Paleomagnetic and rock magnetic investigations during Leg 209 were designed primarily to
Paleomagnetic measurements were performed on discrete minicore and cube samples and, where practical, on continuous pieces of the archive halves. The azimuths of core samples recovered by rotary drilling are not constrained. All magnetic data are therefore reported relative to the following core coordinates: +x (north) is into the face of the working half of the core, +y (east) points toward the right side of the face of the working half, and +z is down (Fig. F9).
The remanence of archive halves was measured using a pass-through 2G Enterprises direct-current Superconducting Quantum Interference Device (DC-SQUID) rock magnetometer (model 760R). The magnetometer is equipped with an in-line alternating-field (AF) demagnetizer (2G model 2G600) where samples can be demagnetized to peak fields of 80 mT. Both the magnetometer and AF demagnetizer are interfaced with a computer and are controlled by the 2G Long Core software (Core Logic, version Leg207.3). The maximum intensities that could be measured while still maintaining an accurate count of flux quanta (i.e., where the number of flux counts returns to zero after the measurement) depends on the velocity of measurements. Archive halves were typically measured at a tray velocity of 1–5 cm/s to prevent the loss of flux counts. Because the response functions of the SQUID sensors have a full width of ~10 cm at half height, data within 5 cm of piece boundaries or voids are significantly affected by edge effects. The current version of the Core Logic software allows bypassing measurements in these intervals. We did not perform measurements within 4 cm of a piece end. Although this approach means that no data are collected for pieces smaller than 8 cm or near piece ends, the time savings allows more detailed measurement or more demagnetization steps elsewhere. Archive halves were typically measured at an interval of 2 cm for natural remanent magnetization and after 10- and 20-mT demagnetization.
A standard 2.5-cm-diameter minicore sample or ~9-cm3 cube was generally taken from each 4.5-m cored interval for shipboard study. These discrete samples were chosen to be representative of the lithology and alteration mineralogy, and an effort was made to utilize samples for which geochemical and physical properties were also measured. The remanence of discrete samples was measured primarily using the 2G SQUID magnetometer, with only a small number of samples measured using the Molspin spinner magnetometer (using PMagic software). The shipboard 2G magnetometer is normally set up to measure as many as seven discrete samples, which are separated by 20 cm on a sample tray 150 cm long. Because remanent intensities of the Leg 209 samples may vary by several orders of magnitude, all discrete samples were spaced 35 cm apart to effectively eliminate any contamination of one measurement from the signal of a neighboring sample.
The calibration of the 2G magnetometer for discrete sample measurements was verified by measuring six standards that were the subject of an interlaboratory cross-calibration experiment (L. Tauxe, pers. comm., 2001). The intensities of these standards measured with the shipboard magnetometer agreed well with the accepted values (on average, the shipboard results are 1% higher). As expected from the uncertainties in positioning samples with respect to the magnetometer/SQUID axes, the remanence directions for the standards showed significant deviations (up to 9°) from the accepted values when measurements were made in one single position. When measurements were averaged over 10 different positions, the standards showed average deviations of ~2° (Table T9). However, the scatter of directions was not uniform, suggesting that a systematic bias resulted from misalignment between the tray and SQUID axes (Fig. F10). The dispersion of the remanence directions was high when samples were rotated about the magnetometer x- or y-axis. When samples were rotated about the z-axis, the scatter of directions decreased but the angular deviation with respect to the accepted standard values increased. This can be explained as the result of an offset in the x- and y- tray axes with respect to the magnetometer/SQUID axes. A counterclockwise rotation of ~7° about the z-axis was applied to the sample holder in order to correct its positioning with respect to the sensors. After this repositioning, measurements over the 10 different positions yielded average directions with an angular deviation <1° (Table T10). Furthermore, single measurements yielded directions within 2° of the accepted value, independent of the orientation of the sample with respect to the magnetometer axes. In order to provide the most robust directional data, all discrete samples were measured in three positions (such that the magnetization component parallel to each of the sample coordinate axes was measured once with each SQUID sensor) and the data averaged.
Most discrete samples were subjected to stepwise AF demagnetization using a DTech (model D-2000) AF demagnetizer capable of peak fields up to 200 mT. A small number of samples were also thermally demagnetized by the Schonstedt Thermal Specimen Demagnetizer (model TSD-1). Sufficient stepwise AF or thermal demagnetizations were performed to isolate characteristic remanent magnetization components and to quantify magnetic overprints. Characteristic directions were fit using principal component analysis (Kirschvink, 1980).
In addition to standard paleomagnetic measurements, the anisotropy of magnetic susceptibility was determined for most discrete samples using the Kappabridge KLY-2 (Geofyzika Brno) and a 15-position measuring scheme. Unfortunately, the computer interface for the Kappabridge was not functional. The 15 measurements were therefore manually recorded, and the susceptibility tensor and associated eigenvectors and eigenvalues were calculated off-line following the method of Hext (1963). All bulk susceptibility values reported for discrete samples are from the Kappabridge and have been corrected for the true cylindrical or cubic sample volume. For a small number of samples, the anisotropy of ARM was also determined. For each of the sample axial directions (i.e., +x, +y, +z and –x, –y, –z), the remanence after a baseline AF demagnetization step (parallel to the subsequent ARM direction) was measured and subtracted from the axial ARM. The remanence anisotropy tensor was then calculated in a manner analogous to that used for the susceptibility tensor.
Whole-core magnetic susceptibility (
) was measured at 2.5-cm intervals on all sections using the Bartington Instruments susceptibility meter (model MS1: 80-mm loop, 4.7 kHz, SI units) mounted on the multisensor track. Susceptibility measurements were made on the whole core following the establishment of the final curated positions and placement of spacers and are therefore directly comparable with the pass-through measurements of the archive halves.
