The natural remanent magnetization (NRM) of archive core sections of RCB cores from Site 1196 was measured at 5-cm intervals, where practical, using the pass-through cryogenic magnetometer. Many sections were not measured in their entirety because they contained fragments that had possibly rotated within the core barrel. In Hole 1196B, an additional measurement difficulty was that the diameter of the recovered ADCB cores was too large to fit in the magnetometer. To measure these cores intact, intervals <6 cm in length were rotated so that they would fit into the magnetometer. The downhole +z-component then became the +y-component of the instrument. This reorientation has been noted in the ODP database. Unfortunately, this precluded the use of the most favorable longer intervals from measurement. NRM was measured throughout using a 5-mT and 30-mT alternating-field (AF) demagnetization.
In addition, discrete samples were collected from Hole 1196A. These were used to aid the interpretation of the long-core record of magnetization by providing additional measurements of polarity and basic magnetic characterization.
The principal difficulty at Site 1196 was poor recovery. The sediments were, for the most part, reasonably magnetized with average NRM intensities of 10-3 A/m, and the magnetization showed well-defined trends on AF demagnetization (Fig. F34). The characteristic remanent magnetization (ChRM) appeared after 30-mT demagnetization. The inclination and the total intensity are presented for Hole 1196A in Figure F35. The poor recovery precludes detailed magnetostratigraphic interpretation, although intervals of reversed polarity predominate toward the top of the recovered section. The magnetization intensity increases markedly in some of the red-stained lithologies and in the dark material at the bottom of the hole.
Data from 1196B were collected on RCB cores to a depth of 104 mbsf and below, for an interval of ~100 m on cores drilled with the ADCB. Because the sampled ADCB cores were measured in an unorthodox configuration, only the y-component is plotted and, in this coordinate system, a positive y is equivalent to a reversed polarity. The results are shown in Figure F36 with the inclination from Hole 1196A for the uppermost 200 mbsf. Despite some intervals in which there are conflicting results between Holes 1196A and 1196B, the data from Hole 1196B are consistent with the interpretation of a predominantly reversed polarity in this interval.
A relatively large number of discrete samples were studied at this site in an attempt to correlate magnetic properties with the range of lithologies encountered and to investigate the possible expression of diagenesis in magnetic properties and magnetization. Standard rock magnetism analysis was carried out on discrete samples (see "Paleomagnetism" in the "Explanatory Notes" chapter), and samples with sufficiently strong intensity of magnetization were subjected to principal component analysis. Of the few samples magnetized strongly enough to permit analysis of NRM, most were very stable against AF demagnetization. An extreme example of one of the red-stained intervals is shown in Figure F37.
The downhole variation of NRM, anhysteretic remanent magnetization (ARM), and isothermal remanent magnetization (IRM) reveals that in the uppermost 200 mbsf the sediments generally have a relatively low magnetic content and are weakly magnetized (Fig. F38). The ratio of IRM acquired in a 100-mT field to IRM is close to one in almost all of the samples measured, indicating magnetite as the dominant phase. The notable exception is the interval between 610 and 626 mbsf, where the red-stained lithologies predominate and where intensity values are extreme (Fig. F39). The ratio of IRM after 40-mT demagnetization to the initial IRM exhibits overall values near to one-tenth, with the only exception again in the interval between 610 and 626 mbsf where values reach one. This pattern is consistent with the dominance of magnetite in most of the section, and hematite as the dominant magnetic phase between 610 and 626 mbsf.
Beneath the red cemented sequence, Sample 194-1196A-70R-3, 94-96 cm, comes from the dark siliciclastic unit at the base of the recovered section and appears to be an excellent paleomagnetic recorder, as indicated by its AF demagnetization behavior. The acquisition of IRM suggests that magnetite is the dominant magnetic phase present.
Because of good recovery at Site 1199, it was possible to produce a limited magnetic stratigraphy. The average magnetization of Site 1199 sediments is ~10-3.5 A/m. Intensities vary between 10-4.5 and 10-2.5 A/m. These variations tend to occur across polarity transitions; however, a remarkable upcore intensity increase of two orders of magnitude occurs at about 103 mbsf (Fig. F40B). This level coincides with the change from Subunit IA to Subunit IB (see "Lithostratigraphy and Sedimentology"). It also coincides with a polarity transition from reversed to normal. The sediments showed a well-defined and stable magnetization trend on AF demagnetization (Fig. F41). As a result, we obtained what appears to be a ChRM with a slight normal overprint isolated between 20- and 30-mT demagnetization steps.
Measured inclination and total intensity after 30-mT demagnetization are presented for Site 1199A in Figure F40. Because data were collected on RCB cores, the data sets are shifted toward normal polarities as seen at Site 1196. Intervals with highly negative inclination were considered normal, whereas intervals with gentle-negative to positive inclination were considered reversed polarity. Poor core recoveries make interpretation difficult for a few intervals, as shown by hatched rectangles in F40. For all these cases, observed polarity continues across nonrecovery intervals.
Data from Site 1196, where intensive rock magnetic studies were performed, indicate that for the top 150 mbsf, the dominant magnetic mineral carrying remanence is magnetite. Some rock magnetic analyses performed at this site showed magnetic behavior similar to that of Hole 1196A. The magnetite is characterized by small values of R (point of intersection of IRM acquisition and IRM demagnetization), indicating the interacting nature of the magnetite, which suggests a diagenetic history perhaps related to zones of dolomitization (see "Lithostratigraphy and Sedimentology"). No effect of this history on the direction of magnetization has been observed. Instead, preliminary results suggest that the dolomitization has no impact on the preservation of the original ChRM.