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doi:10.2204/iodp.proc.341.102.2014

Paleomagnetism

Paleomagnetism studies aboard the JOIDES Resolution during Expedition 341 comprised routine measurements of the natural remanent magnetization (NRM) of archive-half sections before and after alternating field (AF) demagnetization. Remanence measurements and AF demagnetizations were performed using a long-core superconducting rock magnetometer (SRM) (2-G Enterprises model 760-R). This instrument is equipped with a direct-current superconducting quantum interference device (DC-SQUID) and has an inline AF demagnetizer capable of reaching peak fields 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 can in some circumstances sense a magnetization over a core length of 30 cm. The noise level of the cryogenic magnetometer is ~2 × 10^–10 Am² or 2 × 10^–5 A/m for a ~100 cm³ split core sample. The practical noise level is, however, affected by the magnetization of the core liner (~2 × 10^–5 A/m) and by the background magnetization of a clean measurement tray (<5 × 10^–6 A/m). Using the software package SRM Section developed during IODP Expedition 321T, the tray was cleaned and measured as needed and the background magnetization subtracted from subsequent core measurements.

The remanent magnetization of archive halves of all core sections was measured unless precluded or made unreliable by drilling-related deformation. Measurements were made at intervals of 1, 2.5, or 5 cm with leader and trailer lengths of 15 cm. The measurement interval, number of demagnetization steps, and peak field used reflected the quality, the demagnetization characteristics of the recovered sediments, the severity of the drill string magnetic overprint, the desire to keep peak AFs low (usually 20 mT), and the need to maintain core flow through the laboratory. Higher AFs of up to 40 mT were occasionally used for sections recovered using the XCB and RCB systems. Using a track speed setting of 10 cm/s, one step of a three-axis AF demagnetization and subsequent section measurement at 2.5 cm intervals took ~5.5 min. For measurement at intervals of 2.5 cm without AF demagnetization, ~3 min was needed. If time allowed, a four-step demagnetization and measurement scheme with peak fields of 20 mT was deployed following NRM measurement. To speed up core flow, the 5 and 15 mT steps were often dropped. Low peak demagnetization fields ensure that archive halves remain useful for shore-based higher resolution U-channel or cube studies of magnetic remanence.

Measurements were undertaken using the standard IODP magnetic coordinate system (+x = vertical upward from the split surface of archive halves, +y = left-hand split surface when looking upcore, and +z = downcore). Data were stored using the standard IODP file format and manually checked for quality. Void depths and otherwise disturbed intervals were manually noted on the “cryomag log sheets” and later taken into account. Lonestones and other visible debris that might hinder paleomagnetic measurements were removed prior to SRM measurement (and replaced after) or accounted for when processing data.

During APC coring, full-length nonmagnetic, full-length steel, and half-length steel core barrels were used (see “Coring and drilling operations”). Orientation was attempted at one hole for each APC-cored site using the FlexIT orientation tool on selected APC cores beginning at Core 3H. This instrument uses three orthogonally mounted fluxgate magnetometers to record orientation with respect to magnetic north of the double lines scribed on the core liner. The tool also has three orthogonally mounted accelerometers that monitor the movement of the drill assembly and help determine when the most stable, and thus most useful, core orientation data are gathered. Declination, inclination, and temperature are recorded internally at regular intervals until the tool’s memory capacity is filled. Tools were switched every 8–12 h or when necessary. The azimuthal reference line is the double orientation line on the core liner and remains on the working half after the core is split.

Where shipboard AF demagnetization appears to have isolated the characteristic remanent magnetization (ChRM), paleomagnetic inclinations, and/or declinations of the highest demagnetization step, typically 20 mT was used to make an initial designation of magnetic polarity zones. The Astronomically Tuned Neogene Timescale (ATNTS2012) (Hilgen et al., 2012) within the Geological Timescale 2012 (Gradstein et al., 2012) was used as a reference for the ages of correlative Cenozoic polarity chrons.

The magnetic susceptibility of whole-core sections was measured on two separate core logging systems. Whole-core sections were measured on the STMSL to rapidly acquire magnetic susceptibility data for stratigraphic correlation (see “Stratigraphic correlation”). After whole cores were or were not warmed to room temperature, except for those in gassy sediments, measurements were made as part of the WRMSL (see “Physical properties”). Additionally, point-source magnetic susceptibility measurements were made on the archive halves of cores using the SHMSL. When time allowed, the partially demagnetized NRM intensities of selected core intervals were normalized by the WRMSL to assess the potential for deriving shore-based estimates of relative paleointensity of geomagnetic field.

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