Paleomagnetic work during Leg 182 consisted of long-core measurements of the natural remanent magnetization (NRM) of archive-half core sections before and after alternating field (AF) demagnetization, as well as NRM measurements and progressive demagnetization of discrete samples collected from the working half of core sections.
Long-core remanence measurements and AF demagnetization were performed using a computer-interfaced, pass-through, cryogenic direct-current superconducting quantum interference device (DC-SQUID), magnetometer (2-G Enterprises Model 760-R), and an AF demagnetizer (2-G Enterprises Model 2G-600) capable of reaching peak inductions of 80 mT at 200 Hz. The background noise level of the magnetometer is on the order of 10-10 Am2. The large volume of core material within the sensing region of the magnetometer, which is ~100 cm3, permits the measurement of cores with remanent intensities as weak as ~10-5 A/m. The data acquisition program collects magnetic moment data in the cgs system (emu). Data were later reduced to produce moments in SI units, with intensities in amperes per meter calculated using the magnetometer volume response functions.
Because of concerns over the reliability of remanence measurements of weakly magnetized carbonate rocks, the noise level was estimated from repeated runs of empty trays and was found to lie within the range reported from other ODP legs (10-9-10-10 Am2). During transit the reliability of the measurements of the cryogenic magnetometer was further tested on discrete samples of washcore of a homogeneous mixture of carbonate mudstone. Cubic samples (~10 cm3) were given a horizontal anhysteretic remanence (ARM) diagonally across a cube face. The intensity of the ARM was progressively increased using inductions of 5 to 50 mT, resulting in magnetic moments per unit volume (intensity) ranging between ~5 × 10-5 and 2.5 × 10-3 A/m. Two measurements were made in the cryogenic magnetometer, with an inversion of the horizontal axes of the sample. The antipodality of the remanence in both orientations as well as the ratios of the horizontal components x and y were plotted vs. the intensity of the ARM. From this experiment it was concluded that antipodality of >170° was observed only when the sample intensity was >~2 × 10-4 A/m. The ratios of the horizontal axes are near the expected unit value for intensities >2 × 10-4 A/m, although moments measured when the samples were oriented in archive coordinates (sample +X/+Y direction = sensor +X/+Y direction) were consistently higher (by ~5%) than moments measured in working coordinates. There appears to be no simple explanation for this observation.
Archive halves of all core sections were measured, unless coring or drilling-related deformation was noted. The demagnetization sequence applied to each core section was often dictated by the flow of core through the core laboratory. A five-step demagnetization scheme consisting of NRM and AF demagnetization at 5, 10, 15, and 20 mT was used in the first material recovered, but was later reduced to two or three steps. In most cases peak inductions of 20 mT were sufficient to isolate a characteristic magnetization and determine magnetic polarity, ensuring that the archive halves remain useful for shore-based high-resolution studies of magnetic remanence. For data collection a 5-cm measurement interval with 10-cm headers and trailers was first selected, although it was later increased to 10 cm in high-sedimentation-rate intervals. The large leader and trailer distance was used to allow possible future deconvolution of the long-core data. Measurements within 10 cm of the ends of each section may be compromised by edge effects, with false apparent low intensities and inaccurate directions occurring where the response function of the SQUID sensors (12-15 cm) averages empty space with the core signal. Directional data were collected using ODP orientation conventions (the sample +X axis is oriented toward the double line inscribed on the core liner of the working half). Core orientation of APCs was achieved using a Tensor tool mounted on the core-barrel assembly. Core photographs were examined to delete remanence data from disturbed or missing intervals. For each site we produced a set of tables that contain the cleaned data. The raw data are available in the JANUS database.
Discrete samples were collected from working halves in standard ~7-cm3 plastic cubes with orientation marks on the bottom of the sampling cube pointing upcore. Intervals of coring- or drilling-related core deformation were avoided. The shipboard long-core cryogenic magnetometer was used to measure the NRM of the discrete samples. Samples were measured in a tray designed to hold a maximum of seven samples. Discrete samples were also used for rock magnetic experiments, including isothermal remanent magnetization (IRM) acquisition and AF demagnetization of the saturation IRM. An Analytical Services Company pulse magnetizer was used for this purpose.
Low-field MS was measured for all whole-core sections as part of the MST analysis (see "Physical Properties"). The MST susceptibility meter (a Bartington Model MS-2 with an MS2C sensor; coil diameter = 88 mm, operating frequency = 0.565 kHz) has a nominal resolution of 2 × 10-6 SI. Susceptibility was determined at 8-cm intervals using a 10-s integration time. The "units" option was set on SI units, and the values were stored in the JANUS database in raw count units. To convert to SI volume susceptibilities (bulk), these values should be multiplied by a correction factor to account for the volume of material that passed through the susceptibility coils. The correction factor for a standard ODP core is ~0.66 (= 1/1.5). No correction was applied to any figures illustrating magnetic susceptibilities in the "Paleomagnetism" sections in this report. Magnetic susceptibility of discrete samples obtained from the working half was measured using a Bartington MS2 susceptibility meter with a dual frequency MS1B Sensor. Magnetic susceptibility was used as a first-order measure of the amount of ferrimagnetic material and as a correlation tool.
Where AF demagnetization successfully isolated the characteristic component of remanence, paleomagnetic inclinations were used to define polarity zones. The revised time scale of Cande and Kent (1995), as presented in Berggren et al. (1995b), was used as a reference for the ages of Cenozoic polarity chrons.