Paleomagnetic and rock magnetic investigations aboard the JOIDES Resolution during Leg 189 included routine measurements of natural remanent magnetizations (NRM). Initially archive-half sections were used, but later whole cores were used. Both were measured before and after alternating-field (AF) demagnetization to 20 mT. Low-field magnetic susceptibility (k) measurements were made with the MST. NRMs and a limited set of rock magnetic observations were made with discrete samples. A nonmagnetic APC core barrel assembly was used for alternate cores in selected holes and the magnetic overprints in core recovered with this assembly were compared with those obtained with standard assemblies.
The remanence measurements made during Leg 189 were conducted 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, and +z = downcore; see Fig. F8).
The output of the 2G magnetometer is given in magnetic moment in electromagnetic units with a background noise of ~10-7 Gauss cm (SI × 10-10 A/m2). This gives an approximate limit in intensity of magnetization required for reliable measurements of standard half- or whole-core samples, reaching the order of 10-6 A/m, or 0.001 mA/m. However, it should be noted that the magnetization of core liners can exceed this intensity. For standard discrete samples, the weakest measurable intensity is more than an order of magnitude greater than the long cores because of the smaller volume of material in the pickup coils.
Natural remanent magnetization was initially measured on all archive-half sections. However, after comparisons between half-core and whole-core measurements revealed that core splitting sometimes induced significant overprints, whole cores were measured for the remainder of the leg. Core flow through the laboratory was modified with the long-core measurement being made before the MST run, which sped up processing significantly.
Long-core measurements were made at 5-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 20 mT, and when time permitted, a 10-mT step was also measured.
Discrete samples were collected from the working halves in standard 8-cm3 plastic cubes using the extrusion tool. The discrete samples were analyzed on the shipboard pass-through cryogenic magnetometer using a tray designed for measuring six discrete samples. To maintain the ODP convention, the cube face with the arrow on it is placed downward in the tray. Samples were demagnetized by AF using the in-line demagnetizer installed on the pass-through cryogenic magnetometer. They were given anhysteretic and isothermal remanent magnetizations (ARM and IRM), and these were demagnetized to establish the magnetic characteristics of the recovered core. 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 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), 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 take into account the volume of material that passed through the susceptibility coils. Except for measurements near the end of each section, the correction factor for a standard full ODP core is about 0.66 (= 1/1.5). The end effect of each core section was not corrected.
During APC coring, full orientation was achieved with the Tensor multishot tool rigidly mounted onto a nonmagnetic sinker bar. 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 double orientation line.
Where magnetic cleaning successfully isolated the characteristic remanent magnetization (ChRM), paleomagnetic inclinations were used to define magnetic polarity zones. On some occasions, it was possible to recover a satisfactory magnetic stratigraphy even when the inclination was of a single polarity because of a persistent overprint. On such occasions, there were indications of the magnetic stratigraphy in the intensity and associated minor differences in the inclination (Fig. F8). To recover the magnetostratigraphy, the z-component alone was used. The z-component was biased in one direction but showed a clear alternating signal superposed upon this. By removing the bias, the magnetization with alternating sign, which carries the magnetostratigraphic signal, is made clearer. Interpretations of the magnetic polarity stratigraphy, with constraints from the biostratigraphic data, are presented in each of the site chapters. The revised time scale of Cande and Kent (1995), as presented in Berggren et al. (1995a, 1995b), was used as a reference for the ages of Cenozoic polarity chrons.