We measured the natural remanent magnetization (NRM) of most archive-half sections from Hole 1135A every 5 cm with the pass-through cryogenic magnetometer. Only cores with low recovery or high deformation caused by the rotary drilling method were not measured. After NRM measurements, all sections were partially demagnetized at 10 and 20 mT to remove overprints so that geomagnetic field changes could be characterized. Additional alternating field (AF) demagnetization at 5 and 15 mT was adopted for a few sections. Some sections were AF demagnetized up to 40 mT. We routinely collected two oriented discrete samples (7 cm3), 40 per core section (except for highly disturbed sections). Some samples were subjected to stepwise AF demagnetization at 5- and 10-mT increments up to 70 mT. Because all cores were drilled by the RCB method, only inclination values are useful in paleomagnetic studies at Site 1135.
NRM intensities range between 9.81 × 10-1 and 8.53 × 10-6 A/m (Table T5). The average and median values of NRM intensity are 2.00 × 10-3 and 8.38 × 10-4 A/m, respectively. We observe strong NRM intensities at one or more sampling points; these correspond to rock pebbles contained in nannofossil ooze (Unit II, see "Lithostratigraphy"). We also observe scattered directions of the remanent magnetizations and high susceptibilities at the same depth (see "Physical Properties"). Directions from strong NRM contain no paleomagnetic information because we assume that the rock pebbles are ice-rafted debris that fell down the hole from higher stratigraphic levels during drilling (marked by arrows in Fig. F8). Susceptibilities of sediments from Hole 1135A measured with the MST are for the most part negative (indicating diamagnetic material), which is characteristic of carbonate-dominated sediments and corresponds to weak intensities in the sediment. The median susceptibility value is negative in each subunit (Table T5). Reliable paleomagnetic data were obtained from the middle part of Subunit IIA, the upper part of Subunit IIIA, and Subunit IIIC, in which the median value of susceptibility was higher than in the other subunits. Weaker magnetization and/or drilling disturbance caused by RCB coring, however, results in less reliable paleomagnetic information.
We used orthogonal projection plots of progressive AF demagnetization (Fig. F9) to check the reliability of the direction of the remanent magnetization. Reliable paleomagnetic directions were obtained after AF demagnetization at 20 mT (Fig. F9). The median destructive field during stepwise AF demagnetization is in most cases <5 mT (Fig. F9). Discrete sample measurements were not useful because the intensity of NRM of discrete samples is less than the sensitivity of the shipboard pass-through magnetometer (Fig. F9). Directions of remanent magnetization after each AF demagnetization were scattered, and intensities of remanent magnetization did not decrease. Unreliable data, believed to be caused by weak remanent intensities or drilling disturbance, are characterized by scattered or shallow inclinations of remanent magnetization after AF demagnetization at 20 mT.
Because of the low NRM intensities and core disturbance, especially in the upper 100 m, we carefully selected directional data from the cryogenic magnetometer. The selection criteria were that (1) the intensity of remanent magnetization after AF demagnetization at 20 mT was >2 × 10-4 A/m and hence above the noise level of the magnetometer in rough-sea conditions, (2) the inclination was >30°, (3) at least three consecutive values (which corresponds to a 15-cm length of split core) had the same polarity, and (4) there was no significant core disturbance, as described in "Paleomagnetism" in the "Explanatory Notes" chapter.
Selected inclinations from Hole 1135A (Fig. F8) suggest that the normal and reversed chrons between ~180 and 200 mbsf are early Eocene in age according to the biostratigraphy (see "Biostratigraphy"). Both Sections 183-1135A-20R-CC (187 mbsf) and 21R-CC (199 mbsf) lie within nannofossil Zones CP11 and CP10 (see "Biostratigraphy"). The foraminifers indicate that Sections 183-1135A-20R-CC and 21R-CC are within foraminifer Zone AP7. This section correlates with Chron C22 or C23. We found two reversals in the same section (Section 183-1135A-20R-4) between 184.9 and 186.4 mbsf.
The normal and reversed sequence between 252 and 261 mbsf can be correlated to chrons near the K/T boundary. We correlate the proposed reversal in Core 183-1135A-28R at 260 mbsf with the C29n to C29r reversal (Fig. F10), based upon the biostratigraphic results (see "Biostratigraphy"). Sections 183-1135A-27R-CC (252 mbsf) and 28R-CC (267 mbsf) lie within Paleocene nannofossil Zone NA6 and within Maastrichtian (Late Cretaceous) foraminiferal Pt. elegans subzone, respectively. This geomagnetic reversal is observed at 259.7 mbsf, 30 cm above the boundary between white and light green sediment (see "Lithostratigraphy"). The intensity drop at the geomagnetic transition zone at 259.7 mbsf (Fig. F10) may be caused by a low content of magnetic minerals or a drop in the intensity of the geomagnetic field that occurred during the geomagnetic reversal. A peak in susceptibility between 260.0 and 260.2 mbsf is probably caused by the lithologic change at this depth.
It is difficult to unambiguously identify chrons between 360 and 445 mbsf as a result of the low recovery and the low resolution of high southern latitude biostratigraphic data. Nevertheless, we interpret the reversed polarity interval between 402 and 413 mbsf, the normal polarity intervals at ~430 and ~440 mbsf, and the normal polarity interval at ~490 mbsf to lie between Chrons C31r and C34n. We base this interpretation upon Sections 183-1135A-46R-CC (433 mbsf) and 47R-CC (441 mbsf) lying within the Campanian and the age of Sections 183-1135A-51R-CC (478 mbsf) and 52R-CC (491 mbsf) being Coniacian according to nannofossil studies (see "Biostratigraphy"). We correlate the normal polarity interval at ~490 mbsf to Chron C34n.