Paleomagnetic and rock magnetic investigations aboard the JOIDES Resolution during Leg 188 included routine measurements of natural and artificial remanent magnetizations of archive-half sections and discrete samples before and after static alternating field (AF) demagnetization, low-field magnetic susceptibility (k) measurements, and a limited set of rock magnetic measurements aimed at characterizing the downcore variation in the composition, concentration, and grain size of the magnetic carriers (e.g., Verosub and Roberts, 1995).
The shipboard paleomagnetic laboratory is equipped with the following:
The bulk of the remanence measurements made during Leg 188 were carried out using the shipboard pass-through cryogenic magnetometer. 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; +z: downcore) (Fig. F12).
Natural remanent magnetization was routinely measured on all archive-half sections at 4-cm intervals with 15-cm-long headers and trailers. Measurements at core and section ends and within intervals of drilling-related core deformation were removed during data processing. AF demagnetizations were applied to cores at 10, 20, and 30 mT. The low maximum peak demagnetization fields ensured that the archive halves remain useful for shore-based high-resolution (U-channels) studies of magnetic properties.
Discrete samples were collected from the working halves in standard 8-cm3 plastic cubes with the arrow on the bottom of the sampling cube pointing upcore (-z). Our preferred strategy was to sample from the working halves at an interval of one meter; whenever possible, samples were selected from fine-grained horizons. Intervals with drilling-induced core deformation were avoided. The discrete samples were analyzed on the shipboard pass-through cryogenic magnetometer using a tray designed for measuring six discrete samples. Samples were AF demagnetized using the in-line demagnetizer installed on the pass-through cryogenic magnetometer at steps of 0, 10, 20, 30, 40, 50, 60, 70, and 80 mT. A subset of samples was thermally demagnetized using the Schonstedt TSD-1 oven. All of the samples subjected to thermal demagnetization were measured at steps of 0°, 100°, 200°, 300°, 330°, 360°, 400°, 500°, 550°, 600°, 650°, and 700°C. The samples were heated for 90 min at the first demagnetization step to ensure that they had fully dried, then for 40 min at each subsequent step to ensure that they had reached thermal equilibrium. After each step, the low-field magnetic susceptibility was measured to monitor for thermal alteration.
Magnetic susceptibility was measured for each whole-core section as part of the MST (see "Physical Properties") using a Bartington MS2 meter coupled to an MS2C sensor coil with a diameter of 88 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) (Fig. F13), 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 were multiplied by 10-5 and then multiplied by a correction factor to take into account the 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 ~0.67 (= 1/1.5). The end effect of each core section was not corrected.
The low-field magnetic susceptibility was also routinely measured for all the discrete samples, and the data were compared with the whole-core susceptibility log. The frequency-dependent susceptibility, fd(%) = 100 × (klow - khigh) / klow, was monitored to estimate the contribution of superparamagnetic contamination (Bloemendal et al., 1985). Measurements of khigh, however, were sometimes unstable and gave unusually high fd(%) values or negative values.
Further analyses were made on a selected subset of discrete samples. These analyses included the determination of the following:
Estimates of the concentration of magnetic minerals can be obtained from parameters such as k, IRM, and ARM, whereas Bcr, S-ratio, and thermomagnetic curves are more diagnostic of magnetic mineral composition.
Where magnetic cleaning successfully isolated the characteristic component of magnetization (ChRM), paleomagnetic inclinations were used to define magnetic polarity zones. Interpretations of the magnetic polarity stratigraphy, with constraints from the biostratigraphic data, are presented in the site chapters. The revised time scale of Cande and Kent (1992, 1995), as presented in Berggren et al. (1995a, 1995b), was used as a reference for the ages of Cenozoic polarity chrons.