Paleomagnetic investigations during Leg 200 consisted mainly of routine remanent magnetization measurements of (1) archive-half sections and (2) discrete samples from the working-half sections, with both being completed before and after alternating-field (AF) demagnetization. In addition, low-field magnetic susceptibility was measured on whole-core sections using the MST device and on archive-half sections using the AMST. To investigate rock magnetic properties, we also conducted thermal demagnetization, anhysteretic remanent magnetization (ARM), and isothermal remanent magnetization (IRM) experiments on some of the discrete samples. To better evaluate the origin and nature of drilling overprints, we measured the remanent magnetization of one whole-round section of sediment, a few long (>60 cm) whole-round pieces of basalt, and a few working-half sections before and after demagnetization. Such measurements are only permitted under rare circumstances because they result in partial demagnetization of the entire interval being measured. To preserve the natural remanent magnetization in these intervals, the highest peak field used in demagnetization never exceeded 30 mT.
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 AF demagnetizer (2-G Enterprises model 2G600) capable of producing peak fields of 80 mT with a 200-Hz frequency.
The natural remanent magnetization (NRM) was routinely measured every 1 cm along all archive-half sections before and after AF demagnetization. In interpreting the data, we avoided using the measurements within 5 cm from the ends of each section, although we saved these values for future studies that might wish to deconvolve the remanence signal. AF demagnetizations were applied at multiple demagnetization steps on all sections in steps of 1 to 5 mT up to peak fields of 35 to 70 mT as required to remove drilling overprints and resolve the characteristic remanent magnetizations. The narrow measurement interval, the large number of demagnetization steps, and the high demagnetization levels were selected to allow detailed analysis of vector demagnetization paths. This was possible during Leg 200 because the amount of core recovered was relatively small.
The discrete samples were also measured with the cryogenic magnetometer. Typically samples were demagnetized in steps of 1 to 5 mT from 0 to 80 mT. A few of these samples were further demagnetized at fields up to 150 mT using the DTECH AF demagnetizer (model D2000), with subsequent remanence measurements in the cryogenic magnetometer.
To investigate rock magnetic characteristics of some of the discrete samples, ARM and IRM experiments were conducted and then measured using the cryogenic magnetometer. The DTECH AF demagnetizer was used to impart ARMs to discrete samples, using a 100-mT peak alternating field and a 0.05-mT direct-current (DC) field. The ARMs were imparted along the +z-axis of the samples. We then progressively demagnetized the ARMs at 5-mT increments from 0 to 80 mT using the cryogenic magnetometer. IRMs were imparted to the discrete samples by a DC field generated in the ASC (Analytical Services Company) Impulse Magnetizer (model IM-10). We typically measured the remanence of the discrete samples after imparting (1) an IRM of 1000 mT along the +z-axis, (2) a backfield IRM (BIRM) of 100 mT along the -z-axis, (3) a BIRM of 300 mT along the -z-axis, and (4) an IRM of 1000 mT along the -z-axis. The samples were then stepwise AF demagnetized at 5-mT increments from 0 to 80 mT using the cryogenic magnetometer. In addition, we conducted IRM acquisition experiments and thermal demagnetization of IRMs on a few samples.
Low-field magnetic susceptibility was measured for all whole-core sections as part of the MST measurements (see "MST Measurements" in "Physical Properties"). The MST susceptibility meter (a Bartington model MS2 with an MS2C sensor, a coil diameter of 88 mm, and an operating frequency of 0.565 kHz) has a nominal resolution of 2 x 10-6 SI (Blum, 1997). Susceptibility was determined at 1- or 2.5-cm intervals using a 1-s integration time and a 4-s period. The "units" option was set on SI units, and the values were stored in the ODP Janus database in raw meter units. To convert to true SI volume susceptibilities, these should be multiplied by 10-5 and then multiplied by a correction factor to take into account the volume of material that passed through the susceptibility coils. Except for measurements near the ends of each section, this factor for a standard ODP core is ~0.7 (Blum, 1997). No correction was applied for any figures illustrating magnetic susceptibilities in the "Paleomagnetism" sections in the site chapters. Hence, the units are given as raw meter values. Magnetic susceptibility of discrete samples obtained from the working half was measured using a Bartington MS2 susceptibility meter with a dual frequency MS1B sensor. In addition, magnetic susceptibility was measured every 2 cm on the archive halves using the point susceptibility probe (Bartington MS2 susceptibility meter with a MS2F sensor) on the AMST.
Even though results from the shipboard cryogenic magnetometer have been compared with many other laboratories and shown to give consistent results, it is useful to check the calibration of the magnetometer against a known standard at the beginning of each leg. We used a standard purchased from Geofyzika that is an 8-cm3 cube with an intensity of 7.62 A/m (moment 6.096 x 10-5 Am2). All three axes gave results that agree to better than 1% with this standard. In addition, the automated tray positioning was checked by putting the standard at known positions and measuring the tray. The position indicated by the software was found to be good to ±1 cm, which is reasonable given the stretch in the pulley system used to move the sample boat.
