PALEOMAGNETISM

We took pass-through magnetometer measurements on all split-core archive sections. Sediment cores were measured at 5-cm intervals. Coherent basalt pieces that could be oriented unambiguously with respect to the top were measured at 1-cm intervals. We took pass-through magnetic susceptibility measurements on all unsplit core sections at 4-cm intervals.

In order to isolate the characteristic remanent magnetization (ChRM), we subjected the cores to alternating-field (AF) cleaning. The number of AF demagnetization steps and peak-field intensity varied depending on lithology, the natural remanent magnetization (NRM) intensity, and the amount of time available. On average, sediment half-cores were demagnetized using three AF steps in addition to the measurement of NRM. The basalt half-cores were demagnetized using a minimum of six AF steps. The maximum applied field ranged between 20 and 50 mT. We analyzed the results in Zijderveld and stereoplot diagrams, and, where possible, we calculated the ChRM direction using principal component analysis (Kirschvink, 1980). Examples of good-quality AF demagnetization results are shown in Figure F88.

Magnetic susceptibility and NRM intensity variations through the sedimentary section are closely correlated (Fig. F89; Table T12). In general, we find that the magnetic properties of sediments recovered from Hole 1183A correlate with the lithostratigraphy (see "Lithostratigraphy"). Below we describe the magnetic characteristics of the lithostratigraphic subunits:

1. Subunits IA and IB have low NRM intensity and magnetic susceptibility. The few discrete peaks of high NRM and susceptibility values could in many cases be tied directly to the visible presence of pyrite nuggets. By analogy to previous studies of sediments from the Ontong Java Plateau (ODP Leg 130), we speculate that primary magnetite in these subunits has suffered diagenetic reduction to pyrite, thereby destroying the primary remanent magnetization. For these reasons, reliably defining the primary remanent magnetization of Subunits IA and IB was impossible.
2. Subunit IC has relatively high NRM intensity and magnetic susceptibility, caused by the presence of numerous ash beds with high concentrations of magnetic minerals. Glass from the ash layers appears fresh in smear slides, indicating only minor diagenetic alteration. Burrowing has disseminated the ash layers of Subunit IC. The subunit proved ideal for paleomagnetic work, leading to precise determinations of the ChRM direction.
3. Recovery from Subunit IIA was low. Although the NRM intensity of the intervals that could be measured was low, we were able to define the ChRM direction from the few intact cores.
4. Subunit IIB has high NRM intensity and magnetic susceptibility, related to input of volcanic material. Although Subunit IIB is somewhat altered, the ChRM direction is easily defined.
5. Subunit IIIA has low NRM intensity. Once again, the ChRM direction is easily defined using stepwise demagnetization.
6. Subunit IIIB contains dark clay-rich chalks with goethite. We suspect the magnetic remanence in these black chalks to be secondary and have chosen not to include these data in our analysis.

Based on biostratigraphic data (see "Biostratigraphy"), we were able to correlate certain parts of the magnetic polarity recorded in the sediments with the geomagnetic reversal timescale (Berggren et al., 1995). In particular, the polarity intervals in Cores 192-1183A-17R to 25R could be tied with reversals in the late Eocene-early Oligocene (e.g., Chrons C12N and C12R). We also obtained reasonable correlations of polarity intervals in Cores 192-1183A-32R to 48R with the middle Eocene-Campanian polarity chrons. In some cases, polarity reversals were recorded in an individual core (e.g., Core 192-1183A-39R contains the Cretaceous/Paleogene boundary; Fig. F90), and we were able to tie the depth interval closely to the geomagnetic polarity timescale. Elsewhere, entire cores recorded a single polarity, and we had difficulty uniquely identifying the corresponding chron. Figure F90 shows the polarity intervals recorded in Cores 192-1183A-36R through 41R and our proposed correlation with the polarity timescale.

