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doi:10.2204/iodp.proc.346.102.2015

Paleomagnetism

Paleomagnetic samples and measurements

Paleomagnetic studies during Expedition 346 focused on measuring the natural remanent magnetization (NRM) of archive-half sections. NRM of archive-half sections of the first or the longest hole at each site were measured before and after alternating field (AF) demagnetization with a 20 mT peak field at every 5 cm interval resolution. Because of increased core flow and limited measurement time at the paleomagnetism station, NRM of archive-half sections of the other holes at each site were usually measured only after 20 mT AF demagnetization at every 5 cm interval resolution.

We typically collected one oriented discrete sample per core from working-half sections for measuring NRM after complete stepwise AF demagnetization, bulk magnetic susceptibility, and anisotropy of magnetic susceptibility (as time permitted). Discrete samples were usually taken from Hole A at each site and from the lower part of the stratigraphic section of the deepest hole if the deepest hole is different from Hole A. Discrete samples were extracted by pushing plastic Natsuhara-Giken cubes (with 2 cm external edge length and an internal volume of ~7 cm³) into working-half sections with the arrow marker on the cube pointing to the upward direction of the core (Fig. F12A). The sample x-axis is toward the double lines on the working-half sections and the up arrow marks the negative z-axis (Fig. F12C). To avoid potential acquisition of anhysteretic remanent magnetization (ARM) during AF demagnetization, which was observed in discrete sample measurements especially at high AF peak fields (see “Paleomagnetism” in the “Site U1422” chapter and “Paleomagnetism” in the “Site U1423” chapter [Tada et al., 2015a, 2015b]), we followed the Expedition 318 Scientists (2011) protocol to demagnetize and measure the discrete samples multiple times at each demagnetization level for discrete samples from some of the sites when time allows. At each demagnetization step, discrete samples were first demagnetized along all three axes with the samples placed in the “top-toward” orientation (Fig. F12D) followed by measurement. The samples were then demagnetized along all three axes at the same level with the samples placed in the “top-right” orientation followed by measurement. Finally, the samples were demagnetized again at the same level with the samples placed in the “away-up” orientation followed by measurement. For each demagnetization step, averaging the measurements acquired with the samples placed and demagnetized at the three orthogonal orientations should cancel any ARM acquired during AF demagnetization. When there was no time for this protocol, the NRM of the discrete samples was measured with the sample placed in the top-toward or “+z-axis toward magnetometer” orientation on the discrete sample tray.

Remanence measurements for all half-core sections and a majority of the discrete samples were made using a 2G Enterprises Model-760R superconducting rock magnetometer (SRM) equipped with direct-current superconducting quantum interference devices (DC-SQUIDs) and an in-line, automated AF demagnetizer capable of reaching a peak field of 80 mT. The coordinate system used for the SRM is shown in Figure F12C and F12D. The spatial resolution measured by the width at half-height of the pickup coils response is <10 cm for all three axes, although they integrate magnetization over a core length up to 30 cm. The magnetic moment noise level of the cryogenic magnetometer is ~2 × 10^–10 Am². The practical noise level, however, is affected by the magnetization of the core liner and the background magnetization of the measurement tray, resulting in magnetizations of ~2 × 10^–5 A/m that can be reliably measured.

At the beginning of every working shift (approximately every 12 h), we physically cleaned the sample tray with isopropyl alcohol and wiped the backside of the sample tray with antistatic solution. The sample tray was then AF demagnetized with a peak field of 80 mT, followed by remanence measurement to monitor any changes in the sample tray during the course of the expedition and to maintain accurate tray correction values.

NRM measurements of the archive-half core sections were made every 5 cm along the split-core sections, as well as over a 15 cm interval before the sample passed the center of the pick-up coils of the SQUID sensors and a 15 cm interval after the samples had passed through it. Data collected from the two 15 cm intervals beyond the sample are referred to as the leader and trailer measurements and serve the dual function of monitoring the background magnetic moment and allowing for future deconvolution analysis. Typically, we measured NRM before any demagnetization and after AF demagnetization with a peak field of 20 mT. Because core flow (the analysis of one core after the other) through the laboratory dictates the available time for measurements, which was ~2 h per core for Expedition 346, we did not always have time for the optimal number of demagnetization steps.

A suite of discrete samples, selected to characterize typical intervals or to help determine poorly resolved magnetostratigraphy, were subjected to progressive AF demagnetization and measured at 5 mT steps to a peak field of 60 mT and then 10 mT steps to 80 mT when time permitted. This was done to determine whether a characteristic remanent magnetization could be resolved and, if so, what level of demagnetization was required to resolve it. When core flow allowed, we used the SRM with in-line AF demagnetizer to measure NRM of discrete samples before and after AF demagnetization. Discrete samples were placed on a discrete sample tray with each sample separating the adjacent ones by 20 cm to avoid convolution effects of the SRM sensor responses. During busy core flow, we used an ASC D-2000 AF demagnetizer together with an AGICO JR-6A spinner magnetometer (Fig. F13A, F13B) for remanence measurement of discrete samples before and after AF demagnetization. During measurement, discrete samples were placed in the “automatic folder” of the JR-6A as shown in Figure F13C. Following measurement, the JR-6A data were transformed from the instrument coordinate system to the core coordinate system.

