The natural remanent magnetization (NRM) of the archive halves of each core section initially was measured and then remeasured after alternating-field (AF) demagnetization at selected levels. Sections obviously affected by drilling disturbance were not measured. Core 202-1235A-1H was AF demagnetized at peak fields of 15, 20, and 25 mT. Cores 202-1234A-2H through 8H were AF demagnetized at 20 and 25 mT. Cores 202-1235A-9H through 20H and all cores from Holes 1235B and 1235C were only AF demagnetized at 25 mT.
Initial NRM intensities were generally high (Fig. F20), ranging from 0.5 to 2.1 A/m, although a few intervals, discussed below, had significantly lower intensities. The high NRM intensities are due, at least in part, to a drill string magnetization overprinting the natural remanence. This overprint, characterized by steep positive inclinations (averaging +79° in the upper 50 mcd of Hole 1235A), was incompletely removed even by demagnetization at peak AF fields up to 25 mT. After demagnetization, the NRM intensities are lower, with an average of ~0.01 to 0.1 A/m (Fig. F20). The ratio of NRM measured after 25-mT AF demagnetization to the initial NRM, ~0.04, is similar in magnitude to that observed at Site 1234 but about a factor of three lower than that observed at Site 1233. As with Site 1234, the lower ratio indicates that the high initial values are a function of a strong overprinting field (likely due to the drill string) affecting coarser (and more magnetically viscous) siliciclastic sediments. The inclinations are also affected by the drill string overprint, with average values after 20- or 25-mT AF demagnetization reduced to approximately -15° in the upper 50 mcd (Fig. F21). This is much shallower than would be expected for the latitude of Site 1235 (about -55° for an axial geocentric dipole during normal polarity), indicating that a significant drill string overprint remains. Calcareous nannofossil data suggest that the base of the cored sequence is younger than 0.26 Ma (Zone NN21) (see "Biostratigraphy"); therefore, these sediments were deposited/acquired their NRM during the Bruhnes Chron (0-0.78 Ma) and should be of normal polarity (negative inclinations). The positive inclinations observed below 50 mcd (Fig. F22) suggest that the overprint of these sediments is even more severe.
Anomalously large initial NRM intensities were measured in some of the cores from Hole 1235A (Fig. F21A), and the inclinations in these cores even after 25-mT AF demagnetization were still positive (overprinted). These cores were collected when the APCT tool was used. The APCT tool was used on Cores 202-1235A-3H, 6H, 9H, and 12H and on Cores 202-1235B-1H, 4H, and 8H. For Section 2 of each core, mean values for the intensity (NRM0mT) and inclination (Inc0mT) prior to demagnetization, the intensity (NRM25mT) and inclination (Inc25mT) after demagnetization at 25 mT, and the ratio of the demagnetized to undemagnetized intensities (NRM25mT/NRM0mT) were determined (Fig. F23). Subsequently, we have documented that this pattern of behavior is also apparent for the intermittently stronger NRM overprints at Sites 1232, 1233, and 1234 (see Lund et al., this volume).
In Hole 1235C, a nonmagnetic core barrel was used in place of one of the two steel core barrels for odd-numbered cores. The NRM0mT and Inc25mT measurements for the first six cores of Holes 1235C showed an alternating pattern of high NRM0mT and positive Inc25mT in the even-numbered cores (normal core barrel) and relatively low NRM0mT and negative Inc25mT in the odd-numbered cores (nonmagnetic barrel) (Fig. F21). Therefore, nonmagnetic core barrels may substantially reduce the overprint acquired when coring viscous remanent magnetization-susceptible sediments, possibly improving the paleomagnetic data in such settings. The nonmagnetic core barrel was alternated with a normal (magnetic) core barrel for all remaining sites during Leg 202 to further assess its effect on a variety of sediment types.
Inclinations after 25-mT AF demagnetization are normally negative (Fig. F22), but intermittent intervals of very shallow negative or positive inclinations are due to unremoved overprints. The declinations in Figure F22 have been averaged within each core and rotated to a zero average. As noted before at Sites 1233 and 1234, the declinations seem to be much less affected by the drilling overprint. The declination patterns observed may reflect paleomagnetic secular variation (Fig. F22). The declinations can generally be correlated among the three holes at this site (Fig. F24), although not as much detail is initially discernable as was noted at Sites 1233 or 1234. Clearly, caution must be used because this record suffers from magnetic overprints and distinct intervals appear to be affected by time-dependent magnetic dissolution.
Several intervals below 60 mcd have NRM intensities that are more than one order of magnitude lower than the background sediments, from ~0.05 to 2 x 10-3 A/m (Fig. F20). This pattern is analogous to that found at the base of Site 1233 and at Site 1234 and may reflect time-dependent magnetic mineral dissolution associated with early sediment diagenesis.