The high quality of paleomagnetic data from many of the sedimentary cores allowed a precise determination of the ChRM using principal component analysis. For the analysis of paleolatitudes we accepted only those directions that had maximum angular deviation values <5°. Several cores (e.g., Core 192-1183A-21R; Fig. F91) showed consistent magnetic directions (declination and inclination values) over intervals of 15 cm (i.e., four sample points at 5-cm sampling intervals) to >1 m. We used the method of Kono (1980) on data from these consistent intervals to calculate the mean paleoinclination for each core. Where there were insufficient consistent values for an individual core or the core was short, we combined inclination values from successive cores to produce a paleoinclination for that depth interval. We obtained paleolatitudes from the inclination data using the axial dipole assumption

tan (latitude) = 0.5 × tan (inclination).

In determining paleolatitude, we have interpreted positive inclinations as those corresponding to reversely magnetized samples in the Southern Hemisphere. We used ages determined from the biostratigraphic data (see "Biostratigraphy") supplemented with ages determined from the magnetostratigraphy (see above) to provide ages for the paleolatitudes (Fig. F92). There is a gradual increase in (southerly) latitude with increasing age, which we interpret to reflect the motion of the Pacific plate during the past ~120 m.y. The paleolatitudes obtained are consistent with previously published paleomagnetic data for the Pacific plate (listed by Petronotis and Gordon, 1999). If the paleolatitudes are confirmed by shore-based studies of discrete samples, then they imply a position of ~25°-30°S for the plateau at the time of major igneous activity at ~120 Ma. This is less southerly than the paleoposition implied by the plate reconstruction of Neal et al. (1997), which was made in a stationary hot-spot reference frame.

Magnetic inclinations derived from sediments are sometimes influenced by compaction, producing values that are too shallow. Until the influence of compaction can be determined from shore-based studies, we believe that the inclination values should be treated with caution. However, it is encouraging that the magnetic inclinations obtained from basalt samples in Cores 192-1183A-54R and 55R (Table T13) produce very similar inclination values. The inclination values may also be influenced by any tilting of the sedimentary formations, but we found the sedimentary layers to be horizontal where depositional surfaces were apparent, indicating that there has been no major systematic tilting.

We performed detailed AF demagnetization on all coherent basalt pieces that could be oriented unambiguously with respect to the top of the core. All pieces contained a vertical secondary component most likely induced during drilling. In almost all cases, we were able to remove this secondary magnetization by 10- to 20-mT AF demagnetization and isolate the ChRM direction at higher fields (e.g., Fig. F88B). Another problem encountered during half-core measurements on basalt pieces relates to the broken nature of the basalt cores. Because the short pieces have no relative azimuthal orientation, we observed a magnetic interference of one piece on the next within the relatively large (~15 cm) measurement region of the pass-through magnetometer. This problem is exemplified by data from Section 192-1183A-55R-1 (Pieces 4A-4E) (Fig. F93). Principal component analysis for all measured points in this interval reveals that the pieces have different declinations (i.e., the pieces have no relative azimuthal orientation). Pieces 4A-4C are <10 cm in length, and there is significant interference, leading to low NRM intensities and varying inclinations. Pieces 4D and 4E are sufficiently long (>15 cm), so that, in the middle of these pieces, we can define the characteristic inclination. Note that pieces 4D and 4E both give a consistent estimate of inclination of -47°.

We used the following criteria to select only reliable inclination values. The pieces had to be at least 15 cm long, the maximum angular deviation of the principal component analysis had to be <3°, and the pieces had to be homogeneously magnetized. Of the 181 pieces analyzed, only 25 fulfilled these criteria (Table T13). In order to test the half-core data, we demagnetized one discrete sample from Section 192-1183A-65R-1 (Piece 3). This discrete sample yielded an inclination of -38.8°, in good agreement with -38.7° for the half-core measurement.

Although the NRM intensities are quite variable, we identified a distinct difference between the upper and lower basement units. The upper units (Units 2B-4B) are generally more weakly magnetized and have Koenigsberger ratios (Q in Table T13) of <10. Units 5B to 7, however, are more magnetic and, although variable, have many Q values >10. Whereas on the basis of the NRM intensities we are able to distinguish only two units in the basement samples, stable magnetic directions indicate considerably more variation both between units and, more importantly, within units, indicating that the basement units as defined (see "Igneous Petrology") contain many distinct subunits. Significant changes in inclination values within Units 5B and 7 suggest that these units contain at least three subunits with thicknesses of ~5 m.