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Paleomagnetic and rock magnetic investigations during Expeditions 304 and 305 were primarily designed to (1) determine the characteristic remanence directions for use in tectonic studies, (2) assess the orientation and significance of magnetic fabrics, and (3) evaluate whether the recovered materials constitute a suitable source for marine magnetic anomalies. To accomplish these goals, 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 unknown. All magnetic data are therefore reported relative to the following core coordinates: +x (north on plots) is into the face of the working half of the core, +y (east on plots) points toward the right side of the face of the working half, and +z is down (see Fig. F10).

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 inline alternating-field (AF) demagnetizer (2G model 2G600) that can apply peak fields as high as 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). All remanence data during Expedition 304 were corrected for baseline drift. During Expedition 305, this option in the Core Logic program was inadvertently deselected and, therefore, most remanence measurements were not corrected for baseline drift.

With strongly magnetized materials, the maximum intensity that can be reliably measured is limited by the slow rate of the sensors (i.e., the number of flux counts must return to zero after the measurement). At the slowest track velocity (1 cm/s), it was possible to measure archive halves with a magnetization as high as ~10 A/m. Where even this slowest measurement speed still resulted in residual counts, the data were nonetheless archived because they provide some indication of the magnetization. During Expedition 304, archive halves were measured at variable track velocities (typically 1 cm/s for the initial steps and increased velocities up to 10 cm/s after the remanence had been sufficiently demagnetized). During Expedition 305, archive halves were measured at a constant velocity of 2 cm/s. Although both methods minimize the number of residual flux counts, some core sections had residual flux counts. All data were nonetheless archived because they provide some indication of the magnetization. Some caution is therefore warranted in using the archive half core data from strongly magnetized intervals.

The response functions of the SQUID sensors have a full width of ~10 cm at half height so that data within 5 cm of piece boundaries or voids are significantly affected by edge effects. To minimize such spurious data, during Expedition 304 we did not perform measurements within 3–4 cm of a piece end. The data from Expedition 305 included some measurements near piece ends that were removed during reprocessing of the data at the postcruise meeting. Although this approach means that no data are collected for pieces smaller than ~7 cm, the time saving allows more detailed measurement or more demagnetization steps elsewhere. Archive halves were typically measured at an interval of 2 cm. This sampling interval coincides with that used on the MST (see “Physical properties”) so that susceptibility and remanence data may easily be compared. A standard 2.5 cm diameter minicore sample or a ~9 cm3 cube (physical property samples) 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 also measured using the 2G SQUID magnetometer. Because remanent intensities may vary by several orders of magnitude, all discrete samples were spaced 34 cm apart to effectively eliminate any contamination of one measurement from the signal of a neighboring sample. As noted in the Leg 209 “Explanatory Notes” chapter (Shipboard Scientific Party, 2004), the SQUID sensors and sample tray are not perfectly aligned. A variety of discrete samples were measured in multiple positions to establish the angular difference between these coordinate systems, following the procedures described in the Leg 209 Initial Reports volume (Shipboard Scientific Party, 2004). A counterclockwise rotation of ~7° about the z-axis produced a small improvement in the clustering of remanence directions of discrete samples measured in three positions (such that the magnetization component parallel to each of the sample coordinate axes was measured once with each SQUID sensor). However, we elected not to apply this correction for the following reasons: (1) the mean direction of magnetization of the discrete samples calculated from the three-position data was not significantly different from that obtained prior to rotation of the tray, (2) declinations of archive half data are unconstrained and are therefore not affected by rotation about the z-axis, and (3) no directional standards were available to directly test the accuracy of the above rotation. The three-position measurement scheme effectively corrects for the angular difference in SQUID sensor and sample tray coordinate systems and was therefore adopted for all thermally demagnetized and most AF-demagnetized discrete samples during Expedition 304. Many AF-demagnetized samples during Expedition 305 were also measured using this protocol.

Discrete samples were subjected to stepwise AF demagnetization using a DTech (model D-2000) AF demagnetizer capable of peak fields up to 180 mT. At peak fields >40 mT, samples were typically demagnetized and measured twice (once after demagnetization along the sample +x-, +y-, and +z-directions and again after demagnetization along the –x-, –y-, and –z-directions) to identify and compensate for any bias field in the demagnetizing coil. Many samples, particularly during Expedition 305, were 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 (MS) was determined for most discrete samples using the Kappabridge KLY-2 (Geofyzika Brno) and a 15-position measuring scheme. The susceptibility tensor and associated eigenvectors and eigenvalues were calculated offline following the method of Hext (1963). All bulk susceptibility values (as well as remanent intensities) reported for discrete samples from Expedition 304 have been corrected for the true cylindrical or cubic sample volume. During Expedition 305, remanent magnetization and volume susceptibilities are based on a nominal core volume of 10 cm3. For a small number of samples, the anisotropy of anhysteretic remanent magnetization (ARM) was also determined. For each of the sample axial directions (i.e., +x, +y, and +z and –x, –y, and –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.