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doi:10.2204/iodp.proc.345.102.2014

Paleomagnetism

During Expedition 345, routine shipboard paleomagnetism and magnetic anisotropy experiments were carried out. Remanent magnetization was measured on archive section halves and on discrete cube samples taken from the working halves. Continuous archive section halves were demagnetized in an alternating field (AF), whereas discrete samples were subjected to stepwise AF demagnetization, thermal demagnetization, or a combination of low-temperature demagnetization followed by either AF or thermal treatment. Because the azimuthal orientations of core samples recovered by rotary drilling are not constrained, all magnetic data are reported relative to the sample core coordinate system (Fig. F2). In this system, +x points into the working section half (i.e., toward the double line), +z is downcore, and +y is orthogonal to x and z in a right-hand sense.

Archive section half remanent magnetization data

Measurement and filtering

The remanent magnetization of archive section halves was measured at 2 cm intervals using the automated pass-through direct-current superconducting quantum interference device (DC-SQUID) cryogenic rock magnetometer (2G Enterprises model 760R). An integrated in-line AF demagnetizer (2G model 600) capable of applying peak fields up to 80 mT was used to progressively demagnetize the core. Demagnetization was conducted in 5 mT steps up to 80 mT, but data from demagnetizations above 30 mT were usually found to be contaminated by instrument-induced anhysteretic magnetization.

With strongly magnetized materials, the maximum intensity that can be reliably measured (i.e., with no residual flux counts) is limited by the slew rate of the sensors. At a track velocity of 2 cm/s, it is possible to measure archive section halves with a magnetization as high as ~10 A/m (Expedition 304/305 Scientists, 2006; Expedition 330 Scientists, 2011). Although the baseline values measured just prior to and just after the archive section half measurements are not saved in the database, the baseline drift, and thus the number of residual flux counts, can be determined indirectly from the archived directional data. We used LabView software developed by Jeff Gee (WebTabularToMag; Expedition 330 Scientists, 2011) to reconstruct the baseline drift, allowing the residual flux counts to be logged while converting the data for further processing.

The compiled version of the WebTabularToMag LabView software (SRM Section) used during Expedition 345 is SRM version 318. This incorporates two modifications to the program and the Galil motor system (Expedition 330 Scientists, 2011). First, the speed at which the archive section was moved when not measuring was increased to 20 cm/s. Second, simultaneous sampling of the magnetometer axes was incorporated into the magnetometer software. During Expedition 330, these changes together resulted in substantial time savings (on the order of 0.5 h per section with 6–8 demagnetization steps) and also allowed multiple measurements at each interval for weakly magnetized cores. Hence, these modifications were retained for Expedition 345.

The response functions of the pick-up coils of the SQUID sensors have a full width of 7–8 cm at half height (Parker and Gee, 2002). Therefore, data collected within ~4 cm of piece boundaries (or voids) are significantly affected by edge effects. Consequently, data points within 4.5 cm of piece boundaries (as documented in the curatorial record) were filtered out prior to further processing. To further reduce artifacts, any pieces smaller than 10 cm were removed from section trays prior to measuring/demagnetizing and replaced afterward.

Remanent magnetization directions were calculated for each 2 cm measurement using principal component analysis (PCA; Kirschvink, 1980). Note that the intensity reported for such PCA directions represents the length of the projection of the lowest and highest treatment vectors used in the PCA calculation onto the best-fit direction. Because the origin is not included in the PCA calculation and the remanence remaining after the highest treatment potentially may be significant, resulting characteristic remanent magnetization (ChRM) intensity values will be systematically lower than those derived from the remanence at the lowest demagnetization step adopted for the PCA calculation.

Discrete sample data

Measurement and instrumentation

All discrete samples taken from working-half cores for shipboard magnetic analysis were 8 cm³ cubes. Although standard 2.5 cm diameter minicores are more commonly used, cubic samples were preferred, as they should have a more precisely determined vertical reference (based on a saw cut perpendicular to the core length) than the minicores, where the arrow on the split-core face must then be transferred to the long axis of the sample.

