and level parameters were auto-adjusted to optimize visual contrast of the expected downcore temperature variation on the computer screen. Immediately after recovery, the core liner was cleaned and the camera was manually rolled along the core from top to bottom. The ThermaCam Researcher 2001 software, running on a dedicated laptop computer, acquired images from the camera at a rate of 5 frames/s. During trials conducted on the transit to the first site, we found that a ~45-s acquisition time for a 10-m core produced images with minimal blurring and with considerable sequential overlap. While the images were being recorded by the computer, the computer screen would freeze at the first image frame, preventing real-time viewing of the core liner temperatures. At Site 1230, we discovered that the camera itself provides real-time images of the core, and a detachable external screen was therefore used to view the core as it was scanned. The screen span and level parameters were set to optimize visual contrast on the external screen of the expected downcore temperature variation (since the camera and computer spans and levels could be different). Initially, this was 0°-20°C. After hydrates were identified at ~16°C, compared with a background level of >20°C, the span and level were set to show a range of 15°-25°C.
To facilitate depth integration of the IR data with other physical property measurements, a depth scale was constructed using a 10-m x 4-cm aluminum unistrut. A 5-cm spaced numbered scale was painted onto the unistrut using Rustoleum Specialty High Heat oil-based enamel (black #7778). This combination initially produced sufficient thermal contrast for subsequent discrimination of the scale markers. However, the scale was only clearly visible in still images. Images recorded while the camera was moving were too blurred to identify the scale. Holes drilled in the unistrut at 5-cm intervals also proved to be of limited use. Thermal contrast of the holes was very clear, particularly during the day or when a hand was run under the unistrut while the camera was being rolled down the core. However, blurring remained a significant problem. Additionally, producing a single depth-matched downhole record of temperature variation based on the scale required extensive image processing, as only one image could be viewed at time. Several attempts at developing an automated technique proved unsuccessful. The process was further complicated by the varying rate of scan between different camera operators and mechanical problems with the manually operated trolley. At Site 1230, we decided to assign the curated depth of the top of the core to the top of the first image. Then the curated recovered depth was divided by the number of images taken for the core. This interval was sequentially added to the images to assign depths in a core to each image.
In order to develop depth-matched downcore temperature profiles, the following process was established through experimentation over the course of Leg 201. At this writing, only Site 1230 data have been thoroughly examined.
Following this process, core data files were combined to provide downhole temperature profiles. Where recovery was >100% in any core, the overlapping depth interval from the upper core was removed from the composite record.
The MST comprises four physical property sensors on an automated track that sequentially measure volume magnetic susceptibility, wet bulk density, compressional wave velocity, and natural gamma ray radiation on whole-round intact core sections. Measurements are nondestructive of core fabric and are used principally to facilitate shipboard core-to-core correlation, to construct composite stratigraphic sections, and to correlate with downhole tools. Each device has an intrinsic spatial resolution determined by its design specifications (see discussion of each measurement below). Data quality depends on core condition and instrument precision. Optimal MST measurements require a completely filled liner with minimal drilling disturbance. Precision is generally a function of measurement time, especially with respect to magnetic susceptibility, wet bulk density, and natural gamma radiation detection. The final sensor-specific spatial resolution chosen for each site balances the spatial footprint and accuracy with core flow requirements and was particularly critical during rapid core recovery at the shallow-water sites along the Peru margin. In all cases, the scientific objectives of Leg 201 placed primary importance on a high-resolution and precise determination of wet bulk density; therefore, the count times and spatial resolution on this instrument were maximized at the expense of other measurements.
Whole-core volume magnetic susceptibility was measured with the MST using a Bartington MS2 meter coupled to a MS2C sensor coil with a diameter of 8.8 cm operating at 565 Hz. The measurement resolution of the MS2C sensor is 4 cm. The instrument has two precision settings, a minimum statistically significant count time of 1 s and a 10-s count time. During Leg 201, MST magnetic susceptibility was routinely measured at a spacing of 5.0 cm, recording the average of two 1-s (low resolution) data acquisitions for each sample location, unless otherwise stated. The instrument automatically zeros and records a free-air value for magnetic susceptibility at the start and end of each section run. Instrument drift during a section run is then accommodated by subtraction of a linear interpolation between the first and last free-air readings. Drift-corrected magnetic susceptibility data were archived as raw instrument units (SI) and were not corrected for changes in sediment volume.
Determination of wet bulk density is carried out by the GRA densitometer. This system is based on the principle that the attenuation, mainly by Compton scattering, of a collimated beam of gamma rays produced by a 137Cs source passing through a known volume of sediment is related to material bulk density (Evans, 1965). Calibration of the GRA system was completed using known graduated seawater/aluminum density standards. The measurement resolution of the GRA sensor is ~5 mm, with sample spacing generally set at 5.0 cm for Leg 201 cores unless otherwise stated. The minimum integration time for a statistically significant GRA measurement is 1 s. During most legs, a count time of 2 s is used; however, during Leg 201, GRA measurements were acquired over longer periods, from 5- to 10-s integration time in order to reduce scatter and improve precision. A freshwater control was run with each section to measure instrument drift. GRA bulk density data are of highest quality when determined on APC cores because the liner is generally completely filled with sediment. In XCB cores, GRA measurements are unreliable for the determination of true bulk density on their own because of the breakdown of in situ density by the mixing of drilling slurry and core biscuits.
