Demagnetization of natural remanent magnetization (NRM) in archive-half cores and discrete samples was performed to identify polarity changes and to provide data for reorientation of intact core pieces, despite poor recovery of RCB cores. The archive sections of the core were progressively demagnetized at 2-cm intervals using alternating-field (AF) demagnetization up to 40 mT in increments of 10 mT, and occasionally strong AF demagnetization of 45 to 60 mT in increments of 5 mT was applied for investigating polarity changes. Discrete samples were selected from undisturbed core pieces and demagnetized up to 80 mT in increments of 5 mT.
Magnetic susceptibility was measured using a point sensor on the archive multisensor track (AMST) at 2-cm intervals and was compared to susceptibility of whole-round cores measured by the MST (see "Physical Properties") and to magnetic intensity data.
Paleomagnetic declination and inclination of archive cores after 40-mT demagnetization and discrete results are shown in Figure F20. Rapid changes in the inclination and declination were observed in Core 205-1255A-2R from 132.76 to 134.84 mbsf (Sections 205-1255A-2R-1 through 2R-3). Relatively well defined sequences of negative inclinations with declination changes from 132.98 to 133.12 mbsf (interval 205-1255A-2R-1, 28-42 cm) are believed to indicate reversed magnetic polarity. From 133.46 to 134.80 mbsf (Sections 205-1255A-2R-1, 76 cm, through 2R-3, 38 cm), the inclinations show very shallow and wide declinations (Fig. F20). Disagreement of magnetic inclinations between the archive sections and the discrete samples in this interval indicate that these inclinations are questionable. Within Core 205-1255A-3R, a clear polarity change was identified from 144.15 to 145.38 mbsf (Sections 205-1255A-3R-2, 25 cm, through 3R-3, 15 cm) by significant direction changes of the magnetic declination and inclination observed in both archive sections and discrete samples (Fig. F20).
Relatively high magnetic intensities were observed within Core 205-1255A-2R, in which magnetic susceptibilities measured by the AMST and the MST are also slightly elevated (Fig. F21). High intensities also occur at ~143 mbsf; however, no corresponding changes in magnetic susceptibility are observed in this interval. A very small change in the magnetic intensity begins at 144.15 mbsf (Section 205-1255A-3R-2, 25 cm) with a slightly higher peak in the susceptibility, corresponding to the polarity boundary discussed above (Fig. F20). Significantly higher magnetic intensities below 145.58 mbsf (Section 205-1255A-3R-3, 55 cm) correspond to clear peaks in magnetic susceptibility (Fig. F21).
A short interval of negative polarity was identified from 132.98 to 133.12 mbsf (interval 205-1255A-2R-1, 28-42 cm). Polarity identification is not possible from 133.46 to 134.80 mbsf because of variable magnetic declinations and disagreements between archive cores and discrete data. A clear negative polarity associated with an intensity change is identified from 144.15 to 145.38 mbsf (Sections 205-1255A-3R-2, 25 cm, through 3R-3, 15 cm). This clear short negative polarity was not identified during the previous Leg 170 paleomagnetic study at Site 1043 (Kimura, Silver, Blum, et. al., 1997).