Paleomagnetic analyses were performed on the whole-round sections for the APC cores and on the archive halves for RCB cores using the three-axis pass-through cryogenic magnetometer. The core sections were first measured for natural remanent magnetization (NRM). They were then subjected to 5-mT and 30-mT field demagnetization in the alternating-field (AF) coils of the cryogenic magnetometer, and their magnetization was successively measured.
APC cores from Hole 1197A were drilled with a nonmagnetic Russian-made PDC bit in the bottom-hole assembly rather than a standard C3RBI bit, resulting in a good magnetic record. Cores from Hole 1197B were drilled with the RCB. Polarity reversal sequences were observed in intervals of good recovery. However, remagnetization is evident and caused difficulty for the interpretation. Nevertheless, some known chron boundaries can be identified when comparing the magnetic signal with biostratigraphic data (Table T6).
In Hole 1197A, NRM intensity generally varies between 10-3 and 10-2 A/m, with an average intensity of 10-2.5 A/m (Fig. F19). The mean intensity drops to 10-3 A/m after demagnetization at 30 mT. Core-top intensity anomalies were also observed and care was taken to not misinterpret these as polarity sequences. The NRM intensity for Hole 1197B is very low, ~10-4.5 A/m (Fig. F20), and remains constant downcore. Even following demagnetization to 30 mT, a strong normal overprint persists throughout the section, even though the intensity was reduced to 10-5 A/m. This caused problems for the identification of polarity intervals.
Inclination and intensity are shown as a function of depth for the entire drilled section of Hole 1197A in Figure F19. Long intervals of negative inclination from 0 to 24 mbsf, positive inclination between 24 and 44 mbsf, and a short interval of negative inclination below 44 mbsf are clearly identifiable (Fig. F19). Within each of the long intervals, a number of short polarity intervals or excursions also appear. When these polarity interval patterns are compared to the geomagnetic-polarity time scale, the long normal and reversed intervals correlate with the Brunhes and Matuyama Chrons, respectively. However, the correlation in this interval is not in agreement with the biostratigraphic data, which would place the base of the Brunhes Chron at ~15 mbsf, between the Jaramillo Subchron (20-24.5 mbsf) and the Olduvai Subchron (43-50 mbsf).
Magnetic measurements in Hole 1197B are highly corrupted by a strong downward overprint. The weak magnetization of the sediments in this hole precludes the separation of the signal from the overprint by AF demagnetization. Therefore, identification of the polarity interval in Hole 1197B is difficult, even though the recovery was high. By distinguishing intervals of predominantly high negative inclination from intervals with partly zero to positive inclination, some reversed intervals could be recognized using the biostratigraphic data as a guide. In this way, it is possible to identify polarity intervals found in the middle to early Miocene (Figs. F20, F21). However, in most cases, it is difficult to locate the exact boundaries of the polarity reversals.
Below 655 mbsf, strongly magnetized basement rocks with an average NRM intensity of 10-1 to 10-0.5 A/m are encountered (Fig. F20D). These strongly magnetized basement cores have a mean inclination of 60° in a reversed direction. Results of complete thermal demagnetization indicated that the principal minerals carrying magnetization are titanomagnetite and hematite. Two components of magnetization have been identified; one with a mean inclination of -30° and another with 64°, the former being a remagnetization and the latter likely representing primary magnetizations. When compared with the Australian apparent polar wander curve, these two inclinations represent ages of 29 and >102 Ma, respectively.
Between 4 and 50 mbsf, NRM, anhysteretic remanent magnetization (ARM), and isothermal remanent magnetization (IRM) demagnetization of six discrete samples from Hole 1197A were measured. The NRM and ARM results are identical for all of the samples. The IRM acquisition curves for some of the samples show no increase after 300 mT, indicating magnetite as the dominant mineral (Fig. F22A, F22B). For some other samples, an increase in acquisition after 300 mT occurs (Fig. F22C, F22D), which is not strong enough for hematite but suggests a magnetic phase having intermediate coercive force between magnetite and hematite. Complementary determination of ratios between ARM at 0.1 T and susceptibility indicates the possible presence of pyrrhotite, which is in good agreement with the signal of IRM acquisition. When the normalized IRM acquisition, IRM demagnetization, and ARM are plotted together as a function of the applied field (Fig. F22), they intersect at values 0.4 to 0.45, which indicates that the magnetite is predominantly single domain with little interaction. For pelagic limestone, this value is ~0.5 and considered to result from noninteracting single-domain magnetite (Johnson et al., 1975).
The IRM 0.1 T:1 T ratio gives a value of about 0.9 for the top 30 mbsf, indicating that magnetite is the dominant magnetic remanence carrier (Fig. F23B). Below 30 mbsf, this ratio drops to 0.75 to 0.81, indicating the presence of other magnetically hard minerals, possibly pyrrhotite (Fig. F23B). The downcore variation in NRM, ARM, and IRM is a function of magnetite concentration (Fig. F23A). This might suggest that dissolution of fine-grained primary magnetite in reducing diagenetic conditions mostly leads to the formation of pyrite and other iron sulfides, both of which decrease the magnetization of the sediments.
Twelve representative discrete samples have been analyzed from Hole 1197B. Because they were less magnetic, no significant interpretations could be made.