Compressional wave velocity was measured by two methods shipboard. Transverse P-wave velocity was measured on the MST track with the P-wave logger (PWL) for all cores at a routine sample interval of 10 cm. The PWL transmits a 500-kHz compressional wave (P-wave) pulse through the core at a specified repetition rate (50/s). The transmitting and receiving ultrasonic transducers are aligned so wave propagation is perpendicular to the core axis. Ultrasonic transducer separation is measured by two displacement transducers. The recorded velocity is the average of the user-defined number of acquisitions per location (10). Calibration of the displacement transducers and measurement of electronic delay in the PWL circuitry were conducted using a series of acrylic blocks of known thickness and P-wave traveltime. Repeated measurement of P-wave velocity through a core liner filled with distilled water was used to check calibration validity.
Natural gamma ray emissions of sediments are a function of the random and discrete decay of radioactive isotopes, predominantly those of uranium, thorium, and potassium, and are measured through scintillation detectors arranged at 90° to each other and perpendicular to the core axis. The installation and operating principles of the NGR system used on the JOIDES Resolution are discussed by Hoppie et al. (1994). Data from 256 energy channels were collected and archived. For presentation purposes, the counts were summed over the range of 200-3000 keV to be comparable with data collected during previous legs. This integration range also allows direct comparison with downhole logging data, which were collected over a similar integration range (Hoppie et al., 1994). Over the 200- to 3000-keV integration range, background counts, measured using a core liner filled with distilled water, averaged 30 during a 1-hr measurement period. Before taking measurements, each of the four NGR amplifiers were adjusted so the thorium peak was at the highest resolution possible when the other three amplifiers were disabled. The multichannel analyzer was then calibrated by assigning certain channels to the characteristic energies of 40K and the main peak of 232Th (Blum, 1997). The measurement width of the NGR is ~15 cm, with a statistically significant count time of at least 5 s, depending on lithology. Because of the long time required for NGR measurements, sample spacing and count time for NGR measurements varied depending on the age and lithology of the sediment recovered. No corrections were made to NGR data obtained from XCB cores to account for sediment incompletely filling the core liner.
Thermal conductivity measurements of one per core were made using the TK04 (Teka Bolin) system described by Blum (1997). During Leg 201, we employed the single-needle probe (Von Herzen and Maxwell, 1959), heated continuously in full-space configuration. At the beginning of each measurement temperatures in the samples were monitored automatically, without applying a heater current, until the background thermal drift was <0.04°C/min. Once the samples were equilibrated, the heater circuit was closed and the temperature rise in the probe was recorded. The needle probe contains a heater wire and calibrated thermistor. The probe is assumed to be a perfect conductor because of its high conductance relative to the core sediments. With this assumption, the temperature of the superconductive probe has a linear relationship with the natural logarithm of the time after the initiation of the heat:
where,
The thermal conductivity (k) was determined using equation 5 by fitting the temperatures measured during the first 150 s of each heating experiment (for details see Kristiansen, 1982, and Blum, 1997).
The reported thermal conductivity value for each sample is the average of three repeated measurements. Data are reported in watts per meter degree Kelvin, with measurement errors of 5%-10%.
Moisture and density parameters were determined from wet mass, dry mass, and dry volume measurements of split core sediments after Blum (1997). Push-core samples of ~10 cm3 were placed in 10-mL beakers. Care was taken to sample undisturbed parts of the core and to avoid drilling slurry. Immediately after the samples were collected, wet sediment mass (Mwet) was measured. Dry mass and volume were measured after samples were heated in an oven at 105° ± 5°C for 24 hr and allowed to cool in a desiccator. Sample mass was determined to a precision of 0.01 g using two Scientech 202 electronic balances and a computer averaging system to compensate for the ship's motion. Sample volumes were determined using a helium-displacement Quantachrome penta-pycnometer with a precision of 0.02 cm3. Volume measurements were repeated three times, until the last two measurements exhibited <0.01% standard deviation. A reference volume was included within each sample set and rotated sequentially among the cells to check for instrument drift and systematic error. Standard sampling frequency was one per section. However, in many cases during Leg 201 we carried out high-resolution sampling through specific zones of interest. One of the MAD samples was always taken adjacent to any sample for dissolved methane and permeability.
Moisture content, grain density, bulk density, and porosity were calculated from the measured wet mass, dry mass, and dry volume as described by Blum (1997). Corrections were made for the mass and volume of evaporated seawater using a seawater density of 1.024 g/cm3 and a salt density of 2.20 g/cm3.
