The investigation of magnetic properties at Site 1084 included the measurement of bulk susceptibility of whole-core sections and of the natural remanent magnetization (NRM) of archive-half sections and discrete samples. The Tensor tool was used to orient Cores 175-1084A-4H through 17H, Cores 175-1084B-16H through 20H, and Cores 175-1084C-16H through 22H (Table 8). Cores 175-1084B-4H through 15H and 175-1084C-4H through 15H were not oriented because of technical problems with the Tensor tool.
Measurements of NRM were made on all archive-half core sections from Holes 1084A, 1084B, and 1084C. APC sections from Hole 1084A were demagnetized by AF at 10 and 20 mT. XCB sections from Hole 1084A and all sections from Holes 1084B and 1084C were demagnetized by AF at 20 mT only. All discrete samples, one per section from Hole 1084A, were demagnetized by AF at 10, 20, 25, and 30 mT. Magnetic susceptibility measurements were made on whole cores from all holes as part of the MST analysis (see "Physical Properties" section, this chapter).
Magnetic susceptibility ranges between 0 and 8 x 10–5 (SI volume units), and intensity of NRM after 20-mT demagnetization ranges between ~10–2 and ~10–4 A/m (Fig. 14). Long-term changes of the remanent intensity are similar to those of the magnetic susceptibility; highs occur from ~220 to 330 mbsf and from ~500 to 600 mbsf. The exception is in the upper 100 mbsf where the remanent intensity is high, but the magnetic susceptibility is low.
A magnetic overprint was generally removed by 20-mT demagnetization, and a primary NRM was recovered for all APC cores (Fig. 14). For XCB cores, however, a significant magnetic overprint remained after AF demagnetization. Declinations of archive-half cores cluster around –20°, independent of the orientation of the sediments, even where the sediments are extensively biscuited (Fig. 14A, Fig. 15A). This phenomenon is similar to that observed at Sites 1081 and 1082 (see "Paleomagnetism" sections, "Site 1081" and "Site 1082" chapters, this volume), where the magnetic overprint was attributed to the coring process. Declinations of discrete samples taken from the working halves cluster around 180° (Fig. 14A, Fig. 15B). This direction is nearly opposite to declinations from the archive halves, indicating a magnetic overprint of a nearly radial-inward direction. Clustering of declinations is weaker in discrete samples than in half-core samples, suggesting that a primary magnetization was preserved in part of the center of cores. In contrast, inclinations showed distinct polarity biases after 20-mT demagnetization. The inclinations are deflected downward from the downward-oriented magnetic overprint, causing upward inclinations to be shallower than expected and downward inclinations to be steeper than expected. The groupings of normal and reversed polarities are more distinct in the discrete samples (Fig. 16).
The inclinations from APC half-core measurements are steeper than those from discrete samples (Fig. 14, Fig. 16). The average inclination of discrete samples during the Brunhes Chron agrees with that expected from the geocentric axial dipole model (–44° at this site), with the correction for the inclination anomaly (+3°) caused by non-dipole components (Merrill and McElhinny, 1983). The steeper inclinations from half-core measurements may be related to deformation of sediments along the rim of cores (see "Paleomagnetism" section, "Site 1083" chapter, this volume).
We identified the polarity of the NRM from the declinations and inclinations of APC cores and from the inclinations of XCB cores. All major polarity chrons from the Brunhes to the latter part of the Gilbert (~4.4 Ma) were identified at Hole 1084A. Magnetostratigraphic interpretation is summarized in Table 9. The time scale of Berggren et al. (1995) was used. The identification of the Jaramillo Subchron in XCB cores from Hole 1084A at depth was aided by its identification in the APC cores from Hole 1084C, which contained less magnetic overprinting, and supports the polarity interpretations based on inclinations from XCB cores.
Transitional records of the Brunhes/Matuyama polarity change were obtained from Holes 1084B and 1084C. At Hole 1084A, the transition record was unfortunately missing because of a gap in recovery between cores.
In spite of the high sedimentation rate of ~200 m/m.y., no short polarity-reversal event was detected during the Brunhes Chron. Most anomalous directions of NRM seen in Figure 14 were caused by physical disturbance of the sediments that occur at core or section boundaries. Some anomalous directions occur in the middle of sections, but they do not duplicate between holes.