The investigation of magnetic properties at Site 1075 included the measurement of bulk susceptibility of whole-core sections and the natural remanent magnetization (NRM) of archive-half sections. AF demagnetization at 10 mT and 20 mT was conducted for Hole 1075A and AF demagnetization at 20 mT was conducted for Holes 1075B and 1075C. The Tensor tool was used to orient Holes 1075A and 1075B, starting with Core 4H, and Hole 1075C, starting with Core 3H (Table 7). Cores 175-1075A-21H and 22H and 175-1075B-16H through 22H were not oriented because of technical problems with the Tensor tool.
Intensity of NRM after 20-mT demagnetization from all three holes is similar in magnitude, ranging from ~10-5 to ~10-3 A/m (Fig. 21, left panel). Within the upper 40 mbsf, the intensity is on the order of 10-4 A/m, except for the uppermost part where intensities are an order of magnitude stronger. Between 40 and 80 mbsf, the intensity decreases from ~10-4 to ~10-5 A/m with depth, although no decreasing trend was observed in the magnetic susceptibility. Instead, the magnetic susceptibility gradually decreases with depth below 100 mbsf (Fig. 22). The disagreement of the trend between the remanent intensity and magnetic susceptibility suggests that the magnetic minerals that carry the NRM differ from those that dominate the magnetic susceptibility.
In spite of the overall low intensities of magnetization, a relatively stable magnetic component was preserved in sediments from all three holes, which allows the determination of magnetic polarity. The magnetic moments of the cores were one or two orders of magnitude higher than the noise level of the magnetometer. A magnetic overprint with steep positive inclinations, which was probably acquired during drilling, was usually erased by up to 20-mT demagnetization. However, directions of the NRM show relatively large scatter. This suggests that secondary magnetizations still remain and are probably a viscous remanent magnetization and/or chemical remanent magnetization caused by diagenetic growth or dissolution of magnetic minerals.
Magnetic susceptibility measurements were made on whole cores from all three holes as part of the MST analysis (see "Physical Properties" section, this chapter), except for Core 175-1075C-5H, which was not measured because of deformation of the core liner. Magnetic susceptibility was relatively low, on the order of 10-5 (SI volume units; Fig. 22).
We identified the polarity of the NRM from the declinations. Because the data from the Tensor orientation tool were available for many cores, we were able to more confidently interpret reversals in terms of the geomagnetic-polarity time scale (Berggren et al., 1995). Changes of inclination with polarity transitions were vague because of the low latitude of this site (an inclination of -10° is expected from the geocentric axial dipole model) and the magnetic overprint (Fig. 21, right panel).
The Brunhes/Matuyama polarity transition (0.78 Ma) occurs between 81 and 86 mbsf at Hole 1075A, 82-84 mbsf at Hole 1075B, and 84-89 mbsf at Hole 1075C (Fig. 21, middle panel). The polarity transition should occur over an interval of ~1 m, assuming a sedimentation rate of ~100 m/m.y. and a polarity transition with 10 k.y. duration. However, the large scatter of the remanent directions made it difficult to determine the exact position of the boundary. The record of Hole 1075B is least affected by the magnetic overprint and gives the clearest polarity boundaries. The termination and beginning of the Jaramillo Subchron (C1r.1n), the ages of which are 0.99 and 1.07 Ma, respectively, occur at ~96.5 mbsf (Core 175-1075A-11H/12H boundary) and 109-113 mbsf at Hole 1075A, at ~98 mbsf and 110-112 mbsf at Hole 1075B, and at 97-98 mbsf and 108-111 mbsf at Hole 1075C. We interpret the normal polarity interval ~170-180 mbsf at Holes 1075A and 1075B as the Olduvai Subchron (C2n, 1.77-1.95 Ma): the upper boundary (termination) at 167-170 mbsf in Hole 1075B and the lower boundary at 182-185 mbsf at Hole 1075A. The upper boundary is unclear for Hole 1075A (between 154 and 176 mbsf) because of the extremely large scatter of the remanent directions. The lower boundary for Hole 1075B and both the upper and lower boundaries for Hole 1075C could not be determined because of the lack of Tensor tool data.
In spite of the high sedimentation rates of ~100 m/m.y., we obtained no evidence for short-duration polarity flips and/or excursions, such as the Blake event, in the Brunhes Chron. Some data points showing anomalous directions can be seen in Figure 21, but they occur at or near the boundaries of cores and/or sections and most probably are caused by disturbance of the sediments.