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doi:10.2204/iodp.proc.339.102.2013

Paleomagnetism

Samples, instruments, and measurements

Paleomagnetic studies during Expedition 339 focused mainly on measuring the natural remanent magnetization (NRM) of archive-half sections before and after alternating field (AF) demagnetization. We typically collected one discrete sample per section from working-half sections for use in full AF demagnetization experiments. Discrete samples were taken from the first deep hole cored at a site (typically Hole A) and from the rotary-drilled holes that recovered the deeper parts of the cored sections. The samples were collected by inserting a hollow extruder into the middle of the split-core sections and then extruding the sediments into plastic cubes (2 cm × 2 cm × 2 cm, with an internal volume of ~7 cm³) as described in Richter et al. (2007).

All remanence measurements 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 spatial resolution measured by the width at half-height of the pickup coils response is <10 cm for all three axes, although they sense a 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.

NRM measurements of the archive-half core sections were made at 5 cm interval resolution 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. These 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 a 20 mT peak field AF demagnetization. In a few instances, when time permitted, we also measured the NRM after a 10 mT AF demagnetization step. Because core flow (the analysis of one core after the other) through the laboratory dictates the available time for measurements, which was ~2–3 h per core for Expedition 339, we did not always have time for the optimal number of demagnetization steps.

During Expedition 339, a number 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. This was done to determine whether a characteristic remanent magnetization (ChRM) could be resolved and, if so, what level of demagnetization was required to resolve it.

Low-field magnetic susceptibility was measured on whole-core sections using the Special Task Multisensor Logger (STMSL) and WRMSL, and on archive-half core sections using the SHMSL (see “Physical properties” and “Stratigraphic correlation”). The WRMSL and STMSL are equipped with a Bartington Instruments MS2C sensor with an internal diameter of 80 mm, which corresponds to a coil diameter of 88 mm. The sensor has 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 should be 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 is typically measured every 2.5 cm for the whole-core sections and every 2–5 cm for the split-core sections.

Coring and core orientation

Cores were collected using nonmagnetic core barrels, except at depths where overpull tension 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 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 procedure for the FlexIt tool is 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 recorded are those used to constrain the core orientation. When the APC fires, the core barrel is assumed to maintain the same orientation, although this and past expeditions have found evidence that the core barrel can rotate and/or the core liner can twist as it penetrates the sediments. Generally, the core barrel is pulled out after a few minutes except for cores collected with the advanced piston corer temperature tool (APCT-3) (see “Downhole measurements”), for which the core barrel remains in the sediments for ≥10 min.

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. Inclination and declination of NRM after 20 mT peak field AF demagnetization were used for the determination of magnetozones during Expedition 339. For cores with FlexIt tool data, declinations were first corrected for core orientation. Magnetostratigraphy for each site was then constructed by correlating the magnetozones with the GPTS. Biostratigraphic age constraints significantly limit the range of possible correlations of the magnetozones with the GPTS. The GPTS used during Expedition 339 is based upon the Neogene timescale of Lourens et al. (2004).

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