Paleomagnetic investigations aboard the JOIDES Resolution during Leg 198 were mainly of two types: routine measurements of the natural remanent magnetization (NRM) of archive-half core sections, before and after alternating-field (AF) demagnetization, and low-field magnetic susceptibility (k) measurements. Discrete samples were collected during the cruise from working halves of core sections in standard (7 cm3) plastic "cubes" with the orientation arrow on the sample pointing upcore. The sampling frequency was generally three samples per section from one hole per site and one or two samples per section from duplicate holes. Intervals of drilling-related core deformation were obviously avoided, where apparent. Apart from a few test samples, remanence measurements of discrete samples were not taken because sediment sample magnetizations are too weak to be measured reliably with shipboard equipment. The discrete samples will be analyzed on shore with three objectives: (1) to ground truth the shipboard polarity stratigraphies, (2) to gain insight into the magnetic properties of the sediments and determine the mineralogy and grain size of remanence carriers, and (3) to determine paleolatitudes for tectonic studies.
The remanent magnetization of archive halves of core sections was measured using the shipboard 2-G Enterprises (model 760R) long-core cryogenic magnetometer. This instrument is equipped with direct-current super-conducting quantum interference devices (DC SQUIDs) and has an in-line, automated AF demagnetization system. The pick-up coils of the cryogenic magnetometer measure the core over a width of a little more than 30 cm, although ~85% of the remanence is sensed from a 20-cm width. Because of this window size and the 5-cm sampling interval, adjacent measurements made on Leg 198 cores are not strictly independent. A background resolution limit is imposed on the measurement of sample remanence by the magnetization of the core liner itself, which is ~3 x 10-5 A/m. The standard ODP magnetic coordinate system was used (+x: vertical upward from the split surface of archive halves, +y: left along split surface when looking upcore, and +z: downcore; see Fig. F6).
NRM was routinely measured on all archive-half sections at 5-cm intervals, starting 20 cm above the core section top and 20 cm below the core section base. The large leader and trailer distance (20 cm) allows future deconvolution of the core data. For shipboard analyses, measurements within 10 cm from the ends of each section were disregarded because of end effects. After measurement of the NRM, AF demagnetization was routinely applied using peak fields of 5, 10, and 20 mT. The maximum peak field (20 mT) was set at this low level to prevent compromising the archive halves of core sections for further (shore based) studies. Specifically, we plan postcruise sampling of archive halves of core sections using u-channels to resolve the magnetic stratigraphy (including paleointensity stratigraphy) at a resolution beyond the limits of the shipboard magnetometer.
During APC coring, full orientation was attempted using the Tensor Multishot 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 the 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, paleomagnetic inclinations were used to make an initial designation of magnetic polarity zones. The geomagnetic polarity timescale (GPTS) of Cande and Kent (1995) provides the polarity zone template for the Late Cretaceous and Cenozoic. Biostratigraphic constraints should allow us to correlate polarity zones to the GPTS and, hence, to assign initial ages to the sedimentary sequence. For Early Cretaceous polarity intervals prior to the Cretaceous Long Normal Superchron, the GPTS of Channell et al. (1995) was used as the polarity zone template and to provide ages of polarity chrons.
Although the great majority of Leg 198 paleomagnetic measurements were of sedimentary cores, a few were made on igneous rocks cored in Hole 1213B. As for sediment core measurements, the igneous cores were measured at 5-cm intervals. Only sections with long, whole pieces were measured, but because of piece breakage and some small pieces, some measurements are spurious. For interpretation, only the measurements located in the interior zones of the long pieces were used. AF demagnetization was used to remove low coercivity overprints, with the usual steps (10, 15, and 20 mT) augmented up to 30-35 mT.
Magnetic susceptibility was measured for each whole-core section as part of the MST analysis (see "Physical Properties"). Susceptibility is measured on the MST using a Bartington MS2 meter coupled to a MS2C sensor coil, with a diameter of 8.8 cm operating at 0.565 kHz. The sensor was set on SI units, and the data were stored in the Janus database in raw meter units. The sensor coil is sensitive over an interval of ~4 cm (half-power width of the response curve), and the width of the sensing region corresponds to a volume of 166 cm3 of cored material. To convert to true SI volume susceptibilities, these values should be multiplied by 10-5 and then multiplied by a correction factor to account for the actual volume of material that passed through the susceptibility coils. Except for measurements near the ends of each section, the correction factor for a standard full ODP core is ~0.68. The end effect of each core section is not adequately corrected using this procedure.