Remanent magnetization was measured using the shipboard 2-G Enterprises (Model 760R) long-core cryogenic magnetometer that is equipped with direct current superconducting quantum interference devices (DC SQUIDs) and has in-line, automated alternating-field (AF) demagnetization capability. The sense coils of the cryogenic magnetometer measure over a width of a little more than 30 cm, although ~85% of the remanence is sensed from a 20-cm width of a core section. Measurements were made at 5-cm intervals in basalt and other basement materials and at 2-4 cm in sedimentary sequences. A background resolution limit is imposed on measurement of rock remanence by the magnetization of the core liner itself, which is ~3 × 10-5 A/m.
The measurement coordinate system, which is standard for ODP legs, was used: +x = upward (downward) from the split face of the archive (working) half of the core; +y = left looking upcore along the split surface of the archive half; +z = downcore.
AF demagnetization on the archive halves was performed routinely with the automated cryogenic magnetometer and attached AF coils; most core sections were demagnetized at 5, 10, 15, 20, 25, 30, and 35 mT; additional steps of 3, 8, 40, 45, and 50 mT also were tested. Discrete samples were not demagnetized on board, however, because of an anhysteretic remanence magnetization (ARM) that is imparted by the in-line demagnetization apparatus of the cryogenic magnetometer. The ARM probably results from the poor shielding from magnetic fields in the demagnetization region (Fig. F8); the fields are a result of two prominent leaks in the magnetic shielding at the two points where the magnetic shielding cans are bolted together. Comparisons of sequential demagnetization steps show that in many samples, the ARM becomes prominent just as the drill-string remanence is removed. In more magnetically susceptible samples (e.g., medium-grained basalts), the ARM becomes visible at low demagnetizing fields (between 8 and 10 mT). In less susceptible rocks, such as chilled margins of pillow basalts, the ARM can remain obscure until 35-40 mT (Fig. F9).
Magnetic susceptibility is measured routinely as a part of several shipboard procedures. The susceptibility meter for all measurements was the same, a Bartington Instruments model MS-2. Whole-core sections were run through the MST (see "Physical Properties"), on which is mounted a Bartington Instruments susceptibility ring sensor (MS-2C), an 8-cm ID coil through which the whole core (~5.5-6.5 cm) passes; susceptibility was measured every 5 cm. Susceptibility was measured again on the archive half of all cores with a Bartington probe sensor (MS-2F), a cylindrical probe of 2-cm length attached to the AMST (see "Sedimentologic Description"). This sensor can probe to a depth of ~2 cm. The AMST permits measurements at only evenly spaced intervals along core sections and automatically excludes missing sections of core; spacing can vary from 2 to 10 cm. AMST measurements again were made every 5 cm on basalts and other basement rocks and every 2 to 4 cm for sedimentary rocks. Susceptibilities of discrete samples also used a Bartington MS-2 meter in combination with a hollow cylindrical sensor, the MS-1B dual frequency sensor (ID = 3.6 cm).
Comparison among the three types of susceptibility measurements has been undertaken numerous times during previous ODP legs, most recently Leg 183. Shipboard Scientific Party (2000) report that the variation with depth by the three independent susceptibility measurements was consistent throughout the leg but that the MST and AMST data sometimes showed lower values than those of discrete samples. The most significant differences were in the AMST measurements. They speculate that the lower values were probably the result of gaps in core sections, although sensor calibrations also may not have been uniform. Gaps in core material create low values, which are significant additions to the whole-core signatures because of the volume of the ~6.5-cm diameter of whole cores. Although scientists from Leg 183 were unable to reprogram the AMST software to ignore measurements from missing sections or disturbed intervals of core, the software was reprogrammed just before the start of Leg 185 to skip missing sections.
A seafloor punch core taken for microbial studies recovered a very homogeneous brown clay; the homogeneous nature made this material ideal for testing the instruments. The seven sections of the core, Core 185-801D-1W, were measured with each instrument, as a whole core, a half core, and then as discrete samples taken from each section. Figure F10A shows the direct output of each of the three susceptibility devices. The discrete and AMST sensors gave quite similar values. The MST data must be corrected for the difference between diameter of the core and the inner diameter of the ring sensor. The correction requires an estimate of the differences in diameters and the use of a table or graph of correction factors supplied by Bartington Instruments to determine the correction factor. The measured susceptibility values are divided by the correction factor. Use of this correction method improves the agreement between the results from the MST and the other methods (Fig. F10B). Lack of perfect agreement is most likely because of imperfect assessment of the whole-core volume.
Measurements of natural remanent magnetization (NRM) and magnetic susceptibility were routinely conducted on the igneous and sedimentary rocks on board the JOIDES Resolution during Leg 185. NRM was measured on all archive halves and on discrete samples from the working halves. Susceptibility was measured for all whole cores using the MST, on all archive halves using the AMST, and on all discrete samples with the dual-frequency sensor. Stepwise AF demagnetization was conducted on all archive halves of the cores; however, discrete samples were not demagnetized on the ship because of the ARM problem. They will be demagnetized onshore at the University of Wyoming.
Basalt data are presented as the direct output of the long-core magnetometer; principal component analysis of this data will be performed onshore. Together with the discrete sample analyzed onshore, this complete data set will be presented in the Leg 185 Scientific Results volume of the ODP Proceedings. Magnetostratigraphic results from Site 1149 are based on the time scales of Berggren et al. (1995) for the Cenozoic, Gradstein et al. (1995) for the Late Cretaceous, and Channell et al. (1995) for the Early Cretaceous. Paleolatitudes for the Pacific plate will be determined from a complete discrete sampling of the sedimentary section at Site 1149. Discrete samples will be measured onshore to utilize thermal demagnetization, and the results will be compiled with those obtained from Hole 801B of Leg 129 (Steiner and Wallick, 1992) for the Scientific Results volume.