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doi:10.2204/iodp.proc.342.102.2014

Paleomagnetism

Samples, instruments, and measurements

Paleomagnetic investigations during Expedition 342 focused mainly on measuring the natural remanent magnetization (NRM) of archive section halves before and after alternating field (AF) demagnetization. We also collected one discrete sample per section from most Hole A working section halves for use in bulk magnetic susceptibility, anisotropy of magnetic susceptibility (AMS), and AF demagnetization experiments. Discrete samples were taken in plastic Natsuhara-Giken sampling cubes (7 cm³ sample volume) (Fig. F6A, F6B). Cubes were pushed into the working half of the core by hand with the “up” arrow on the cube pointing upsection. For indurated intervals, cubes were cut with a table saw and trimmed to fit in the plastic containers. The coordinate system of the discrete samples follows the right-hand rule and is oriented so that the cube is pushed toward the double lines on the core liner and the +z-axis points downsection.

Remanence measurements of archive section halves were made using a 2G Enterprises Model-760R superconducting rock magnetometer (SRM) equipped with direct-current superconducting quantum interference devices (SQUIDs) and an in-line, automated AF demagnetizer capable of reaching a peak field of 80 mT. The coordinate system for the SRM is shown in Figure F6C. Prior to every working shift (i.e., every ~12 h), we washed the length of the sample track with antistatic solution and the top and bottom of the sample tray with isopropyl alcohol. The magnetization of the sample tray was measured at the beginning of every shift to monitor any changes to the sample tray during the course of the expedition and to maintain accurate tray correction values. Every section half was wrapped in plastic wrap prior to measurement. Finally, we regularly checked the tuning of the SQUIDs and made adjustments when necessary. This procedure ensured that the magnetometer was kept as clean and noise free as laboratory conditions permitted.

We measured NRM every 2.5 cm along the archive section halves after 0 and 20 mT AF demagnetization. Additional intermediate steps were deemed unnecessary for the goals of shipboard analysis. We did not measure sections that were entirely visibly disturbed. Similarly, when analyzing SRM data we removed measurements from within 7.5 cm of section ends and within intervals with drilling-related core disturbance as described by the lithostratigraphers.

Remanence of the discrete samples was measured on an AGICO JR-6A spinner magnetometer. Discrete samples were placed into the “automatic holder” of the JR-6A as shown in Figure F7. Following measurement, the JR-6A data were transformed from the instrument coordinate system to the core coordinate system. AF demagnetization of discrete samples was made with an ASC D-2000 AF demagnetizer. AF demagnetization of these samples was usually conducted at a peak field of 20 mT. Some selected discrete samples were step-wise demagnetized up to 60 or 80 mT if core flow permitted and if full demagnetization was necessary for directional interpretations.

AMS measurements were made on an AGICO KLY 4S Kappabridge instrument using the AMSSpin LabVIEW program designed by Gee et al. (2008) and adapted for use with the shipboard KLY 4S. The KLY 4S Kappabridge measures anisotropy of magnetic susceptibility by rotating the sample along three axes, stacking the data, and calculating the best-fit second-order tensor. It also measures the volume-normalized, calibrated bulk susceptibility (χ). Tensor elements were converted to eigenparameters (eigenvectors V₁, V₂, and V₃ with associated eigenvalues τ₁, τ₂, and τ₃, in which τ₁ is the maximum and τ₃ is the minimum [terminology of Tauxe, 2010]). These parameters can be interpreted in terms of particle alignment within the sample (Fig. F8). Normal sedimentary fabrics are oblate with vertical axes of minimum susceptibility (e.g., Fig. F8B). Disturbance by slumping or other deformation generally yields triaxial fabrics (e.g., Fig. F8D). AMS data were analyzed using the Pmag.Py Python software package (Tauxe, 2010; freely available at magician.ucsd.edu/Software/PmagPy).

