At Holes 899A and 899B, magnetic measurements using the pass-through cryogenic magnetometer were performed on all archive halves of cores, and the magnetic susceptibility of all cores was measured at 3-cm intervals. Sections of sediment were progressively demagnetized at 10-cm intervals using peak fields of 10 or 15 mT, as well as sections of the serpentinite breccias at 2- to 5-cm intervals with a peak field of 15 mT. Thirty-six discrete samples taken from the working halves of cores from the two holes were progressively alternating-field (AF) and thermally demagnetized to verify the magnetostratigraphic results from the cryogenic magnetometer.
As drilling at Hole 899A started with drill pipe that had not been used for a relatively long time, many of the cores from Hole 899A exhibit strong signs of drilling-induced remagnetization, as evidenced by the ubiquitous normal polarity and almost vertical inclinations in the NRM measurements. Fortunately, AF demagnetization to 15 mT can generally remove this strongly induced, but relatively "soft," over-print and isolate the stable component of magnetization. At Hole 899 A, core recovery averaged only 33%, and the first four cores were highly disturbed by drilling. Pass-through cryogenic magnetometer measurements identified five magnetic reversals from 160.0 to 210.0 mbsf (Fig. 28). According to the shipboard biostratigraphic age determinations (see "Biostratigraphy" section, this chapter), sediments at about 162.0 mbsf may be 8.3 Ma old (late Miocene). This suggests that the shift from normal to reversed polarity at 160.5 mbsf may fall within the C4A Chron (8.91-8.02 Ma). An exact polarity chron assignment, however, is impossible at this time because of the absence of adequate age control. Below 210 mbsf, sediments are too weakly magnetized to be measured reliably.
The magnetic intensity of sediments from Hole 899B was consistently very weak, essentially the same magnitude as the noise level of the cryogenic magnetometer and the sample holder. A weak, and magnetically "soft", reversed remanent magnetization was observed in Core 149-899B-6R (273.4-281.5 mbsf), but will have to await shore-based work for confirmation. Between 282 and 320 mbsf, the NRM intensities decrease to only 0.1 mA/m, and the measurements of magnetic directions become noisy and inconsistent. Discrete sample demagnetizations show no stable remanence. Therefore, it was not possible to determine the polarity of sediments recovered from this interval on board the ship. From 323 to 360 mbsf, the NRM intensities increase by an order of magnitude. The inclinations after AF demagnetization suggest a period of predominantly normal polarity. Currently, the poor recovery in Hole 898A and weakly magnetized sediments in Hole 899B do not permit us to construct a magnetostratigraphy.
From 369.9 to 482.9 mbsf in Unit IV (see "Lithostratigraphy and Petrology" section, this chapter), magnetic remanence in both pass-through and discrete sample measurements is apparently very stable. Eight samples were AF-demagnetized using peak fields ranging from 40 to 90 mT, depending on the coercivity of the sample. In addition, 14 samples were thermally demagnetized up to 600°C. A stable component of magnetization usually can be identified by both techniques (Fig. 29). We were surprised that, with the exception of small pieces of core, this interval shows a stable normal field direction. Although a normal polarity remagnetization might conceivably explain this feature, we deem this unlikely based on the magnetic behavior of cores from below this interval and on the preliminary shipboard biostratigraphic ages. Biostratigraphic ages suggest that cores below 364.6 mbsf are of pre-Latest Cretaceous age, and preliminary paleomagnetic results from Core 149-899B-28R (484.2 mbsf) show negative inclinations (reversed polarity?). Hence, the rocks from 369.8 to 482.9 mbsf may have acquired remanent magnetization during the Cretaceous Long Normal Superchron (83-124 Ma). However, the relatively highly oxidized material in this interval indicates the magnetization in these rocks may be complex and renders any polarity evaluation suspect. The age and origin of the magnetic signal in this interval are not well understood at this time.
Below 482.9 mbsf, paleomagnetic results that are useful for magnetostratigraphic studies were obtained only from some of the discrete samples taken from long continuous pieces of cores. As shown in Figure 30, thermal demagnetization of Samples 149-899B-28R-1, 86-88 cm, and -30R-2, 8-10 cm, removed a "soft" component, probably of viscous origin, at low to intermediate temperatures (200°-400°C). After further demagnetization up to 585°-600°C, the stable component of magnetization was revealed. The unblocking temperatures and coercivities indicate that the dominant magnetic mineral in these samples is magnetite.
There appear to be two polarity reversals between 484.2 and 549.9 mbsf (Fig. 31), as suggested by the inclination changes in some thermally demagnetized discrete serpentinite samples from the mass flow deposits of Subunit IIIB (see "Lithostratigraphy and Petrology" section, this chapter). We do not yet know when they occurred.
Figure 32 shows the downhole profiles of magnetic susceptibility for Holes 899A and 899B. In Unit I (see "Lithostratigraphy and Petrology" section, this chapter), the magnetic susceptibilities are about 3 × 10-4 SI units, similar to those observed at Sites 898 and 897.
Several susceptibility maxima were observed within this unit, most of which correspond to the terrigenous sands at the bases of turbidite sequences. The magnetic susceptibility of the pelagic sediments from 230 to 360 mbsf (Cores 149-899B-1R to -14R, in Unit II) was low, averaging about 1 × 10-4 SI units, which is fully consistent with the observed weak NRM intensities from these cores.
A sharp increase in susceptibility occurs around 364 mbsf (Section 149-899B-15R-3), which corresponds to the boundary between Subunits IIIA and IIIB (see "Lithostratigraphy and Petrology" section, this chapter). A similar pattern was observed at Holes 897C and 897D. Whole-core susceptibility measurements for Hole 899B vary in a similar fashion to the NRM signal. The magnetic susceptibility of Units III and IV averages about 1 × 10-4 SI units, which is about two orders of magnitude higher than those of overlying Unit II (Fig. 32).