Paleomagnetic studies aboard the JOIDES Resolution during Leg 202 comprised routine measurements of the natural remanent magnetization (NRM) of archive-half sections before and after alternating-field (AF) demagnetization and low-field magnetic susceptibility (k) measurements.
The remanent magnetization of the archive halves of all core sections was measured, unless precluded by drilling-related deformation. The number of demagnetization steps applied to each core section was controlled by time constraints and core flow through the laboratory with a two- or three-step demagnetization scheme (taking ~9-14 min) applied to most 1.5-m-long core sections. The standard three-step measurement scheme involved the measurement and stepwise demagnetization of the NRM and AF demagnetization at peak fields of 10- or 20-mT and 20- or 25-mT steps. The low maximum peak demagnetization fields ensure that the archive halves remain useful for shore-based high-resolution (U-channel) studies of magnetic remanence. The large leader and trailer distance (20 cm) was used to allow future deconvolution of the long-core data. For shipboard analysis, we masked measurements within 10 cm from the ends of each section.
Long-core remanence measurements and AF demagnetizations were performed using a long-core cryogenic magnetometer (2-G Enterprises model 760-R). This instrument is equipped with a direct-current superconducting quantum interference device (DC-SQUID) and has an in-line AF demagnetizer capable of reaching peak fields of 80 mT. The pickup coils of the cryogenic magnetometer have a width at half-height of <10 cm for all three axes, though they may sense a magnetization over an interval of 30 cm. Measurements were made at 5-cm sample intervals starting 20 cm above the core section top and ending 20 cm below the base. Background resolution is affected by the magnetization of the core liner of ~8 x 10-6 A/m. The standard ODP magnetic coordinate system was used (+x = vertical upward from the split surface of archive halves, +y = left hand split surface when looking upcore, and +z = downcore).
During APC coring, full orientation was attempted using the tensor orientation tool, which 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 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).
Where magnetic cleaning appears from shipboard data to have isolated the characteristic remanent magnetization (ChRM), paleomagnetic inclinations and/or declinations (depending on site latitude) 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 Cenozoic polarity chrons.
The magnetic susceptibility was measured for each whole-core section as part of the MST analyses. The MST susceptibility meter (a Bartington MS2C meter with an 88-mm coil diameter and a 0.565-kHz frequency) was set on SI units, and the output values were stored in the Janus database. The width at half-height of the response function of the susceptibility coil is ~4 cm (Blum, 1997), although the sensing region corresponds to a volume of 166 g/cm3 of cored material. To convert the stored values to SI units of volume susceptibility, they should be multiplied by 10-5 and by a correction factor to take into account the volume of material that passed through the susceptibility coils. This factor for a standard ODP core is ~0.66, based on laboratory/ship comparisons.
When time allowed, the partially demagnetized NRM intensity of selected core intervals were normalized by whole-core magnetic susceptibility measured on the MST to assess the potential for deriving estimates of relative geomagnetic field paleointensity. Though far from an ideal method, shipboard normalization provides a quick test of a sediment's potential for shore-based paleointensity studies.