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doi:10.2204/iodp.proc.335.102.2012

Paleomagnetism

During Expedition 335, routine shipboard paleomagnetic 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 either stepwise AF demagnetization, thermal demagnetization, or a combination of low-temperature demagnetization followed by either AF or thermal treatment. Given the low recovery during Expedition 335, experiments were also conducted on a suite of discrete samples cut from gabbroic rocks recovered during Expedition 312. 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. 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 (Fig. F2).

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 inline 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 60 mT were found to be contaminated by instrument-induced anhysteretic magnetization and were discarded.

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, 2012). 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, 2012) 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 LabView software (SRM_SAMPLE) used during Expedition 335 is SRM version 318. This incorporates two modifications to the program and the Galil motor system (Expedition 330 Scientists, 2012). First, the speed at which the archive section was moved when not measuring has been increased to 20 cm/s. Second, simultaneous sampling of the magnetometer axes has been 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 have been retained for Expedition 335.

The response functions of the pick-up coils of the DC-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 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 may be significant, resulting 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 fiducial arrow on the split-core face must then be transferred to the long axis of the sample. For core sections recovered during Expedition 335, discrete samples for shipboard analysis were severely restricted by the limited number of oriented core pieces (i.e., those where the way up of the piece is known unambiguously). Samples from the legacy working-half cores from Expedition 312 were likewise limited by the degree of sample depletion of oriented pieces resulting from extensive postcruise sampling by the scientific community.

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 (SRM) that showed significant scatter in remanence directions measured in different sample orientations (see “Reliability of discrete sample measurements using the 2G superconducting rock magnetometer”). 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 on the order 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), capable of peak fields up to 200 mT. Fourteen AF demagnetization steps were used, with 5 mT steps up to 40 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 a Schonstedt Thermal Demagnetizer (model TSD-1), capable of demagnetizing samples up to 700°C. The total magnetic field along the length of the TSD-1 access tube is illustrated in Figure F17, demonstrating that the sample cooling chamber between 68 and 106 cm (measured from the outside of the access door) has a maximum field of <20 nT. Each sample boat for thermal demagnetization included as many as nine 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 MS2F point magnetic susceptibility meter) after every heating step to monitor thermal alteration of magnetic minerals during heating.

A subset of discrete samples was subjected to low-temperature demagnetization (Merrill, 1970; Dunlop, 2003; Yu et al., 2003) prior to subsequent AF or thermal demagnetization in order to test the efficiency of low-temperature demagnetization in removing substantial secondary drilling-related magnetizations. Low-temperature demagnetization involves cooling samples in a liquid nitrogen bath (T = 77°K) 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 rocks 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 a three-layer cylindrical mu-metal shield with removable end caps. Precise measurement of the field inside the shield was difficult to achieve because the cable from the three-axis fluxgate magnetometer probe prevents the end caps from being fitted tightly to the shield. Nevertheless, holding the three end caps in place manually indicated that the internal field is <25 nT (with a ship orientation of 230° and shield long-axis orientation of ~320°). This was sufficiently low to allow the low-temperature demagnetization treatment to be performed successfully.

Reliability of discrete sample measurements using the 2G superconducting rock magnetometer

The large-aperture 2G SRM is designed principally to measure the magnetization of whole-core and section half samples. Measurements of discrete samples on the SRM are known to be affected by factors such as the alignment of the DC-SQUID sensors and discrete sample tray (Shipboard Scientific Party, 2004) and inhomogeneity in sample shape or in the distribution of remanence carriers. Uncertainties in the reliability of SRM discrete sample measurements were highlighted during Expedition 330 (Expedition 330 Scientists, 2012) using a suite of reference samples previously analyzed at the Scripps Institution of Oceanography. Here we further assess the reliability of discrete measurements made in different SRM sample positions across a range of intensities of magnetization. The SRM discrete measurement software allows all 24 possible orientations of a cubic sample (relative to the DC-SQUID axes) to be measured in turn. We used this capability to repeatedly measure anhysteretic remanent magnetizations (ARMs) imparted in different directions on a set of seven previously demagnetized samples (using AF demagnetization to stepwise decrease ARM intensities between four measurement runs). At each intensity level, samples were measured in the full set of 24 SRM positions. To provide an independent determination of the direction and intensity of remanence at each level, samples were also measured using the AGICO JR-6A spinner magnetometer using the automatic three-position holder and high (87.7 rev/s) rotation speed. Repeat measurements with this instrument showed <1° directional variability and <1% intensity variability. In addition, the practical noise level of the JR-6A during shipboard operations (4.0 × 10^–6 A/m) was at least one order of magnitude lower than that of the SRM (4.4 × 10^–5 to 9.6 × 10^–5 A/m). Hence the JR-6A determinations provide suitable reference directions to compare with the SRM measurements.

Results for six samples are shown in the equal area stereographic projections in Figure F18 (the seventh sample was magnetized along its z-axis and is omitted for clarity). The variation of circular standard deviation (CSD) with remanence intensity for all samples is shown in Figure F19. In order to allow visual comparison of the resulting distributions at each remanence level, SRM data (black symbols in Fig. F18) have been rotated around a vertical axis to align the JR-6A reference directions with azimuths of 045°, 135°, 225°, and 315° in decreasing order of remanence intensity (red triangles in Fig. F18). At all intensity levels there is considerable scatter in remanence directions and intensities across the 24 measuring positions. Even when intensities are of the order of 1 A/m, averaging of data from multiple positions is required to yield directions of magnetization within 2° of the JR-6A reference values for all samples. Orientation-related scatter increases dramatically with decreasing intensity, becoming extreme at intensities < 20 mA/m (with CSD values >10°, reaching a maximum of 56°). Such intensities are routinely encountered at high treatment levels during demagnetization experiments (and as natural remanent magnetization intensities in sedimentary rocks). Given the uncertainties in determining remanence directions demonstrated by this experiment and the need for multiple measurements to provide meaningful results even at high intensities, it is clearly preferable to use the AGICO JR-6A system for discrete sample analyses.

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 software AMSSpin (Gee et al., 2008). The susceptibility tensor and associated eigenvectors and eigenvalues were calculated offline 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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