Based on tests conducted during Leg 186, the background noise level of the magnetometer in the shipboard environment is ~2 x 10-9 Am2 (Shipboard Scientific Party, 2000b). We repeated tests during Leg 200 for an empty split-core tray (also referred to as the sample boat) and for the tray that holds discrete samples (Figs. F11, F12). Several discrete trays are available for use, but we only used the tray provided by 2-G Enterprises. This tray is 150 cm long and has holders for samples with 2 cm x 2 cm bases at 10, 30, 50, 70, 90, 110, and 130 cm and holders for samples with 2.6 cm x 2.6 cm bases at 20, 40, 60, 80, 100, 120, and 140 cm. It sits in the sample boat when used. We first measured the sample boat before and after cleaning it (Fig. F11A, F11B), where the cleaning consisted of spraying the boat with window cleaner and wiping it clean followed by AF demagnetization at 60 mT. The noise is noticeably reduced by cleaning, although in both cases it is less than ±2 x 10-9 Am2. We also repeated the cleaned sample boat measurement several times over a 2-day period and got very similar results for all three axes, indicating that the signal shown in Figure F11B is not random noise of the instrument but a coherent signal related to the cleaned sample boat. When the discrete sample tray is placed in the sample boat, the noise level increases. It is slightly greater than ±2 x 10-9 Am2 before cleaning (Fig. F12A) and slightly less after cleaning (Fig. F12B). The clean discrete sample tray measurement was repeated a few days later with the addition of empty plastic cubes in the tray, resulting in noise that exceeded ±3 x 10-9 Am2 (Fig. F12C). Thus, the practical noise level is related to the magnetization of the plastic cubes (for discrete samples) and to dirt on the sample boat or tray. Some of this noise can be removed by subtracting the tray magnetization from sample measurements, but again dirt on the tray can accumulate. Even with diligence, it is difficult to keep the trays clean given the amount of core material measured during a typical leg. For long core measurements, it is not practical to clean and remeasure the empty tray before each new sample. Thus, the correction for the tray magnetization may not improve the accuracy of the measurements significantly. Furthermore, the split-core sections are in a plastic core liner that has been stored in dusty conditions prior to coring and that resides in a metal core barrel just prior to core collection. Therefore, the noise associated with the core liners will likely be several times greater than the noise associated with the sample boat.
We conclude that under favorable conditions the noise level will be ~±2 x 10-9 Am2. For a split core, given the large volume of core material within the sensing region of the magnetometer, which is ~100 cm3, the minimum measurable remanent intensities will be greater than ~2 x 10-5 A/m. For discrete samples, which typically have volumes of 6-10 cm3, the minimum measurable remanent intensities are greater than ~4 x 10-4 A/m. Results from measurements during several cruises indicate that accurate measurements are likely to be obtained when both split-core and discrete samples have intensities about two to five times higher than the background noise level, or when they have intensities greater than ~10-4 A/m and 10-3 A/m, respectively.
The standard ODP paleomagnetic coordinate system was used. In this system, +x represents the vertical upward from the split-core surface of archive halves, +y is left along split-core surface when looking upcore, and +z is downcore (Fig. F13A).
Given the limited APC coring that was conducted, we did not have the opportunity to use the Tensor tool for azimuthal core orientation during Leg 200. For hard rocks it is possible to combine images of the exterior of the core with images of the borehole wall from the Formation MicroScanner (FMS) logging tool. To allow for this possibility, the exterior of all basalt cores were scanned with the new digital camera system. We first marked a line on the top of each core piece indicating where the core would be split into archive and working halves. The whole-core pieces were scanned in this orientation, and then each piece was rotated counterclockwise 90° about the -z-axis and scanned. The process was repeated for 180° and 270° rotations, resulting in four images of each piece, with the hope that the four images could be correlated with the four images provided by the sensor pads on the FMS.
Oriented discrete samples were taken from the working half of each section (typically one or two per section for shipboard analyses) using plastic cubes with a 2 cm x 2 cm x 2 cm exterior dimension and an aluminum extruder. The interior volume of a cube is ~7 cm3. Each plastic cube has an arrow on the bottom face. The sediment is collected in the extruder with a 2 cm x 2 cm cross-section opening (Fig. F13B). Usually a few millimeters of sediment is extruded and removed from the bottom of the extruder with a spatula, leaving a flat surface of sediment and just enough sediment in the extruder to fill the plastic cube. The sediment is then carefully extruded into the plastic cube. An orientation mark on the extruder aids in ensuring the upcore orientation of the sample is maintained, and a mark on the plunger in the extruder aids in ensuring that enough sediment is collected to fill the plastic cube.
In lithified intervals, we drew an arrow on the split-core face pointing uphole and used the rock saw to cut the sample. When measuring the sample, we placed the side with the arrow down in the tray with the arrow pointing along the -z-axis, or uphole, direction. This makes the discrete sample orientation the same as that of the archive halves, which allows for a more direct comparison of the discrete sample results with archive-half results.
We also cut small chips or cubes (~0.5-1.5 cm3) and drilled ~1.1-cm3 minicores (1.2 cm diameter and ~1 cm long) from hard rock cores to assess differences in the drilling overprint from the periphery of the core to its interior. These were oriented with an arrow pointing uphole and drawn along the split-core face or along the face closest to and parallel (or tangent for cylinders drilled along the z-axis) with the split-core face. These were measured in the magnetometer with the arrow pointing along the -z-axis, or uphole, and with the side with the arrow facing up or down, as noted for specific tests. In all cases, the arrow on the discrete samples points upcore.
Several types of secondary magnetization were acquired during coring, which sometimes hampered magnetostratigraphic and paleomagnetic interpretation. The most common was a steep downward-pointing overprint attributed to the drill string. For the Leg 200 cores, we also observed a bias for 0° declinations, which has been observed during many previous cruises and has been interpreted as a radially inward overprint (Fig. F13A). We sampled the periphery and interior of core pieces to better document the nature of the overprints for different lithologies cored at the two sites. Other overprints related to diagenesis or tectonic deformation were observed. Details of these and other magnetic complexities are presented in the site chapters.
Where AF demagnetization successfully isolated the characteristic component of remanence, paleomagnetic inclinations were used to define polarity zones. Ages for reversals are from the revised Cenozoic timescale of Cande and Kent (1995).