Paleomagnetic studies conducted on the JOIDES Resolution during Leg 175 consisted of remanent magnetization measurements of archive-half sections before and after alternating-field (AF) demagnetization and magnetic susceptibility measurements of whole-core sections. Part of the discrete samples obtained from working-half sections were also measured.
Measurements of remanent magnetization were carried out using an automated pass-through cryogenic direct-current-superconducting quantum interference (DC-SQUID) magnetometer (2-G Enterprises model 760-R) with an in-line AF demagnetizer (2-G Enter-prises model 2G600), capable of producing peak fields of 80 mT with a 200 Hz frequency. The background noise level of the magnetometer onboard environment is about 3 × 10-10 Am2. The large volume of core material within the sensing region of the magnetometer, which is on the order of 100 cm3, permits accurate measurements of cores with remanent intensities as weak as ~10-5 A/m.
The standard ODP magnetic coordinate system was used: positive-x (vertical upward from the split surface of archive halves), positive-y (left along split surface when looking upcore), and positive-z (downcore).
The natural remanent magnetization (NRM) was measured before and after AF demagnetization for all archive-half sections at 5-cm intervals. Corrections for the end effect were not applied. AF demagnetizations were applied at 10 and 20 mT for cores from the first hole of each site. Other cores were demagnetized by AF at 20 mT only. Generally, discrete samples were demagnetized by AF at four steps: 10, 20, 25, and 30 mT. Seven discrete cube samples were placed on the sample boat at a time spaced at 20-cm intervals.
Magnetic susceptibility was measured for each whole-core section as part of the MST analysis (see "Physical Properties" section, this chapter). The MST susceptibility meter (a Bartington MS2 meter equipped with an MS2C sensor with a coil diameter of 88 mm and an inducing-field frequency of 0.565 kHz) was set on SI units, and the values were stored in the JANUS database in raw meter units. To convert to true SI volume susceptibilities, these values should be multiplied by 6.6 x 10-6, which corrects for the volume of material that passed through the susceptibility coils. This correction was applied for all figures illustrating magnetic susceptibilities in the "Paleomagnetism" sections of the site chapters (this volume).
Core orientation of the APC cores was achieved with a Tensor tool mounted on the core barrel. The Tensor tool consists of a three-component fluxgate magnetometer and a three-component accelerometer rigidly attached to the core barrel. 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 double-line orientation mark on the core liner relative to magnetic north. Determining orientation was not usually attempted for the top three cores (~30 mbsf) until the bottom-hole assembly (BHA) was sufficiently stabilized in the sediment.
Oriented discrete samples were taken from the working half of each section (one per section) using 7 cm3 plastic cubes (designed at Kyoto University) and a stainless-steel sampler. The cube is symmetrical in shape and was designed to fit standard 1-in sample holders of paleomagnetic and rock-magnetic instruments. The cubes were capped to prevent dehydration. Orientation arrows for the samples were marked on the bottom of the cube.
Where AF demagnetization successfully isolated the primary component of remanent magnetization, paleomagnetic inclinations and declinations relative to magnetic north, derived from the Tensor tool, were used to assign a magnetic polarity to the stratigraphic column. Interpretations of the magnetic polarity stratigraphy, with constraints from the biostratigraphic data, are presented in the site chapters of this volume. The time scale of Berggren et al. (1995b) was used.
We encountered several types of secondary magnetization acquired during coring, which sometimes hampered magnetostratigraphic interpretation. Details of the magnetic overprints are presented in the site chapters of this volume.