The investigation of magnetic properties at Sites 1110, 1111, and 1112 included the measurement of (1) bulk susceptibility of whole core sections, (2) point susceptibility and remanent magnetization of archive half core sections, and (3) anisotropy of magnetic susceptibility and remanent magnetization of discrete samples. No magnetic properties were measured at Site 1113 because of poor recovery.
Magnetic susceptibility measurements were made on whole core sections as part of the multisensor track (MST) analysis (see "Physical Properties"), and on half core sections as part of the archive multisensor track (AMST) analysis. The MST and AMST susceptibilities values (uncorrected for volume) at Sites 1110, 1111, and 1112 ranged between on the order of 10-4 and 10-3 SI (Figs. F21A, F21B).
Results of the measurements of magnetic susceptibility and its anisotropy (AMS) on discrete samples are listed in Table T9. Samples above ~8 mbsf at Holes 1110A, 1110B, and 1112A showed a relatively constant mean susceptibility of ~4-5 × 10-4 SI. At Hole 1111A, two samples at ~68-71 mbsf showed relatively higher susceptibilities (~4 × 10-3 SI), whereas two at ~126-129 mbsf showed relatively lower susceptibilities (~1 × 10-4 SI). The degree of anisotropy (Pj) values ranged between ~1.02 to 1.08, reflecting a low degree of anisotropy. The shape parameter (T) values of all samples except one were positive, which indicated predominantly oblate susceptibility ellipsoids. Limited data precluded defining a trend in the orientation of susceptibility axes from all samples. Two samples at ~68 and 71 mbsf from Hole 1111A showed a higher susceptibility and a greater degree of Pj than the other samples. These same two samples from Hole 1111A showed steep Kmin axes with subhorizontal Kmax axes.
Measurements of remanent magnetization were made on relatively undisturbed sections from archive-half cores and on discrete samples taken from working half core sections. Results are shown in Figure F22A, F22B, F22C, and F22D.
In Hole 1110A, intensity of remanent magnetization after AF demagnetization at 25 mT ranged from values on the order of 10-3 A·m-1 to values on the order of 10-2 A·m-1. In Hole 1110B, very low intensities on the order of 10-5 A·m-1 occurred near the top of the recovered section, but values increased rapidly so that by 0.5 mbsf intensities were similar to those observed in Hole 1110A. Holes 1111A and 1112A showed intensity values similar to those observed at Site 1110.
The trends of intensity data (Fig. F22) compared with susceptibility data (Fig. F21) were not similar at any of the sites. The disagreement between the trends of remanent intensity and magnetic susceptibility suggests that the magnetic minerals that carry the remanent magnetization differ from those that dominate the magnetic susceptibility, at least for the intervals measured.
Demagnetization behavior of discrete samples from all sites generally showed two components of magnetization (Fig. F23). The soft component, which was removed by 5 or 10 mT alternating-field (AF) demagnetization, showed inclinations that were steep downward and probably represented an overprint acquired from the drill string.
Seven out of 10 measured discrete samples yielded a characteristic remanent magnetization (ChRM) at demagnetization levels between ~15 and 25 mT. Three samples either showed a curved demagnetization trajectory, indicating a considerable overlap in the coercivity spectra of the soft and ChRM components, or a trajectory that did not decay toward the origin on vector demagnetization plots. The ChRM of the seven samples with stable demagnetization behavior showed inclinations between ~-20º and -50º (Table T10).
The polarity of the remanent magnetization after AF demagnetization at 25 mT for Site 1110 was determined primarily from the inclinations. Scatter within and between sections was relatively low except near the bottom of the hole where greater scatter was associated with coarse sands and pebbles. Directions were corroborated by discrete sample analysis (Fig. F22A, F22B). Only the Brunhes normal polarity chron (C1n) was recorded at this site, consistent with the paleontologic data.
Evidence for an excursion of the magnetic field during the Brunhes was indicated between ~6.5 and 7.2 mbsf in Hole 1110A (Fig. F22A), which may represent the Blake event (~0.128 Ma; Harland et al., 1990). Evidence of this event was not observed in Hole 1110B because recovery was limited to ~5 mbsf (Fig. F22B).
Data are shown for Holes 1111A and 1112A in Figure F22C and F22D.