Shipboard paleomagnetic measurements in Holes 1133B and 1133C consisted of long-core measurements at 5- to 10-cm intervals of the natural remanent magnetization (NRM) and the remanence after alternating field (AF) demagnetization at 20 mT, as described in "Paleomagnetism," in the "Explanatory Notes" chapter. Long-core measurements established a tentative Pleistocene magnetostratigraphy to a depth of ~30 mbsf in Hole 1133B, which includes the Brunhes (0-12 mbsf), Matuyama (12-24 mbsf), and Gauss (24-? mbsf). The interpretation is problematic because the same result was not reproduced in Hole 1133C, which yields uniform normal polarity magnetizations to a depth of 20 mbsf and anomalously shallow magnetizations of both polarities below this depth.
Intensity of the NRM is very low, with a median of ~1.5 × 10-4 A/m. High values are observed in the uppermost 5 mbsf, although the highest intensities (and susceptibilities) are observed in a package composed of wackestones, observed in both holes at ~30 mbsf. The NRM inclinations are predominantly of moderately positive inclination (Fig. F8). After partial demagnetization (20 mT), inclinations in Hole 1133B define intervals of negative and positive inclination. In contrast, the record for Hole 1133C yields magnetizations of predominantly negative inclination to a depth of ~20 mbsf, at which point inclinations are shallow and no meaningful polarity record can be obtained. Declinations in both holes are scattered about 0°/360°, the direction of the fiducial line in sample coordinates. This probably reflects the acquisition of a drilling-induced remanence. Measurements on discrete samples were unsuccessful because of the overall low intensity of the remanence.
Rock magnetism analysis consisted of measurements of weak-field susceptibility at two frequencies, progressive isothermal remanent magnetization (IRM) acquisition, and AF demagnetization of anhysteretic remanent magnetization (ARM). Decay of the NRM upon AF demagnetization is typical of a cubic phase, either magnetite or greigite. Representative samples were given a 400-mT IRM and subsequently demagnetized (Fig. F9). Inductions of 400 mT were not sufficient to reach saturation, suggesting that magnetic sulfides are present and may be important remanence carriers. Alternating field decay of the IRM suggests that in some of the cored intervals, contributions from single-domain size particles is less important than that from multidomain particles (Cisowski, 1981). This is also manifest in the lower ARM:IRM ratios observed at this site (<0.1). Magnetic susceptibility is of low negative values and is therefore dominated by the contributions of the diamagnetic carbonate matrix.
The long wavelength pattern of normal polarity from 0 to ~12 mbsf, reversed polarity from 12 to ~24 mbsf, followed by a return to dominantly normal polarity appears to be a manifestation of the Brunhes-Matuyama-Gauss, Pliocene-Pleistocene sequence, lacking a good definition of the Jaramillo and Olduvai normal zones in the Matuyama (Fig. F8). As noted above, this interpretation is problematic because the same result was not reproduced in Hole 1133C. Therefore, possible correlation with the geomagnetic polarity time scale in the Pliocene-Pleisto-cene is uncertain.