Remanent magnetization of discrete samples was measured exclusively with the JR-6A spinner magnetometer following tests of the reliability of discrete measurements on the 2G superconducting rock magnetometer conducted during Expedition 335 that showed significant scatter in remanence directions measured in different sample orientations (Expedition 335 Scientists, 2012). For samples measured on the spinner magnetometer, the automated sample holder was used, providing the most accurate discrete sample remanent magnetization directions and intensities. Measurements of the empty automatic sample holder after subtracting the stored holder magnetization yielded intensities of 4.0 × 10^–6 A/m, representing the practical noise limit of the system.

Discrete samples were subjected to stepwise AF demagnetization using the DTech AF demagnetizer (model D-2000), which is capable of peak fields up to 200 mT. Fifteen AF demagnetization steps were used, with 5 mT steps up to 50 mT and 10 mT steps up to a maximum peak field of 100 mT. The residual magnetic field at the demagnetizing position in this equipment was ~25 nT.

Discrete samples were thermally demagnetized using an ASC Scientific thermal demagnetizer (model TD-48 SC) capable of demagnetizing samples up to 700°C. The total magnetic field along the length of the TD-48 SC access tube is illustrated in Figure F18, demonstrating that the sample chamber from 30 cm onward (measured from the edge of the access opening) has a maximum field of <50 nT. Each sample boat for thermal demagnetization included as many as 22 samples, and sample orientations were varied at alternative steps to allow any interaction between adjacent samples to be identified. Samples were held at the desired temperature for 40 min prior to cooling in the low-field chamber. Magnetic susceptibility was measured (using a Bartington MS2C magnetic susceptibility sensor) after every heating step to monitor thermal alteration of magnetic minerals during heating.

Discrete samples were subjected to low-temperature demagnetization (LTD) (Merrill, 1970; Dunlop, 2003; Yu et al., 2003) prior to subsequent AF or thermal demagnetization in order to remove substantial secondary drilling-related magnetizations. LTD involves cooling samples in a liquid nitrogen bath (T = 77K) and allowing them to warm back up to room temperature in a very low field environment. This cools the samples to below the Verwey transition of magnetite (Dunlop, 2003), resulting in a loss of magnetic remanence by multidomain grains upon subsequent warming to ambient temperature. This technique was employed in shore-based paleomagnetic analysis of discrete samples from gabbroic rock recovered from Atlantis Massif in IODP Hole U1309D (Morris et al., 2009) and successfully removed a large proportion of the drilling-related magnetization that is presumed to be carried by coarse, multidomain magnetite grains. During shipboard experiments, a suitable low-field environment was provided by nesting the two available cylindrical mu-metal shields to produce a six-layer shield with an internal field <10 nT (with shields aligned parallel to the ship orientation of 110°–120°). This was sufficiently low to allow LTD treatment to be performed successfully.

Anisotropy of low-field magnetic susceptibility

In addition to standard paleomagnetic measurements, the anisotropy of low-field magnetic susceptibility was determined for all discrete samples using the KLY 4S Kappabridge with the AMSSpin software (Gee et al., 2008). Each sample was measured three times to assess repeatability. 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 based on a sample volume of 8 cm³.

Inclination-only analysis

For azimuthally unoriented cores, the simple arithmetic mean of inclination data will be biased to shallower values (e.g., Kono, 1980; McFadden and Reid, 1982; Arason and Levi, 2010). To compensate for this bias, we have used the inclination-only statistics of Arason and Levi (2010) to calculate the overall mean inclination for the cored interval and appropriate subintervals.

Although the Arason and Levi technique is more robust than previous inclination-only methods (e.g., Kono, 1980; McFadden and Reid, 1982), this technique nonetheless fails to converge under certain circumstances. For example, if inclinations are steep and the scatter is substantial or if dual polarities (also with steep inclination) are present, no maximum likelihood estimate is possible. Hence, this method is unsuitable for analysis of steep drilling-induced magnetization.

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