Shipboard paleomagnetic measurements in Holes 1131A and 1131B consisted of long-core measurements at 5- to 10-cm intervals of the natural remanent magnetization (NRM) and the remanence after alternating field (AF) demagnetization at 20 mT, as described in "Paleomagnetism" in the "Explanatory Notes" chapter. Measurements were performed on archive halves of all APC and XCB cores, except for intervals affected by core disturbance. Long-core measurements established a magnetostratigraphy to a depth of ~320 mbsf, which includes the Brunhes and uppermost Matuyama Chrons. Below this, core reorientation by drilling disturbance disrupts the record and interpretation of magnetic polarity is uncertain. In partially lithified materials below 200 mbsf, measurements on discrete samples as well as individual biscuits provide more conclusive polarity determinations than long-core measurements in which remobilized sediment carries a strong spurious magnetization. Discrete samples were also collected from representative core material and were subjected to progressive AF demagnetization up to 30 mT. These samples were also used for anhysteretic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) acquisition and demagnetization experiments.
The intensity of initial remanence is relatively low with a median of ~2 × 10-4 A/m. High values occur in the uppermost 20 mbsf, and anomalous spikes are observed at the top of several cores. The NRM is of shallow to moderately steep negative inclination with a downhole trend toward positive inclinations (Fig. F6). After partial demagnetization (20 mT), magnetizations above ~280 mbsf are of steeply negative inclination and scatter is reduced significantly, indicating that a magnetization of steeply positive inclination is preferentially removed. In addition, after demagnetization, intensities display a gentle downhole trend to lower values. Superimposed on this trend there are two clear signals. The first is dominated by oscillations with wavelengths of 10-20 m, commonly corresponding to inclination variations. These oscillations may thus be related to geomagnetic field behavior (paleosecular variation). The other signal is a spurious signal associated with anomalies at core ends.
Below 250 mbsf, lower recovery and core disturbance in partially lithified materials results in intervals in which measurements yield lower inclination values than expected for the present latitude. These shallow inclinations are less common in discrete samples, suggesting that they may be caused by contamination by a drilling-induced overprint.
Measurements of discrete samples reveal a characteristic magnetization that is isolated after removing a spurious overprint, which we interpreted as a drilling-induced remanence. Median destructive fields range from 20 to 30 mT. In Sample 182-1131A-31X-1, 27-34 cm (Fig. F7A), a soft component with a steep downward inclination is removed first, isolating a characteristic magnetization of normal polarity that decays to the origin. Sample 182-1131A-48X-1, 45-47 cm, (Fig. F7B) is characteristic of deeper cores. In this sample, a prominent soft steep magnetization is removed with inductions of 5 mT, isolating a characteristic magnetization of lower intensity, although unequivocally of reverse polarity. Line fits were used to define the direction of the characteristic magnetization. These samples yield high maximum angular deviation values.
The magnetic Tensor tool was used in APC Cores 182-1131-3H to 7H in both holes. Within-core declinations at Hole 1131B are relatively well grouped, although between-core declinations are generally scattered. Declinations from Hole 1131A, however, show significant within-core scatter (Fig. F8). After correcting azimuths at Hole 1131B using the Tensor tool, declinations fall near the expected field direction for cores for which a nonmagnetic shoe was used (see "Appendix: Magnetics Experiment"). As with other sites reported in this volume, for XCB cores for which azimuthal orientation is not available, declinations are preferentially along the core fiducial line.
Normal polarity magnetizations are persistent in Cores 182-1131A-1H through 33X (Fig. F6). Within this normal polarity, we observed intensity fluctuations in NRM after 20-mT demagnetization similar to those observed at Sites 1127 and 1130. Once again, although these NRMs have not been normalized, they are of a similar periodicity to variations in the dipole moment of the geomagnetic field (Valet and Meynadier, 1993).
Rock magnetism analysis consisted of measurements of weak-field susceptibility at two frequencies, progressive IRM acquisition, and AF demagnetization of ARM. Rock magnetic properties are rather uniform within the cored interval. Decay of the NRM upon AF demagnetization is typical of a cubic phase, either magnetite or greigite. Representative samples were given a 400-mT IRM and subsequently demagnetized (Fig. F9). Inductions of 400 mT are not sufficient to reach saturation, suggesting that greigite is present and may well be the main remanence carrier in at least some cored intervals. Alternating field decay of the IRM is typical of weakly interacting single-domain grains (Cisowski, 1981). Rapid viscous decay of the saturation IRM was observed in samples from Cores 182-1131A-10X, 13X, and 17X, suggesting that particles near the superparamagnetic-single domain threshold are present. All samples display high ARM:IRM ratios, which suggest a single-domain grain size. Single-domain magnetite may also carry part of the remanence.
Moderately to steeply positive inclinations in Core 182-1131A-33X (at ~290 mbsf) are interpreted as reversed magnetizations. A precise location for the boundary is impossible because of incomplete recovery and core disturbance; discrete samples place the boundary between 280 and 300 mbsf, whereas long-core measurements place it between Sections 182-1131A-32X-2 and 33X-1. After a short interval of reverse polarity, normal polarity magnetizations are observed again in Section 182-1131A-34X-6. This pattern of reversal stratigraphy is interpreted as the uppermost Matuyama and Brunhes epochs, possibly with the top of the Jaramillo Subchron at 308 mbsf. This interpretation implies a lower sedimentation rate during the upper lower Pleistocene than during the Brunhes epoch. Below this depth, long-core data are of poor quality, although discrete samples from Cores 182-1131A-45X to 48X define normal-reverse-normal transitions between 440 and 420 mbsf. These, however, cannot be uniquely correlated with the geomagnetic polarity time scale.