The investigation of magnetic properties at Site 1077 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 Holes 1077A and 1077B, starting with the fourth core (Table 7).
Measurements of NRM were made on all archive-half core sections from Holes 1077A, 1077B, and 1077C. Sections from Hole 1077A were demagnetized by AF at 10 and 20 mT, and sections from Holes 1077B and 1077C were demagnetized by AF at 20 mT only.
Magnetic susceptibility measurements were made on whole cores from all three holes as part of the MST analysis (see "Physical Properties" section, this chapter). Magnetic susceptibility is on the order of 10-5 (SI volume units; Fig. 11).
The intensity of NRM after 20-mT demagnetization from Holes 1077A and 1077B is similar in magnitude, ranging from ~10-5 to ~10-3 A/m (Fig. 12, left panel). Within the upper 80 mbsf, the intensity is on the order of 10-3 to 10-4 A/m. Between 80 and 140 mbsf, the intensity fluctuates between ~10-4 A/m and ~10-5 A/m, after which it remains relatively constant (on the order of 10-5 A/m) with depth. The magnetic susceptibility does not show these trends but remains fairly constant at about 4 x 10-5 throughout Holes 1077A and 1077B.
A primary magnetic component was preserved in the majority of the sediments from Holes 1077A and 1077B, which allowed us to determine the magnetic polarity. Directions of the NRM below ~120 mbsf, however, show relatively large scatter, suggesting the presence of secondary magnetization. Part of a magnetic overprint acquired during coring still remains after 20-mT demagnetization (see below). Viscous remanent magnetization and/or chemical remanent magnetization caused by diagenetic growth or dissolution of magnetic minerals may also contribute to the secondary magnetizations.
We identified the polarity of the NRM from the declinations and inclinations. Data from the Tensor orientation tool were available for most of Holes 1077A and 1077B, which facilitated interpretation of reversals in terms of the geomagnetic time scale. Changes of inclination with polarity transitions were difficult to interpret because of the relatively low latitude of this site (an inclination of -23° is expected from the geocentric axial dipole model) and the magnetic overprint (Fig. 12, right panel).
The Brunhes/Matuyama polarity transition (0.78 Ma; Berggren et al., 1995) occurs between 115 and 120 mbsf at Hole 1077A and between 112 and 118 mbsf at Hole 1077B (Fig. 12, middle and right panels). The thickness of sediments, which records a polarity transition, should be ~1.4 m at these holes, assuming that the sedimentation rate is ~140 m/m.y. and a polarity transition completes within 10 k.y. The large scatter of the remanent directions and the presence of unremoved secondary components, however, made it difficult to determine the exact position of the boundary. Below the Brunhes/Matuyama boundary, an interval of relatively well-grouped northward declinations occurs from ~128 to 137 mbsf at Hole 1077A and from ~128 to 138 mbsf at Hole 1077B. We tentatively interpret this as the Jaramillo Subchron (C1r.1n). This interpretation, however, does not agree with biostratigraphic data (see "Biostratigraphy and Sedimentation Rates" section, this chapter). There is a possibility that this interval contains coring magnetization, as discussed below. The scatter of directions is very large from ~137 to 155 mbsf at Hole 1077A and from 138 to 160 mbsf at Hole 1077B; thus, it is difficult to determine the polarity of magnetization.
At this site, we encountered significant magnetic overprinting caused by the coring process. Figure 13 shows the NRM of Cores 175-1077B-17H through 22H (below 148 mbsf). Within each core, the inclination becomes positive and steeper upcore with a parallel increase in intensity. Deeper sections in each core show relatively well-grouped declinations, but shallower sections show larger scatter. These tendencies emerge at about 40 mbsf and become more pronounced with depth below seafloor.
Physical deformation of sediments caused by the core liner being dragged along the sediment during coring and/or sediment expansion after recovery can affect the direction and intensity of NRM (Shipboard Scientific Party, in press). Figure 14A illustrates that the effect of drag along the core-liner on the magnetic inclination is opposite in the archive and working halves: in this example, inclination steepens within the working half and shallows within the archive half as a result of coring disturbance along the rim. The angle between the split surface of the cores and the horizontal component of NRM varies with the orientation of cores. The orientation and intensity of the overprint are thus expected to depend on the orientation of cores in this model. In Holes 1077A and 1077B, however, the inclination always shifts to positive and downward in the upcore direction, independent of the core orientation. It is also difficult to explain the increase in intensity within each core by this model. It is thus concluded that this type of deformation is not responsible for the magnetic overprint.
During previous Ocean Drilling Program (ODP) legs, coring-induced magnetization (CIM) of radially inward and steep-downward directions has often been reported in sediments recovered with the APC (e.g., Curry, Shackleton, Richter, et al., 1995; Shipboard Scientific Party, 1996). During some legs, the CIM was much stronger in intensity than the primary NRM, and it could not be erased by AF demagnetization. This made it impossible to recover a meaningful geomagnetic signal from the sediments. Efforts to seek the cause and solution of the CIM have been made during recent legs (Fuller et al., 1997; Fuller and Garrett, in press). The magnetic overprint observed at this site could be explained by an upward increase in the CIM within each core. The acquisition of a steep downward overprint makes the inclination become positive and steeper, and it is independent of core orientation because of the radially inward component (Fig. 14B). Upcore increase of the acquisition of CIM suggests that a shearing stress during penetration under a magnetic field from the core barrel and the cutting shoe could produce the CIM, because sediments in the upper part of each core suffer from shear for a longer period of time than those from the lower part. This model can also explain the fact that the CIM is less evident in softer sediments at shallower depths below the seafloor.
To our knowledge, this is the first report of within-core variation of CIM, which could facilitate a better understanding of the origin of CIM. The upcore increase of intensity was already documented as an anomalous feature of the CIM during Leg 154 (Shipboard Scientific Party, 1995), but no trend in remanent directions was found because the very strong CIM dominated the NRM.