After alternating-field (AF) demagnetization to 20 mT, the natural remanent magnetization (NRM) of whole-round sections from Hole 1169A was measured at 5-cm intervals using the pass-through cryogenic magnetometer. An exception was made for cores whose liners were deformed, to avoid possible damage to the magnetometer. These deformed sections were measured as archive-half cores. The nonmagnetic core barrel assembly was used for odd-numbered cores, starting with Core 3H. The comparison between results from cores collected with the nonmagnetic corer and those from the standard corer is discussed in the "Appendix" chapter as are results of experiments investigating the effect of core splitting on magnetization and other coring-related magnetic experiments. The Tensor tool was used to orient the APC cores beginning with the third core at each hole, but the variability in the declination values of the cores from Hole 1169A precluded the orientation of cores.
Discrete oriented samples were routinely collected from Hole 1169A; two samples were taken from each working-half core. These samples were used to aid in the interpretation of the long-core record of magnetization by providing additional measurements of polarity and basic magnetic characterization. Most of them were demagnetized at 5, 10, 15, 20, 30, 40, and 50 mT to permit principal component analysis. For rock magnetic characterization, anhysteretic remanent magnetization (ARM) was induced in 0.2-DC and 200-mT AC fields and isothermal remanent magnetization (IRM) in a DC field of 1 T. Some discrete samples were progressively saturated up to 1.3 T to study the hardness of the IRM.
The long-core measurements were predominantly normal (Fig. F7) throughout the hole, which reflected the ubiquitous flow-in disturbance of the hole. The affected cores exhibit extreme deformation, with features extended in the upcore direction (see "Lithostratigraphy"). The associated magnetization appears to be consistently upward. The sediment must therefore be remagnetized in the upward direction by the present field and then oversteepened so that the behavior is not entirely passive, as in the passive markers of structural geology. The extreme upward inclination caused by the deformation serves as a useful marker of sediment plastic deformation.
Many core section extremities presented an anomalous reversal of inclination and intensity increase, part of which appears to record magnetic contamination during section cutting and capping. The strong remagnetization in the normal direction was helpful in the identification of contamination. It appeared important in carbonate sediments marked by a weak intensity on the order of 10-4 or 10-5 A/m. For the remaining sites of Leg 189, where weak intensities are also expected, this effect will have to be taken into consideration in evaluating the reliability of apparent short reversal chrons at the ends of sections.
Sequences of poorly recorded reversals appeared in the upper part of Hole 1169A, where no flow-in was observed (Fig. F7). With the help of biostratigraphic datums (see "Biostratigraphy"), these reversals were identified as being the onset of Brunhes Chron (C1n) at 3.5 mbsf, the termination of Jaramillo Subchron (C1r.1n) at 6 mbsf, the Olduvai Subchron (C2n) between 19.2 and 25.2 mbsf, and the termination of Subchron C2An.1r at 40.6 mbsf.
Most of the progressive demagnetization carried out on discrete samples did not permit identification of reliable paleodirections of the magnetic field. The NRM, ARM, and IRM values and NRM:IRM ratio suggest a detrital origin of the magnetic signal (Fig. F8). Among these cores, Core 189-1169A-22X was marked by a microtektites layer (see "Lithostratigraphy"). Rock magnetic analysis on discrete samples taken from Core 189-1169A-22X confirmed this observation. The ratio of ARM:IRM is high, indicating that the magnetic signal is principally carried by fine particles. The sediment containing the microtektites has acquired a stable magnetization (Fig. F8), that is considerably stronger than other sediments. The presence of microtektites, revealed by this relatively strong intensity, is apparent between 198.5 and 212.2 mbsf. This suggests that the microtektites were initially magnetized and have been magnetically oriented in the past field direction. Microscope observations showed spherical microtektites ~200 µm in diameter. The particles are likely to carry a thermoremanent magnetization acquired during initial cooling that is evidently strong enough to orient them in the water column and in the final sediment.
A reliable magnetostratigraphy could not be established at this site because of the extreme sediment deformation (Fig. F7). The few chrons recognized in the upper part of Hole 1169A (Fig. F8) were dependent upon biostratigraphic tie points (Table T10). Reversed magnetizations are observable at the ends of some sections. But, we did not interpret them because of the sensitivity of these weakly magnetized carbonates to magnetic contamination, which appears to be concentrated at the end of cores. Discrete-sample demagnetization revealed a poor magnetic record, so that even in the absence of the deformation, it would probably have proved difficult to obtain a good magnetostratigraphy.