P-wave velocities were also measured at selected locations on split cores, usually near a MAD sample, by the ODP standard insertion probe system comprising two transducer pairs that measure velocities along axial (PWS2) and transverse (PWS1) directions. The insertion probe system determines P-wave velocity based on the traveltime of a 500-kHz wave between a pair of piezoelectric crystals separated by a fixed distance. System accuracy was checked prior to testing the first section of each hole by measuring the velocity in distilled water at a specific temperature.
Routine sampling frequency and location for P-wave measurements was coincident with the discrete MAD samples. Note that the velocity data stored in the Janus database are uncorrected for in situ temperature and pressure. These corrections can be made using the relationships outlined in Wyllie et al. (1956), Wilson (1960), and Mackenzie (1981).
Velocity anisotropy (as a percent) was calculated using axial (VPa) and transverse (VPt) P-wave velocities. Anisotropy is determined from the difference between the average horizontal and vertical velocity using
Formation factor (F) was determined from electrical resistivity measurements taken adjacent to discrete MAD samples on split-core sediments. Four in-line electrodes, 2 cm long and spaced ~1 cm apart mounted on a plastic block, were inserted into the split-core sediments. The two outer electrodes produce an alternating current (5-10 kHz) in the sediment. The resulting potential difference is measured by the two inner electrodes (Wenner array). In samples saturated with saline interstitial water, polarization effects are minimal in this frequency range and the measured resistivity is largely independent of frequency.
At each sampling location two measurements of sediment resistance were made, one oriented axially (Rcore, axial) and the other transverse (Rcore, trans) to the core axis, as with discrete P-wave velocity data collection. Measurement of resistance for room-temperature seawater (Rwtr) was made regularly so that formation factors,
in each direction could be calculated. Temperature measurements for the sediment and seawater were not made, as both were equilibrated to ambient laboratory temperature.
This simple method for determination of formation factor does not take into account surface conductivity effects of the sediment matrix. However, this is not of concern in high-porosity sediments where the conductive pathways depend dominantly on intergranular porosity and pore connectivity, even where the sediment matrix contains significant clays. Previous drilling at the sites cored during Leg 201 indicate that porosities should exceed 50% everywhere from seafloor to total depth.
Using the axial and transverse formation factors from equations 7 and 8, anisotropy can be computed as
During Leg 201, no scientist sailed as a paleomagnetic specialist. However, based on results from previous legs, paleomagnetic measurements were made at sites where better resolution could be provided by updated shipboard laboratory facilities. The data are expected to facilitate both the identification of iron redox intervals and the correlation of cores. Preliminary interpretation of paleomagnetic data was undertaken during the cruise when possible.
Measurements of remanent magnetization were made using an automated pass-through cryogenic magnetometer with direct-current superconducting quantum interference devices (DC SQUIDs) (2-G Enterprises Model 760-R). The magnetometer is equipped with an in-line alternating-field (AF) demagnetizer (2-G Enterprises Model 2G600) capable of producing peak fields of 80 mT with a 200-Hz frequency. The magnetometer and AF demagnetizer are interfaced to a PC-compatible computer and are controlled by the 2-G Long Core software program by National Instruments. Based on tests conducted during Leg 200, the background noise level of the magnetometer in the shipboard environment is ~2 x 10-9 Am2 and the minimum measurable remanent intensities for split cores will be greater than ~2 x 10-5 A/m (Shipboard Scientific Party, 2003).
The natural remanent magnetization was measured in increments ranging from 2 to 10 cm (depending on the specific interest in a particular section) along selected archive-half sections before and after AF demagnetization. AF demagnetizations were applied at multiple demagnetization steps on the initial two sections of a hole in incremental steps of 5 mT up to 20 mT in order to remove drilling overprints.
During APC coring, orientation was achieved using the Tensor multishot tool. Orientation of cores is of particular importance in paleomagnetic studies of paleoequatorial regions, where the paleomagnetic inclination is close to zero. ODP core orientation designates the positive x-axis direction as the horizontal direction (geomagnetic north in a global coordinate reference frame) from the center of the core to the median line between a pair of lines inscribed lengthwise on the working half of each core liner (Fig. F13).
An automatic portable spinner magnetometer (Niitsuma and Koyama, 1994) was used to measure remanent magnetization of the discrete samples collected during Leg 201. The samples were collected from working halves of core sections in 2-cm x 2-cm x 2-cm plastic cubes. The sampling frequency was generally one sample per core from one hole at each site.
The noise level of this magnetometer is roughly equivalent to that of the cryogenic magnetometer for the 8-cm3 discrete samples when 10 repeat spinner magnetometer measurements are averaged (Shipboard Scientific Party, 2003). This magnetometer is equipped with an AF demagnetizer, an anhysteretic remanent magnetizer, and a magnetic susceptibility anisotropy meter.