The investigation of magnetic properties at Site 1078 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 175-1078A-3H through 8H, Cores 175-1078B-4H through 14H, and Cores 175-1078C-3H through 14H (Table 6). No cores from Hole 1078D were oriented.
Measurements of NRM were made on all archive-half core sections from Holes 1078A, 1078B, 1078C, and 1078D, except those sections in which severe and pervasive core disturbance was visible. Sections from Hole 1078A were demagnetized by AF at 10 and 20 mT, and sections from Holes 1078B, 1078C, and 1078D 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). Magnetic susceptibility was relatively constant (between ~6 and 9 × 10-5 with depth (SI volume units; Fig. 20). The intensity of NRM after 20-mT demagnetization from the four holes is similar in magnitude, ranging generally from ~10-5 to ~10-3 A/m (Fig. 21, left panel). The NRM intensity shows fluctuations of more than an order of magnitude with depth, despite a relatively constant magnetic susceptibility.
A primary component of NRM is preserved in sediments from all four holes, except for the intervals where sediment flow-in occurred during core recovery. The magnetization of sediment flow-in shows steep positive inclinations after 20-mT demagnetization (Fig. 22). Before the cores were oriented using the Tensor tool data, the declinations were close to zero. This radial-inward direction (vertical upward from split surface of cores; positive-x in ODP archive coordinates) of the flow-in sediments was acquired in a magnetic field of the core barrel and cutting shoe. Some of the magnetic grains were possibly remobilized during sediment fluidization of the flow-in process and were re-aligned in the ambient magnetic-field direction. Roberts et al. (1996) also reported very steep inclinations acquired by flow-in sediments. Measurement of NRM proved to be a useful tool for detecting flow-in sediments in core sections, particularly in zones of uniform lithology where flow-in sediments are not physically apparent. At Site 1078, more than 10 possible flow-in zones were detected from the anomalous magnetization (marked "F" in Fig. 21). This remagnetization was particularly common at Hole 1078B, where it appeared in seven of 14 cores. Characteristics of the remagnetization associated with sediment flow-in, (the radial-inward and steep-downward direction, as well as coercivity >20 mT), are similar to the magnetic overprint acquired during the coring process (coring-induced magnetization [CIM]; see "Paleomagnetism" section, "Site 1077" chapter, this volume). This suggests that CIM is caused by partial remobilization of magnetic particles caused by penetration-generated shear stress. If this is true, neither AF nor thermal demagnetization will be able to remove the CIM because it could be carried by physically re-aligned single-domain and/or pseudo-single domain magnetic particles and thus have a coercivity spectrum similar to that of the primary component of postdepositional remanent magnetization.
We identified the polarity of the NRM from the declination and inclination. Inclinations and declinations from all four holes indicate that only the Brunhes (C1n) normal polarity Chron (Berggren et al., 1995) is recorded in these sediments (an inclination of -23° is expected from the geocentric axial dipole model; see Fig. 21, middle and right panels). In spite of the high sedimentation rate, based on bio-stratigraphy, short reversal events and/or excursions in the Brunhes Chron (such as the Blake event) were not found. Anomalous directions seen in Figure 21 were caused by the sediment flow-in discussed above or by other physical disturbance of the cores.