The investigation of magnetic properties at Site 1080 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 175-1080A-4H, 5H, and 6H and Cores 175-1080B-3H and 4H (Table 4).
Magnetic susceptibility measurements were made on whole cores from both holes as part of the MST analysis (see "Physical Properties" section, this chapter). The magnetic susceptibility generally ranges from 1 to 15 x 10–5 (SI volume units; Fig.9).
Measurements of NRM were made on all archive-half core sections from Holes 1080A and 1080B. Sections from Hole 1080A were demagnetized by AF at 10 and 20 mT, and sections from Holes 1080B were demagnetized by AF at 20 mT only.
A primary magnetic component was preserved in sediments from Holes 1080A and 1080B, which allowed the determination of the magnetic polarity. Most of the magnetic overprint could be removed by 20-mT demagnetization, except for below ~40 mbsf at Hole 1080A, where the relatively large scatter of directions suggests the presence of a significant secondary magnetization. The intensity of NRM after 20-mT demagnetization is similar in magnitude at both holes, ranging generally from ~10–4 to ~10–3 A/m (Fig. 10, left panel). The intensity decreases sharply at the polarity boundaries identified at both holes. Below ~40 m at Hole 1080A, the intensity decreases from ~10–4 A/m to ~10–5, where magnetic susceptibility also shows some decrease.
We identified the polarity of the NRM from the declinations and inclinations (Fig. 10, middle and right panels). Based on the bio-stratigraphy (see "Biostratigraphy and Sedimentation Rates" section, this chapter), the upper normal-polarity zone (above 9.5 mbsf at Hole 1080A and above 9 mbsf at Hole 1080B) is identified as the Brunhes Chron (C1n, 0.78 Ma; Berggren et al., 1995), the reversed-polarity zone as the Matuyama Chron, and the lower normal-polarity zone between 41 and 51 mbsf at Hole 1080B as the Jaramillo Subchron (C1r.1n, 0.99–1.07 Ma; Berggren et al., 1995). The Jaramillo Subchron was not observed at Hole 1080B, but the intensity decrease at the bottom of the hole suggests that the termination of the Jaramillo Subchron may occur just below the bottom of the hole. The fluctuation of directions and very low intensities from ~12 mbsf at Hole 1080A is interpreted as the onset of the Brunhes/Matuyama transition. At ~9.8 mbsf, the transition record abruptly terminates, which coincides with the presence of silty bands in the sediment. The bio-stratigraphy suggests the presence of a hiatus at the silty bands. At Hole 1080B, the polarity boundary occurs within a gap between Cores 175-1080B-1H and 2H; the transitional record was not observed.
The average sedimentation rate between the Brunhes/Matuyama boundary and the termination of the Jaramillo Subchron is ~150 m/m.y. The high sedimentation rate at this site in the late Matuyama Chron and the NRM with little magnetic overprint could give us a rare opportunity to study detailed magnetic field behavior during this period of time. A steep decrease in the remanent intensity with a fluctuation of direction occurs at 26 mbsf at Hole 1080A and at 24 mbsf at Hole 1080B. This does not coincide with a low in magnetic susceptibility, which occurs ~0.5 m shallower than the intensity low. This is probably a short normal-polarity event or excursion in the late Matuyama Chron, possibly the Kamikatsura event (0.85 Ma; Harland et al., 1990).