The investigation of magnetic properties at Site 1079 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 175-1079A-4H through 14H, 175-1079B-3H through 14H, and 175-1079C-4H through 14H (Table 5).
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 ranges from ~5 to 15 x 10–5 (SI volume units; Fig. 10) and is relatively constant with depth, except for a low between ~3 and 15 mbsf.
Measurements of NRM were made on all archive-half core sections from Holes 1079A, 1079B, and 1079C. Sections from Hole 1079A were demagnetized by AF at 10 and 20 mT; sections from Holes 1079B and 1079C were demagnetized by AF at 20 mT only.
A primary magnetic component was preserved in sediments from all three holes. The intensity of NRM after 20-mT demagnetization from the three holes is similar in magnitude and trend, ranging generally from ~10–4 to ~10–2 A/m (Fig. 11, left panel). Fluctuations of about an order of magnitude are superimposed on an overall decreasing trend with depth. The large intensity variation contrasts with the relatively constant magnetic susceptibility.
The variation pattern of remanent intensity at Site 1079 closely resembles that at Site 1078 (see "Paleomagnetism" section, "Site 1078" chapter, this volume). Between the two horizons of nannofossil events (at 0.09 and 0.26 Ma; see "Biostratigraphy and Sedimentation Rates" section, this chapter, and "Site 1078" chapter, this volume), sediments at Sites 1078 and 1079 can be correlated using the remanent intensity (Fig. 12). The intensity of NRM can be controlled by the strength of the geomagnetic field, the concentration of magnetic minerals, and other rock-magnetic characteristics of sediments including composition, grain size, and interaction of magnetic minerals. If the sediments prove to be uniform rock-magnetically, variations of remanent intensity seen after normalizing the abundance of magnetic minerals using rock-magnetic parameters could be interpreted as relative changes of past geomagnetic-field strength (paleo-intensity). Because relative paleointensity variation during the last ~200 k.y. is relatively well understood (Yamazaki and Ioka, 1994; Guyodo and Valet, 1996), paleointensity can be used as a tool to correlate and estimate the age of sediments (paleointensity stratigraphy). If we assume that the relatively constant magnetic susceptibility of sediments from Sites 1078 and 1079 implies rock-magnetic homogeneity, ages of ~0.11 and ~0.19 Ma may be tentatively assigned to the remarkable lows in remanent intensity (labeled "A" and "B" in Figure 12) by comparison with the established magnetic paleointensity curve. This approach, however, must be thoroughly tested by postcruise rock-magnetic studies.
Magnetic inclinations and declinations from all three 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 (Fig. 11, middle and right panels). Whether the Brunhes is complete cannot be determined from the magnetostratigraphy.
Possible short reversal events and/or excursions in the Brunhes Chron, such as the Blake event, were not found at this site, in spite of the high sedimentation rate (~500 m/m.y.). Most anomalous directions seen in Figure 11 occur at boundaries of cores and/or sections and, thus, probably are caused by physical disturbance of the cores. One possible cause for the absence of short reversal events is extensive bioturbation of sediments at this site. Many deep burrows, some of which continue vertically for more than 1 m, are present in the cores. This suggests that acquisition of postdepositional remanent magnetization (pDRM) occurs over a very wide zone (possibly more than 1 m) in shallow-water sediments, which reduces the resolution of past geomagnetic-field variations and filters out short reversal events. This contrasts with deep-sea sediments, where the lock-in depth of pDRM is generally considered to occur within ~20 cm (deMenocal et al., 1990), or possibly within a few centimeters below seafloor (Tauxe et al., 1996), and low sedimentation rates limit the resolution.