PALEOMAGNETISM

The natural remanent magnetization (NRM) of the archive half of each core section was initially measured then remeasured after alternating-field (AF) demagnetization at selected levels. Sections obviously affected by drilling disturbance were not measured. Core 202-1234A-1H was AF demagnetized at peak fields of 10, 15, and 20 mT. Cores 202-1234A-2H and 3H were AF demagnetized at 15 and 20 mT. Cores 202-1234A-4H through 22X were AF demagnetized at 20 and 25 mT. All other cores (202-1234B-1H through 19X and 202-1234C-1H through 8H) were measured after a single demagnetized step of 25 mT.

Initial NRM intensities were high, typically between 0.1 and 1 A/m except for a few discrete intervals discussed in more detail below (Fig. F17). The NRM intensities after demagnetization at 25 mT ranged from 1 x 10^-3 to 8 x 10^-2 A/m (Figs. F17, F18). The high NRM intensities prior to demagnetization are characterized by a steep positive inclination (averaging +76° in the upper 50 mcd of Hole 1234A) that is consistent with a drill string magnetic overprint. Though substantially reduced, this overprint was incompletely removed after AF demagnetization up to 25 mT. Within the upper 50 mcd, the inclinations after AF demagnetization at 20 or 25 mT were about -30° on average (Figs. F17, F19), much shallower than expected for the location of Site 1234 (expected inclination based on an axial geocentric dipole 56°). Before demagnetization, declinations tend to have values close to 0°/360°. This bias is likely the result of a drilling-induced magnetic overprint. Demagnetized (25 mT) declinations in the upper ~80 mcd deviate significantly from the 0°/360° values, and declination variability can be correlated between holes. Both of these are indicators that most of the overprint has been removed from the 25-mT declinations. Demagnetized (25 mT) inclinations have anomalously positive inclinations below 50 mcd and demagnetized (25 mT) declinations have a strong bias toward 0°/360° below 80 mcd, indicating the continued severe effect on the NRM of the drill string overprint below these depths (Fig. F17). The fact that the declination overprint is deeper than its inclination counterpart suggests that declination is more resistant to the drill string overprint. For more details on the overprint see Lund et al., this volume.

Biostratigraphic constraints (see "Biostratigraphy") based on calcareous nannofossils indicate that the base of the cored sequence is younger than 0.26 Ma (Zone NN21); therefore, these sediments should all have normal polarity (negative inclinations) associated with the Bruhnes Chron (0-0.78 Ma).

Within the uppermost 60 mcd, high positive inclinations associated with the drill string overprint are at least partially removed through AF demagnetization (Fig. F19). Inclinations are dominantly negative, as would be expected from sediment deposited during a normal polarity interval, and variations are consistent in all three holes (Fig. F19). The most significant of these variations is an interval of anomalous directions recorded between 21 and 23 mcd (Fig. F19). Steep positive inclinations and declinations ~180° opposed to the background declination values are observed (Fig. F20B). This interval, which is ~1 m thick in Core 202-1234-3H, is interpreted to be a magnetic field excursion and is remarkably similar in both inclination and declination to an excursion found at Site 1233 (Fig. F20). This interval may represent the Laschamp Excursion (~41 ka). The comparison of selected inclination and declination data with Site 1233 (Fig. F20) suggests that regionally correlative directional paleomagnetic secular variation can be observed at Site 1234. Clearly, caution must be used because this record suffers from magnetic overprints and distinct intervals appear to be affected by time-dependent magnetic dissolution.

Several intervals below 60 mcd have NRM intensities that are more than one order of magnitude lower than background, from ~0.05 to 2 x 10^-3 A/m (Fig. F18). These intervals may represent times when magnetic mineral dissolution associated with more significant early sediment diagenesis was occurring.