The investigation of magnetic properties at Site 1087 included the measurement of magnetic susceptibility of whole-core sections and the natural remanent magnetization (NRM) of archive-half sections. The Tensor tool was used to orient Cores 175-1087A-3H through 27H (except for 17H), 175-1087B-3H through 8H, 175-1087C-4H through 27H (except for 20H), and 175-1087D-1H through 11H (Table 6).
Measurements of NRM were made on all archive-half core sections from Holes 1087A, 1087B, 1087C, and 1087D. All sections were demagnetized by AF at 20 mT. Magnetic susceptibility measurements were made on whole cores from all holes, except 1087D, 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, except for below 432 mbsf at Hole 1087C, where the intensity is ~10–2 A/m. Magnetic susceptibility ranges between 0 and 5 x 10–5 (SI volume units) for the upper 432 mbsf and between 10 and 20 x 10–5 below 432 mbsf. The trends in variations in remanent intensity and magnetic susceptibility are not parallel, except for the sudden increase below 432 mbsf.
APC suffered 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 clustering of declinations around 0° before orientation. In contrast, inclinations showed distinct polarity biases after 20-mT demagnetization, from which an interpretation of the magnetic polarity was possible. However, the uppermost parts of many cores showed anomalous steep negative inclinations, even after the data from physically disturbed sediments were discarded. One explanation is that some of the disturbance was not physically apparent.
XCB cores from Hole 1087C between 250 and 400 mbsf show extremely strong scatter in both declinations and inclinations, and we could not identify the polarity of NRM (Fig. 8C). However, biases in inclination possibly caused by polarity reversals can be recognized. XCB cores below 400 mbsf show a clustering of declinations around 0°, which indicates a radial-inward magnetic overprint. The direction is different from the nonaxisymmetric declination of –20° to –30° observed in XCB cores of previous sites (see "Paleomagnetism" sections, "Site 1081," "Site 1082," "Site 1084," and "Site 1085" chapters, this volume). Inclinations below 400 mbsf are biased toward positive polarity.
We identified the polarity of the NRM mainly from the inclinations (Fig. 8) of APC cores from Holes 1087A and 1087C. Further study of all holes drilled at this site may clarify the preliminary interpretations presented here. Considering constraints from the bio-stratigraphy (see "Biostratigraphy and Sedimentation Rates" section, this chapter), we interpreted the polarity reversal sequence from Chrons C1n to C2Ar (~ 4 Ma). Magnetostratigraphic interpretation is summarized in Table 7 using the time scale of Berggren et al. (1995).
Interpretation of the Brunhes/Matuyama boundary was problematic. Two possible interpretations are presented in Figure 8. If the upper limit for the Brunhes/Matuyama boundary is used (27 mbsf; Table 7), then the Jaramillo event may be responsible for the anomalous directions between 27 and 39 mbsf. The Jaramillo, therefore, is only tentatively interpreted in Figure 8 and Table 7, based on the lower limit for the Brunhes/Matuyama boundary (39 mbsf; Table 7). The biostratigraphy suggests that turbidites may be present within the upper ~60 mbsf (see "Biostratigraphy and Sedimentation Rates" section, this chapter), which may account for the disagreement between the magnetostratigraphy and the biostratigraphic interpretations within this interval. Between ~60 and 200 mbsf, the magnetostratigraphic interpretation agrees well with the biostratigraphy. Between about 200 and 250 mbsf, however, the inclinations are highly scattered and the declinations are clustered around 0° before orientation, which precluded polarity interpretation.