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doi:10.2204/iodp.proc.303306.110.2006

Paleomagnetism

Paleomagnetic analyses during Expedition 306 consisted of long-core measurements of the natural remanent magnetization (NRM) of archive-half sections before and after alternating-field (AF) demagnetization. In addition, low-field volume magnetic susceptibility was measured on whole cores using the MSCL and MST devices.

NRM measurements and AF demagnetizations were performed using a three-axis (x, y, and z) 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 (2G Enterprises model 2G600). 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 effective sensor length (i.e., area under the normalized SQUID response function) of axes x, y, and z are 6.071, 6.208, and 9.923 cm, respectively. Experiments conducted during ODP Legs 186 and 206 showed that the background noise level of the magnetometer is about ±2 × 10^–9 Am² and that reliable measurements could be obtained for intensities greater than ~10^–5 A/m for split-core sections (Shipboard Scientific Party, 2000, 2003b).

Archive halves of all core sections were measured unless precluded by drilling-related deformation. Measurements were made at intervals of 5 cm, starting 15 cm above the top and ending 15 cm below the base of each section. These additional data points were measured to allow deconvolution of the records in later shore-based studies. When performing shipboard data analyses (e.g., for site reports), data corresponding to the top and bottom 10 cm of each core section were edited to account for edge effects. The number of demagnetization steps used to remove the drill string magnetic overprint and isolate a useful magnetic moment reflected time constraints. A three-step demagnetization scheme consisting of 0 (NRM), 10, and 20 mT was used initially. When core flow through the laboratory needed to be increased, only the first (NRM) and last (20 mT) demagnetization steps were used, reducing the measurement time by ~5 min per core section. The use of peak AF demagnetization fields lower than 20 mT ensure that archive halves remain useful for shore-based studies.

Measurements were undertaken using the standard IODP magnetic coordinate system described in Figure F9 (+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 autosave option was not used. The sample interval was set to 5 cm, leader and trailer lengths were both set to 15 cm, and a drift correction was applied. The program options for skipping voids and gaps at the top of the section were not used. Void depths and otherwise disturbed intervals were instead 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 least once per shift (e.g., 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 “Introduction”). Full orientation was attempted using the Tensor orientation tool beginning at Core 3 of all holes when available. 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 (Shipboard Scientific Party, 2003a). 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 20 mT, were used to define magnetic polarity zones. The revised timescale of Cande and Kent (1995) with updated age estimates for the Cobb Mountain (1.190–1.215 Ma) (Channell et al., 2002) and Reunion (2.115–2.153 Ma) (Channell et al., 2003) Subchronozones 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 track systems. Whole-core sections were measured on a MSCL to rapidly acquire magnetic susceptibility data for stratigraphic correlation (see “Stratigraphic correlation”). After whole cores warmed to room temperature, measurements were made as part of the MST analyses (see “Physical properties”).

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