The investigation of magnetic properties at Site 1085 included the measurement of bulk susceptibility of whole-core sections and the natural remanent magnetization (NRM) of archive-half sections and discrete samples. The Tensor tool was used to orient Cores 175-1085A-16H through 27H, 175-1085B-3H through 26H, and 175-1085B-31H through 35H (Table 6). Cores 175-1085A-4H through 15H, 175-1085A-28H through 33H, and 175-1085B-27H through 30H were not oriented because of technical problems with the Tensor tool.
Measurements of NRM were made on all archive-half core sections from Holes 1085A and 1085B. APC core sections from Hole 1085A were demagnetized by AF at 10 and 20 mT. XCB sections from Hole 1085A and all sections from Holes 1084B were demagnetized by AF at 20 mT only. Measurements of discrete samples were performed only when half-core measurements indicated strong enough intensities to be measured on board after AF demagnetization (~10–4 A/m in half-core measurements). Discrete samples from APC cores were demagnetized by AF at 10, 15, 20, and 25 mT, and those from Cores 175-1085A-58X through 64X were demagnetized at 0, 10, 20, 25, 30, and 35 mT. Magnetic susceptibility measurements were made on whole cores from all holes as part of the MST analysis (see "Physical Properties" section, this chapter).
The intensity of NRM after 20-mT demagnetization is between ~10–3 and 10–5 A/m in the upper 537 mbsf (Fig. 14) and abruptly increases to ~10–2 A/m below 537 mbsf. The magnetic susceptibility shows a similar sudden increase from ~10 to 40 x 10–5 (SI volume units) at ~540 mbsf. Except for this, long-term variations of remanent intensity and magnetic susceptibility do not coincide. For example, magnetic susceptibility increases below ~400 mbsf, but remanent intensity does not.
Except for the uppermost ~40 mbsf, the APC cores show significant coring-induced magnetization (CIM) with a radial-inward direction (see "Paleomagnetism" sections, "Site 1077" and "Site 1081" chapters, this volume). The CIM is evident from the disagreement of the declinations between (archive) half-core measurements and discrete samples from working halves: the former tend to cluster around 0° and the latter around 180° before orientation (Fig. 14A). In contrast, inclinations showed distinct polarity biases after 20-mT demagnetization, from which we could interpret the polarity (Fig. 14A, right panel).
Below ~430 mbsf at Hole 1085A, the declinations of XCB cores cluster at ~–20°, independent of the orientation of the sediments. This direction was pervasively observed in all XCB cores at previous sites (see "Paleomagnetism" sections, "Site 1081," "Site 1082," and "Site 1084" chapters, this volume). Below 537 mbsf, where the remanent intensity is strong (~10–2 A/m), inclinations of discrete samples clearly separated into positive and negative groupings after AF demagnetization (Fig. 15). We interpret these distinct groupings as the polarity of NRM. Between 430 and 537 mbsf, some structures are visible in the inclination data of half-core measurements, but it is difficult to assign a polarity. The remanent intensity of this depth interval is too weak to measure the discrete samples on board. Between 305 and 430 mbsf, the tendency of the declinations to cluster around –30° is very weak, suggesting a weaker magnetic overprint. However, the inclinations did not show any polarity bias and precluded a magnetostratigraphic interpretation.
We identified the polarity of the NRM mainly from the inclinations of APC cores (Fig. 14). The major polarity chrons from the Brunhes to the earliest part of the Gilbert (~5.5 Ma) could be identified, although the quality of the record is not good because of the severe magnetic overprint. The magnetostratigraphic interpretation is summarized in Table 7. The time scale of Berggren et al. (1995) was used. This interpretation agrees well with the biostratigraphic ages (see "Biostratigraphy and Sedimentation Rates" section, this chapter).
The inclinations of discrete samples from XCB cores below 537 mbsf suggest that the sediments recorded more than 10 polarity reversals. Biostratigraphic ages range from ~11 to 15 Ma. Magnetic polarity reversals were frequent during this period of time, which supports our interpretation of the inclinations, although it is difficult to correlate it with the time scale at present.