PALEOMAGNETISM

The natural remanent magnetization (NRM) of archive halves of APC and XCB cores recovered at Site 1232 was measured using the shipboard pass-through cryogenic magnetometer. Measurements were made at 5-cm intervals. Sections obviously affected by drilling disturbance were not measured. Cores 202-1232A-1H through 20X and 34X were initially measured and then remeasured after demagnetization at peak alternating fields (AF) of 10, 15 and 20 mT. Cores 202-1232A-23X through 30X and Cores 1202-232B-1H through 6H were initially measured and then remeasured after demagnetization at peak AF fields of 10 and 20 mT. Cores 202-1232A-31X through 33X and 35X through 39X were initially measured and then remeasured after demagnetization at a peak AF field of 20 mT.

Initial NRM intensities were extremely high, ranging from 0.5 to >3.5 A/m (Fig. F15). Intensities in Hole 1232A cores were lower in the XCB section (mean 0.6 A/m) than in the APC section (mean 1.4 A/m). The high NRM intensities are consistent with a magnetic overprint that has been attributed to the drill string. This overprint, characterized by steep positive inclinations averaging 71° for Hole 1232A (Fig. F15), was substantially reduced (typically by 75%) by demagnetization at peak AF fields as low as 10 mT. After 20-mT AF demagnetization, inclinations observed (Fig. F15) were closer to the expected axial-geocentric-dipole inclination (-59°) for the site latitude (~40°S). Although the positive overprint is substantially reduced through AF demagnetization, some overprint still remains, as evidenced by mean inclinations of -30° with a median value of -39° after 20-mT AF demagnetization (Fig. F15). High NRM intensities are still observed after 20-mT AF demagnetization, with lower values in the XCB section (0.14 A/m) than in the APC section (0.21 A/m). Declinations prior to demagnetization are consistently near 0° and 360°. These directions are consistent with a drill string overprint that is removed in the APC but not the XCB cored section after demagnetization (Fig. F15). The tensor orientation tool was employed below the second core of the APC section of both Holes 1232A and 1232B.

The magnetic polarity at Site 1232 is normal and attributed to the Brunhes Chron (0-0.78 Ma). However, many intervals of positive (reversed) or shallow inclination are observed (Figs. F15, F16). These anomalous inclinations do not reflect geomagnetic field behavior, but rather artifacts of the drill string magnetic overprint combined with decimeter-scale changes in grain size and magnetic susceptibility (see "Lithostratigraphy"). Intermittent coring deformation of the silt-rich intervals may also contribute to the anomalous inclinations.

Initial NRM intensity and magnetic susceptibility values are generally in phase within the uppermost 130 mcd of all holes showing similar variability at the decimeter to dekameter scale (Fig. F16). The large-scale (tens of meters) variability is generally consistent with the incidence/thickness of turbidites at Site 1232 (see "Lithostratigraphy"). Higher intensities are present in stratigraphic intervals having more/thicker turbidites, whereas lower intensities are present in stratigraphic intervals having fewer/thinner turbidites. This pattern is less obvious in the XCB cored interval, possibly due to incomplete recovery (Fig. F15). The high intensity of the drill string overprint and the similarity between the NRM intensity and magnetic susceptibility (Fig. F17) may reflect the coarse-grained, highly terrigenous nature of these turbidite-rich sediments with the drill string-overprinted NRM accentuated by the coarser magnetic grains. This relationship is generally lost after AF demagnetization through the removal of much of the strong viscous remanent magnetization (Fig. F17).