The investigation of magnetic properties at Site 1081 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 Cores 175-1081A-4H through 16H and 18H, Cores 175-1081B-4H through 21H, and Cores 175-1081C-3H through 17H (Table 7).
Measurements of NRM were made on all archive-half core sections from Holes 1081A, 1081B, and 1081C. Sections from Hole 1081A were demagnetized by AF at 10 and 20 mT, and sections from Holes 1081B and 1081C were demagnetized by AF at 20 mT only. Magnetic susceptibility measurements were made on whole cores from both holes as part of the MST analysis (see "Physical Properties" section, this chapter).
The intensity of NRM after 20-mT demagnetization decays gradually with depth from ~10–2 to ~10–5 A/m in the upper 150 mbsf (Fig. 23, Fig. 24). It rapidly increases with depth to 10–2 A/m between 160 and 190 mbsf at Hole 1081A, then decreases gradually with depth again. The trend in magnetic susceptibility is similar to the remanent intensity, except for the uppermost 30 mbsf and between 210 and 260 mbsf, but the amplitude of the variations is much smaller, ranging between 3 and 15 x 10–5 (SI volume units).
A primary NRM record was recovered after 20-mT demagnetization from sediments in the upper 120 mbsf (Fig. 23, Fig. 24). APC cores, however, below 120 mbsf from all three holes and all XCB cores from Hole 1081A (between 137 and 144 mbsf and below 154 mbsf) were pervasively overprinted during coring. Declinations of APC cores below ~120 mbsf show a weak grouping at 0° before orientation (Fig. 23), indicating significant coring-induced magnetization of a radially-inward direction (see "Paleomagnetism" section, "Site 1077" chapter, this volume). All XCB cores show declinations of about –30° and inclinations of ~45°, even where sediments are extensively biscuited (Fig. 23). The pervasiveness of the declination, which is independent of the orientation of sediments, indicates that the magnetization was acquired during coring, but it is difficult to explain the nonaxisymmetric geometry. A magnetic overprint similar to that found at this site was observed in XCB cores during previous ODP legs (declination of +20°, Shipboard Scientific Party, 1996; declination of –40°, Shipboard Scientific Party, in press).
We identified the polarity of the NRM from the magnetic declinations and inclinations. The Brunhes/Matuyama boundary (0.78 Ma; Berggren et al., 1995) occurs between 48 and 52 mbsf at Hole 1081A, between 51 and 53 mbsf at Hole 1081B, and between 53 and 57 mbsf at Hole 1081C (Fig. 24, middle and right panels). The termination and beginning of the Jaramillo Subchron (C1r.1n), 0.99 and 1.07 Ma (Berggren et al., 1995), respectively, occur at ~68 and 72 mbsf at Hole 1081A, 68 and 75 mbsf at Hole 1081B, and 69 and 78 mbsf at Hole 1081C. The termination and beginning of the Olduvai Chron (C2n), 1.77 and 1.95 Ma (Berggren et al., 1995), respectively, occur at about 102 and 116 mbsf at Hole 1081A, 102 and 114 mbsf at Hole 1081B, and 97 and 112 mbsf at Hole 1081C.
No short reversal events and/or excursions were identified within the Brunhes and Matuyama Chrons. All anomalous directions in the Brunhes Chron were likely caused by disturbance of the sediments because they occur at core boundaries. The scatter of directions between 62 and 70 mbsf at Hole 1081B (Core 175-1081B-8H) is also attributed to sediment disturbance. There are several apparent shifts in direction between the Jaramillo and Olduvai Subchrons (including those labeled "A" and "B" in Figure 24). They can be correlated among the three holes from the magnetic susceptibility. Most of them showed declinations close to zero before Tensor orientation, how-ever, suggesting that these directions represent the radial-inward coring-induced magnetization. This was confirmed for Hole 1081C by measuring the NRM of both archive and working halves for the suspicious sections (175-1081C-9H-5, 10H-3, and 11H-3). The declinations at the pseudo-events were almost 180° opposite between the archive and working halves, indicating a radial-inward magnetization (Fig. 25). It is unclear why the overprint dominates at these particular horizons. The remanent intensity decreases at many of these pseudo-events, suggesting that the primary component is much weaker and thus strongly overprinted.
Sediments from the three holes recorded the Brunhes/Matuyama polarity transition with little magnetic overprint. These records show the characteristic behavior of the Brunhes/Matuyama transition, displayed in Figure 26. Before the main transition between 52.5 and 52.0 mbsf, a decrease in intensity, accompanied by a fluctuation of directions, occurs at 54.2 mbsf. This may represent the "precursor" of the reversal (at ~15 k.y.) before the main transition documented by Hartl and Tauxe (1996). Shortly after the main transition, a large change in declination with a shallow inclination ("rebound") occurs at 51.4 mbsf, which may correspond to the shift reported previously by Clement and Kent (1986) and Quidelleur and Valet (1996). Agreement with this previously reported feature of the Brunhes/Matuyama transition suggests that sediments at this site faithfully recorded the geomagnetic changes. These sediments will thus be useful for studying the detailed behavior of the geomagnetic field during the polarity reversal.