The investigation of magnetic properties at Site 1076 included the measurement of bulk susceptibility of whole-core sections and the natural remanent magnetization (NRM) of archive-half sections. The Tensor tool was used to orient Hole 1076A for Cores 3H through 16H, Hole 1076B for Cores 4H through 7H, Hole 1076C for Cores 18H through 22H, and Hole 1076D for Cores 3H through 5H (Table 7). The remaining cores from each hole were not oriented because of technical problems with the Tensor tool.
Measurements of NRM were made on all archive-half core sections from Holes 1076A, 1076C, and 1076D. Cores 175-1076B-1H through 3H were measured, but the remaining cores from Hole 1076B were not measured because of pronounced sediment disturbance. Sections from Hole 1076A were demagnetized by alternating field (AF) at 10 and 20 mT, and sections from Holes 1076B, 1076C, and 1076D were demagnetized by AF at 20 mT only.
Magnetic susceptibility measurements were made on whole cores from all four holes as part of the MST analysis (see "Physical Properties" section, this chapter), except for Cores 175-1076B-4H through 7H, which were not measured because of technical difficulties with the MST. Magnetic susceptibility was relatively low, on the order of 10-5 (SI volume units; Fig. 11).
The intensity of NRM after 20-mT demagnetization from the three measured holes is similar in magnitude, ranging from ~10-5 to ~10-3 A/m (Fig. 12, left panel). Within the upper 50 mbsf of Holes 1076A and 1076C, the intensity is on the order of 10-3 to 10-4 A/m. The NRM decreases gradually (from ~10-4 to ~10-5 A/m) between 40 and 90 mbsf, below which it increases rapidly to 10-3 A/m and once again decreases gradually with depth. A similar trend was observed in the magnetic susceptibility, which increases rapidly below ~95 mbsf (Fig. 11).
A relatively stable magnetic component was preserved in sediments from all three holes, which allowed the determination of the magnetic polarity. A magnetic overprint with steep positive inclinations, which was probably acquired during drilling, was usually erased by 20-mT demagnetization. Directions of the NRM below ~140 mbsf, however, 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.
We identified the polarity of the NRM from the declinations and inclinations. Data from the Tensor orientation tool were available for most of Hole 1076A, which facilitated interpretation of reversals in terms of the geomagnetic time scale. Changes of inclination with polarity transitions were difficult to interpret 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. 12, right panel).
The Brunhes/Matuyama polarity transition (0.78 Ma; Berggren et al., 1995) occurs between 132 and 138 mbsf at Hole 1076A and between 136 and 140 mbsf at Hole 1076C; the Brunhes/Matuyama boundary was not identified at Hole 1076D (Fig. 12, middle panel). The thickness of sediments, which records a polarity transition, should be ~1.5 m at these holes, assuming that the sedimentation rate is ~150 m/m.y. and a polarity transition completes within 10 k.y. However, the large scatter of the remanent directions and the incomplete Tensor data made it difficult to determine the exact position of the boundary. The termination and beginning of the Jaramillo Subchron (C1r.1n), the ages of which are 0.99 and 1.07 Ma (Berggren et al., 1995), respectively, occur at ~156-158 and 167-170 mbsf at Hole 1076A and at about 165-167 and 175-178 mbsf at Hole 1076C.
We observed a possible short reversal event in the Brunhes Chron. It occurs at all four holes: 16-22 mbsf at Hole 1076A (the bottom is not clear because of a gap); 20 mbsf to the bottom of Hole 1076B (23 mbsf); 20-26 mbsf at Hole 1076C; and 20-25 mbsf at Hole 1076D. Figure 13 displays this record from Cores 175-1076D-2H through 4H. All holes except Hole 1076C show declinations of almost 180° opposite the average Brunhes declination. This is possibly the Blake event, considering the calcareous nannofossil age of 90 ka at 16 mbsf at Hole 1076A (see "Biostratigraphy and Sedimentation Rates" section, this chapter). The Blake event is considered to have occurred during the earlier part of the δ18O Stage 5, at ~120 ka (Tric et al., 1991; Tucholka et al., 1987). The duration of the Blake event is not well constrained, but it is probably 10 k.y. or less. The occurrence of the event through several meters in the sediments suggests an extremely high sedimentation rate (~500 m/m.y.) during Stage 5 at this site.
Another rapid change in declinations was observed in Core 175-1076A-8H (74 mbsf). It is, however, uncertain whether this is of geomagnetic field origin or whether it reflects a coring disturbance. Other anomalous directions, shown in Figure 12, occur at core boundaries and are thus considered to be caused by sediment disturbance.