The goals of paleomagnetic studies during Leg 191 were twofold: (1) to determine magnetic polarities of cores for correlation with the geomagnetic polarity time scale (GPTS) and (2) to measure paleomagnetic directions for tectonic studies. Paleomagnetic investigations during Leg 191 consisted of measurements of archive-half core sections and discrete samples taken from the working half of the core using the shipboard pass-through cryogenic magnetometer. Archive-half measurements were made for natural remanent magnetization (NRM) and remanent magnetization after alternating-field (AF) demagnetization. Whole-section measurements were made only on APC and XCB cores because of the internal rotation of core segments in RCB cores. Discrete samples were studied from the APC, XCB, and RCB cores. In the APC and XCB cores, discrete samples allowed a more detailed picture of the remanence to be developed. In the RCB cores, these samples were the sole source of paleomagnetic information. To characterize magnetic carriers within the samples, isothermal remanent magnetization (IRM), anhysteretic remanent magnetization, and anisotropy of magnetic susceptibility (AMS) were measured in selected samples.
A 2-G Enterprises pass-through cryogenic direct-current superconducting quantum interference device rock magnetometer (Model 760R) was used to make the majority of paleomagnetic measurements during Leg 191. This pass-through cryogenic magnetometer is equipped with an in-line AF demagnetizer (2-G Model 2G600) that allows for demagnetization of samples up to 80 mT. The magnetometer and AF demagnetizer are interfaced with a PC-compatible computer. The sensor coils of the cryogenic magnetometer measure 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. A background resolution limit is imposed on measurement of rock remanence by the magnetization of the core liner itself, which is about 3 × 10-5 A/m.
Magnetic susceptibility of core sections was measured with two devices. Whole-core sections were measured on the whole-core MST. This apparatus includes a Bartington model MS2 meter with an 80-mm internal diameter MS2C sensor loop (88-mm coil diameter) operating at a frequency of 565 Hz and an AF of 80 A/m (0.1 mT). The specified sensitivity for the MS-2 susceptibility meter is 10-5 SI/10 cm3 volume or 10-8 SI/10 g mass. A second Bartington susceptibility meter is included on the AMST. This meter uses a 15-mm-diameter MS2F probe capable of making measurements of susceptibility at the core surface, giving a greater resolution than the loop sensor of the whole-core MST. The susceptibility sensitivity of the split-core meter is the same as the whole-core meter. Its specified horizontal resolution is 20 mm. Most of the probe sensitivity is within less than a diameter of the probe tip.
Additional instruments in the paleomagnetic laboratory include a DTECH model D-2000 AF demagnetizer capable of demagnetization up to 200 mT and a Schonstedt thermal demagnetizer (model TSD-1) capable of demagnetization up to 700°C. A Geofyzika Brno KLY-2 Kappabridge magnetic susceptibility meter, with an operating frequency of 920 Hz and a magnetic field intensity of 0.3 mT, allows measurements of the magnetic susceptibility and AMS of discrete samples. The specified magnetic susceptibility sensitivity of this instrument is 1 × 10-8 SI volume units, but the noisy environment of the core laboratory reduces the sensitivity to 1 × 10-6 SI. Also in the laboratory is an Analysis Services Company impulse magnetizer model IM-10, capable of applying magnetic fields from 0.02 to 1.35 T. This apparatus imparts an IRM on a sample, and the magnetization characteristics can be used to determine the type of magnetization carrier.
APC cores were oriented using the electronic tensor orientation tool. This instrument hooks to the sinker bar assembly above the core barrel, where it senses the magnetic field direction through a nonmagnetic drill collar. The tensor tool records the magnetic field direction every 30 s in internal memory. Valid orientation measurements were recognized by holding the drill string still for ~5 min while 6-10 consistent measurements were made before shooting the core. After the tensor tool was recovered, orientation data were uploaded into a microcomputer and from there to the Janus database. Orientation corrections were made using the MTF angle (the angle between magnetic north and the double line on the core liner, which is at the base of the working half) in the following calculation:
where
Standard ODP paleomagnetic measurement conventions were used for Leg 191 paleomagnetic studies. The x-axis is positive upward (downward) from the split face of the archive (working) half of the core. The positive y-axis is left facing upcore along the split surface of the archive half, whereas the positive z-axis is downcore (Fig. F5).
Each APC core archive half was routinely measured at 5-cm intervals using the shipboard pass-through cryogenic magnetometer. NRM was measured initially, followed by magnetization after progressive AF demagnetization at 10-mT steps from 10-50 mT. Demagnetization was accomplished using the in-line AF demagnetization coils built into the cryogenic magnetometer. Discrete samples were taken at intervals dictated by core recovery and scientific interest. These samples were usually either AF demagnetized at 5-mT intervals from 0 to 70 mT or thermally demagnetized at 50°C steps from 0° to 700°C.
Magnetic susceptibility measurements were made on APC cores with the MST and the AMST at 2-cm intervals. RCB samples were measured at 5-cm intervals. The quality of these results degraded in XCB and RCB sections, which can be undersized and/or disturbed. Nevertheless, the general downhole trends were useful for stratigraphic correlations. The MS2 meter measures relative susceptibilities that have not been corrected for the differences between core and coil diameters. Susceptibility values were stored in the Janus database as raw data in units of 10-5 SI. The true SI volume of susceptibilities should be multiplied by a correction factor to account for the volume of material that passed through the coils.
Magnetic polarity results were correlated to the polarity reversal sequence, biostratigraphy and fossil stage boundaries, and absolute age using two primary GPTSs. For Cenozoic time, the Berggren et al. (1995b) geochronology was used (Fig. F6). This time scale incorporates the widely used calibration of age and polarity intervals derived by Cande and Kent (1995). For Mesozoic time, the GPTS of Gradstein et al. (1994, 1995) was employed.