Remanence measurement for the archive-half sections and discrete samples were saved as SRM and DSC files and uploaded to the shipboard LIMS database. To process the shipboard paleomagnetic measurement data, we export formatted data files from both Web Tabular Reports and LIMS Reports. During Expedition 346, we used a modified version of the UPmag program (Xuan and Channell, 2009) to process the shipboard paleomagnetic measurement data and plot and compare downcore variations of various paleomagnetic parameters, as well as to analyze demagnetization data on orthogonal and equal area projections.

During Expedition 346, low-field magnetic susceptibility was measured on whole-core sections using the STMSL and the WRMSL and was measured on archive-half core sections using the SHMSL (see “Physical properties”). The WRMSL and STMSL are equipped with Bartington Instruments MS2C sensors with an internal diameter of 80 mm, which corresponds to a coil diameter of 88 mm. The sensors have a nominal resolution of ~2 × 10^–6 SI (Blum, 1997). The “units” option for the meters was set on SI units, and the values were stored in the database in raw meter units. To convert to true SI volume susceptibilities, these raw units were multiplied by ~0.68 × 10^–5 (Blum, 1997). The SHMSL is equipped with a Bartington Instruments MS2E point sensor that measures the susceptibility of an integrated volume of approximately 10.5 mm × 3.8 mm × 4 mm, where 10.5 mm is the length perpendicular to the core axis, 3.8 mm is the width in the core axis, and 4 mm is the depth. Magnetic susceptibility was typically measured every 2.5, 5, or 10 cm for the whole-core sections and every 1 or 2 cm for the split-core sections. We multiply the SHMSL acquired susceptibility stored in the database by a factor of (67/80) × 10^–5 to empirically convert the measurement to SI volume susceptibilities.

Coring and core orientation

Cores were collected using nonmagnetic core barrels, except at depths where overpull weight was large enough to cause damage to the more expensive nonmagnetic core barrel. In addition, the BHA included a Monel (nonmagnetic) drill collar when the FlexIT core orientation tool was used. The FlexIT tool, which was used for the first hole at each site that succeeded in taking mudline (typically Hole A), 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 and help determine when the most stable, and thus useful, core orientation data were gathered. The tool declination, inclination, total magnetic field, and temperature are recorded internally at regular intervals until the tool’s memory capacity is filled. For a measurement interval of 6 s, which is what we used, the tool can typically be run for ~24 h, although we aimed to switch tools at least every 8–12 h.

Standard operating procedures for the FlexIT tool are described in the IODP “Core Orientation Standard Operating Procedure” manual (available from the IODP Cumulus database iodp.tamu.edu/tasapps/). This involves synchronizing the instrument to a PC running the FlexIT software and inserting the tool inside a pressure casing. The enclosed tool is then installed on the sinker bars that reside above the core barrel. The double lines on the core liner are aligned relative to the tool. Prior to firing the APC, the core barrel is held stationary (along with the pipe and BHA) for several minutes. During this time, the data are recorded to constrain the core orientation. When the APC fires, the core barrel is assumed to maintain the same orientation, although this and past cruises have found evidence that the core barrel can rotate and/or the core liner can twist as it penetrates the sediment.

Magnetostratigraphy

Magnetic polarity zones (magnetozones) were assigned based on changes in inclinations, as well as distinct ~180° alternations in declination that occur along each stratigraphic section. Sediment disturbance because of coring or geological processes (slumping, faulting, etc.) often leads to distorted and unreliable paleomagnetic directional records and largely alters the sediment fabric. We examined high-resolution shipboard core photographs including both LSIMG (core sections) and COREPHOTO (core composites) from LIMS Reports for each core section (see “Lithostratigraphy”) to mark disturbed intervals and avoided using paleomagnetic data from those intervals for magnetostratigraphic interpretations. Once a polarity stratigraphy was established for a given hole, we correlated the pattern to the GPTS. This was done in close collaboration with the shipboard biostratigraphy team.

During Expedition 346, we used inclination and declination of NRM after 20 mT AF demagnetization for the determination of magnetozones. For cores with FlexIT tool data, declinations were first corrected for core orientation. Magnetostratigraphy for each site was constructed by correlating the magnetozones with the most recent GPTS (Gradstein et al., 2012) summarized in Table T11 and Figure F14. Polarity boundary ages for Chrons C1n–C13n in the Gradstein et al. (2012) timescale are orbitally tuned. We followed the chron terminology of Gradstein et al. (2012) listed in Table T11.

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