Low-field magnetic susceptibility was measured on all whole-core sections using the WRMSL and the Special Task Multisensor Logger (STMSL) (see “Physical properties”). Magnetic susceptibility was measured with a Bartington loop meter (Model MS2 with an MS2C sensor. The MS2C meter in the STMSL had a coil with an internal diameter of 90 mm and an operating frequency of 565 Hz. The modified MS2C in the WRMSL had a coil with an internal diameter of 80 mm and an operating frequency of 513 Hz. The “units” option for the meters was set to SI units, and the values were stored in the database and are reported in raw instrument units. To convert to approximate SI volume susceptibilities for the 80 mm loop, instrument units are multiplied by ~0.578 × 10^–5 (1/K_REL; K_REL = 3.45 × (d/D)³; d = core diameter [~70 mm] and D = loop sensor diameter + 8 mm [88 mm]). For the 90 mm loop, the factor changes accordingly. This conversion factor may vary downhole with changes in core diameter and core volume.

Paleomagnetic data collected on the pass-through SRM were uploaded to the LIMS database. Data from discrete samples are tabulated in each site chapter.

Coring and core orientation

Cores were collected using nonmagnetic core barrels and cutting shoes. The FlexIT core orientation tool was used with all APC deployments except when partial strokes were experienced because of hard stratigraphic layers (i.e., cherts) or when core liners repeatedly failed during recovery. The orientation tool uses three orthogonally mounted fluxgate magnetometers to record the orientation of the double lines scribed on the core liner with respect to magnetic north. The tool also has three orthogonally mounted accelerometers to monitor the movement of the drill assembly, which helps determine when the most stable and thus most robust core orientation data were gathered. The tool declination, inclination, total magnetic field, and temperature are recorded at regular intervals until the internal memory of the tool is filled to capacity. These data are downloaded, processed, and uploaded to the LIMS database on the ship by shipboard support staff. Core orientation tool deployment and core orientation data are also indicated in our site reports.

In general, the quality of magnetic data from APC-recovered intervals is much higher than data from XCB-recovered cores. For example, magnetic directions and intensity are substantially less noisy within and between cores in a single hole and more coherent between holes at a given site for APC-recovered intervals than they are from XCB intervals. XCB-recovered intervals often, but not always, have higher magnetic intensities attributable to more pervasive magnetic overprinting during the coring process. These intervals are frequently disturbed by “biscuiting,” which is characterized by intervals of coherent or relatively undisturbed stratigraphy that are a few to tens of centimeters long and are separated by hydrated clay and pulverized rock intervals as thick as several centimeters. The clay layers between biscuits are especially prone to remagnetization. Moreover, rotation (primarily on the vertical axis but random for biscuits smaller than the diameter of the core liner) between biscuits effectively randomizes downhole magnetic declination through these intervals. Therefore, our general shipboard strategy is to interpret only the magnetostratigraphy from APC-recovered intervals; we occasionally interpret the magnetostratigraphy from XCB-recovered intervals when the magnetic data are unusually good and unambiguous.

Magnetostratigraphy

Expedition 342 drill sites are located at ~40°N; therefore, typical magnetic polarity zones can be easily identified by distinct changes in inclination. The present-day normal field in this region, as calculated from the geocentric axial dipole (GAD), has a positive inclination of ~59°. Negative inclination values, therefore, indicate reversed field polarity. Magnetozones identified from the shipboard data were correlated to the GPTS with the aid of biostratigraphic datums.

We used the GTS2012 timescale, which is summarized in Table T7 and Figure F5 (see “Biostratigraphy”). In GTS2012, boundary ages for Chrons C1n–C13n and C24n.1n–C34n are orbitally tuned, whereas those for Chrons C13r–C23r are spline fitted. Cryptochrons not described in Gradstein et al. (2012) are referred from Cande and Kent (1995) and are described in the Remarks column in Table T7. Cryptochron C18n.1n-1, found in sediments from Site U1333 (Expedition 320/321 Scientists, 2010), is also noted in this table.

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