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doi:10.2204/iodp.proc.303306.102.2006

Paleomagnetism

Paleomagnetic studies aboard the JOIDES Resolution during Expedition 303 were composed of routine measurements of the natural remanent magnetization (NRM) of archive-half sections before and after alternating-field (AF) demagnetization and low-field volumetric magnetic susceptibility measurements made on whole cores.

Remanence measurements and AF demagnetizations were performed using a long-core cryogenic magnetometer (2G Enterprises model 760-R). This instrument is equipped with direct-current superconducting quantum interference devices (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 sense a magnetization over a core length up to 30 cm. The magnetic moment noise level of the cryogenic magnetometer is ~10^–4 Am². The practical noise level, however, is affected by the magnetization of the core liner and the background magnetization of the measurement tray, and magnetizations of ~5 × 10^–5 A/m can be reliably measured.

The remanent magnetization of archive halves of all core sections was measured unless precluded or made worthless by drilling-related deformation. Measurements were made at intervals of 5 cm starting at 15 cm above and ending at 15 cm below the base of each section. The number of demagnetization steps and the peak field used reflected the demagnetization characteristics of the sediments, the severity of the drill string magnetic overprint, the desire not to exceed peak fields of 20 mT shipboard, and the need to maintain core flow through the laboratory. One step of three axes AF demagnetization and subsequent section measurement at 5 cm intervals takes ~5 min. Only 3 min are needed for measurement without AF demagnetization. Following NRM measurement, one-step demagnetization and measurement using peak fields of 10 or 20 mT was typically employed (~8 min). If time allowed or the drill string magnetic overprint was not removed, additional steps were added up to 20 mT. Low peak demagnetization peak fields ensure that archive halves remain useful for shore-based high-resolution (U-channel) 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. Data were manually checked for quality and the auto-save option was not used. The sample interval was set to 5 cm, leader and trailer lengths were both set to 15 cm, and no drift correction was applied. The program options for skipping voids and gaps at the top of the section were not used. Instead, void depths and otherwise disturbed intervals were manually noted on the “cryomag log sheets” and later taken into account. All sections were measured using an internal diameter setting of 6.5 cm. Background tray magnetization was measured at the beginning of each shift and subtracted from all measurements.

During APC coring, a nonmagnetic “monel” core barrel was used for all but some overdrilled parts of the section (see “Drilling operations” in “Introduction”). The low levels of drilling-related magnetic overprint can be attributed to the use of nonmagnetic core barrels. Full orientation was attempted using the Tensor (orientation) tool beginning at Core 3 of all holes. The Tensor tool is rigidly mounted onto a nonmagnetic sinker bar attached to the top of the core-barrel assembly. The Tensor tool consists of three mutually perpendicular magnetic-field fluxgate sensors and two perpendicular gravity sensors. The information from both sets of sensors allows the azimuth and dip of the hole to be measured as well as azimuth of the APC core. 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 the shipboard AF demagnetization appeared to have isolated the characteristic remanent magnetization, paleomagnetic inclinations and/or declinations of the highest demagnetization step (typically 10 or 20 mT) were used to make an initial designation of magnetic polarity zones. The timescale of Cande and Kent (1995) with updated age estimates for the Cobb Mountain (Channell et al., 2002) and Reunion (Channell et al., 2003) Subchrons was used to construct age models.

The magnetic susceptibility of whole-core sections was measured on two separate track systems. Whole-core sections were measured on a Fast Track system used to rapidly acquire magnetic susceptibility data for stratigraphic correlation (see “Composite section”). After whole cores warmed to room temperature, magnetic susceptibility measurements were made as part of the MST analyses (see “Physical properties”).

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