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doi:10.2204/iodp.proc.307.102.2006

Paleomagnetism

Paleomagnetic studies aboard the JOIDES Resolution during Expedition 307 comprised 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 (k) measurements made on whole cores. The paleomagnetic measurements at Site U1317 were carried out on the whole-round sections. Tests were carried out on all sections from Hole U1316B to evaluate the feasibility of NRM measurement on whole-round sections. Freezing and splitting (see “Introduction”) can affect the measurements on archive-half sections, especially in carbonate-rich sediments where NRM is very weak. Moreover, by measuring on whole-round sections, the same volume is measured as for the susceptibility measurements, which is an advantage for calibration of paleointensities.

Remanence measurements and AF demagnetizations were performed using a long-core cryogenic magnetometer (2G 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 sense a magnetization over a core length up to 30 cm. The magnetic moment noise level of the cryogenic magnetometer is ~10^–9 emu or 10^–6 A/m for 10 cm³ rock volume. The practical noise level, however, is affected by the magnetization of the core liner (~8 × 10^–6 A/m) and the background magnetization of the measurement tray (~1 × 10^–5 A/m).

The NRM of archive halves or whole-round sections 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 10 cm above and ending at 10 cm below the base. 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. Measurements without AF demagnetization require only 3 min. Following the NRM measurement, a two-step demagnetization was employed (~13 min) using peak fields of 10 and 15 mT or 15 and 20 mT. Low peak demagnetization fields ensure that the core sections 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 split surface when looking upcore, and +z = downcore). Data were stored using the standard IODP file format. 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.

Full orientation was attempted using the Tensor orientation tool. 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 appears to have isolated the characteristic remanent magnetization, paleomagnetic inclinations and/or declinations of the highest demagnetization step (typically 15 or 20 mT) were used to make an initial designation of magnetic polarity zones. The revised timescale of Cande and Kent (1995) was used as a reference for the ages of correlative polarity chrons.

The magnetic susceptibility of whole-round sections was measured on two separate track systems. Whole-core sections were measured on the Fast Track MSCL system to rapidly acquire magnetic susceptibility data for stratigraphic correlation. After whole cores warmed to room temperature, magnetic susceptibility measurements were made as part of MST analyses (see “Physical properties”). When time allowed, the partially demagnetized NRM intensity of selected core intervals was normalized by the MST whole-core magnetic susceptibility to assess the potential for deriving estimates of relative geomagnetic field paleointensity.

Discrete samples were collected from working halves in standard ~7 cm³ plastic cubes with orientation marks on the bottom of the sampling box pointing upcore. Intervals of coring- or drilling-related core deformation were avoided.

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