PALEOMAGNETISM

Natural Remanent Magnetization

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. Cores 202-1240A-1H through 14H were AF demagnetized at 10 and 20 mT. Additionally, Section 202-1240A-1H-1 was demagnetized at 15 mT and Section 2H-3 was demagnetized at 25 mT. Cores 202-1240A-15H through 28H and all cores from Holes 1240B, 1240C, and 1240D were demagnetized at 20 mT. Sections obviously affected by drilling disturbance were not measured.

The NRM intensity before and after demagnetization shows the same general downhole trend with ~1 order of magnitude less intensity after 20-mT AF demagnetization than before (Fig. F23). NRM intensities after AF demagnetization are in the 10-3 to 10-4 A/m range throughout the section. Intensities oscillate downhole, but no downhole trend is observed in contrast to other Leg 202 sites. The overall pattern of NRM intensity is similar to that observed from magnetic susceptibility.

Steep positive inclinations observed prior to demagnetization are due to a drill string magnetic overprint partially, but incompletely, removed after 20-mT AF demagnetization. The inclination values after demagnetization have a positive bias, with steeper (higher) positive values below 70 mcd (Fig. F24). Biostratigraphic datums (see "Biostratigraphy") indicate that this depth corresponds to the approximate position of where the Brunhes/Matuyama boundary should occur; therefore, the steeper positive inclinations could in some way be related to the overprint on top of an interval with reversed magnetization or low intensity (Fig. F23). Because of the equatorial location of Site 1240 and its current ~20° inclination, only small deviations in inclination are expected even from polarity changes.

Because the inclination of the Earth's magnetic field is on average ~0° at the equator (Site 1240 latitude), declination shifts of 180° provide the only means of ascertaining if polarity has reversed. The declinations within individual cores at Site 1240 are generally consistent (show similar directions) when the drill string overprint has been mostly removed. But, when a significant high-inclination drill string overprint remains after AF demagnetization, the declinations become more random in orientation (Fig. F25). Intervals with more randomly oriented declinations are usually associated with lower NRM intensities.

To evaluate the polarity stratigraphy at Site 1240, we have used the Tensor tool to provide azimuthal orientation to the declination data. The Tensor tool was employed for Cores 202-1240A-4H through 28H, 202-1240B-3H through 26H, and 202-1240C-3H through 8H and was used to reorient the declination for each hole (Fig. F26). Oriented declinations near zero are considered to represent normal polarity and those ~180° represent reversed polarity. Based on declination data, the following polarity intervals can be identified by correlation to the geomagnetic polarity timescale (Cande and Kent 1995): Brunhes (1n) Chron, the Jaramillo (1r.1n) and Olduvai (2n) normal polarity subchrons of the Matuyama Chron, and the upper part of the Gauss (2An.1n) Chron. However, most reversal boundaries are not well defined (because of intervals of fluctuating polarity) or are present in core breaks (Fig. F26; Table T14). For example, the Brunhes/Matuyama boundary is not observed in Holes 1240A or 1240B but may be preserved in Core 202-1240C-7H, where a 110° shift in declination is observed. This is complicated because the polarity of sediment below can only be identified as reversed in Core 202-1240A-8H. The Jaramillo (1r.1n) Subchron is also difficult to resolve. The Olduvai Subchron and the upper and lower Olduvai/Matuyama boundaries are particularly well defined in sediments from both Holes 1240A and 1240B. These data suggest that Site 1240 preserves a good paleomagnetic record (Table T15) and that if postcruise demagnetization and analysis are successful, Site 1240 could prove to be a unique paleomagnetic record for the equator.

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