Paleomagnetic measurements on archive-half sections were routinely performed. After measuring NRM, all sections were continuously demagnetized using alternating-field (AF) demagnetization up to 40 mT in increments of 10 mT at 2.5-cm intervals in order to remove magnetic overprints and to provide reorientation for intact pieces in the sedimentary cores. An AF demagnetization of 5 mT was occasionally applied in the demagnetization sequences to investigate drilling-induced isothermal remanent magnetization.
Two or three discrete samples were collected from each working-half section for analyses of postcruise rock magnetism and paleomagnetism. The samples were picked from intact pieces and in the region of clear sedimentary structures. All discrete samples will also be used to aid in the paleomagnetic interpretation of the archive sections' record of magnetization by providing additional measurements of polarity and basic magnetic characterization. The discrete samples were demagnetized up to 80 mT in 5-mT increments to permit principal component analysis.
Magnetic susceptibility of all archive-half sections were also measured every 2 cm by a point susceptibility sensor on the archive multisensor track (AMST). The "high-resolution mode (0.1 range)" of the point sensor measures susceptibility within short intervals of the sections better than the loop sensor of the MST.
The majority of the NRM measurements for archive sections show steep inclinations of ~60° to ~80°N. These steep inclinations partially disappeared in a weak AF demagnetization level of 5 mT. Then several negative inclinations appear after a demagnetization of 10 mT, which suggests that the NRM inclination represents drilling-induced magnetic overprints (Fig. F35). After AF demagnetization of 40 mT, magnetic inclinations of cored sediments show several polarity changes (Figs. F36, F37) that were compared to previous paleomagnetic results at Site 1040 (Kimura, Silver, Blum, et al., 1997). However, construction of magnetostratigraphy and age identification of magnetic polarities in Hole 1254A is not possible because of high variation of the magnetic inclination. Also, scattered magnetic declination data suggest pervasive drilling disturbance in the sediments (Figs. F36, F37). Structural core observation found "spiral" drilling disturbance in several cores at this site (see "Structural Geology"). Additionally, previous work in Hole 1040C identified continuous spirals of magnetic declinations within core sections. Therefore, those deformed sediments indicate that RCB drilling caused pervasive disturbance in cores of soft sediment and spirals of the magnetic declination (Kimura, Silver, Blum, et al., 1997). This places severe constraints on paleomagnetic investigations of the cores from this hole.
Although the magnetic inclinations of the archive halves after AF demagnetization of 40 mT are still unclear, several inclinations appear to exhibit negative polarity (Figs. F36, F37). Some significant spikes of the inclination in which the angle shows <0° may reflect true negative polarities, although several scattered spikes reflect disturbed magnetic remanence caused by the drilling. However, to exactly separate the drilling-induced disturbances from the true polarities is impossible. Relatively clear negative inclinations in which angles continuously range from -70° to -10°S are observed from ~207 to ~221 mbsf (Cores 205-1254A-7R to 8R) and 350 to ~356 mbsf (Core 14R), suggesting that there may be two major negative polarity chrons. Unfortunately, a comparison of these polarities with the previous paleomagnetic investigation in Hole 1040C is not possible because of poor identification of the magnetic polarities at this time.
Scattered magnetic declinations cannot be used for the paleomagnetic study, but a reorientation of structural direction in each portion of spiraled sedimentary sections using declination data after the demagnetization of 40 mT was mostly successful (see "Structural Geology").
High magnetic intensities of the NRM were observed from 184 to 202 mbsf (Sections 205-1254A-5R-1 through 6R-6) and from 310 to ~350 mbsf (Sections 10R-1 through 14R-2) (Fig. F38). Stepwise AF demagnetization revealed that the lower interval has a different magnetic coercivity from the upper one, whereas the maximum values of both NRM intensity anomalies are nearly even.
The upper intensity peak is still observed after 20-mT demagnetization. Relatively high intensity ratio of magnetic intensity after the demagnetization (20 mT) to NRM intensity ranges from ~0.5 to 0.8, which indicates that 50%-80% of the magnetization remains after the demagnetization.
The lower intensity peak of the NRM is rapidly demagnetized within weak AF demagnetization levels, and then the peak disappears completely after 20-mT AF demagnetization (Fig. F38). The intensity ratio indicates an interval from ~320 to 340 mbsf that exhibits very unstable magnetization, which suggests that the magnetic coercivity of the sediments is extremely weak. Sediments above and below the lower peak show ~0.2-0.7 in the intensity ratio, indicating that 20%-70% of the magnetization still remains after the demagnetization, but it is distinctly higher than that of the lower anomaly (Fig. F38).
The fluctuations of magnetic intensity, reflecting the magnitude of magnetization, generally indicate that changes in concentration, grain size, and chemical components of magnetic minerals relate to variability of lithology. Two high-intensity peaks of NRM in the lower anomaly (~320 and 340 mbsf) correspond to sand layers noted in sedimentological observation; however, significant lithologic changes or boundaries of the sedimentary unit were not observed around the other places. As an additional fact, the lower anomaly is overlapping with a fault zone at ~320-330 mbsf and the décollement zone (Fig. F38). More rock magnetic data is required to constrain these interpretations, but this suggests that chemical alteration or reducing conditions may have affected the magnetic minerals within the sediment at the fault and the décollement zones.
Magnetic susceptibility of all whole-round cores was measured by a loop susceptibility sensor on the MST unit, and the measurement of all archive-half cores was conducted using a point sensor of the AMST system (Fig. F39) (see "Paleomagnetism" in the "Explanatory Notes" chapter). Although the magnetic susceptibility measured by the AMST typically shows smaller values than the MST because of the different volume of the archive cores from the whole-round cores, they show almost the same trend in the magnetic susceptibility.
At Site 1254, clear highs in the magnetic susceptibility observed in the intervals from 184 to 202 mbsf (Sections 205-1254A-5R-1 through 6R-6) and from 310 to ~350 mbsf (Sections 10R-1 through 14R-2) correspond to the magnetic intensity peaks. This suggests that the magnetic intensity observed at this hole depends strongly on the magnetic grain mineralogy and concentration. Changes in magnetic susceptibility at Site 1040 also show a slight high from 160 to ~190 mbsf and high susceptibility with high variability below ~275 mbsf. Top depths of both peaks at Site 1040 are ~30 m shallower than those at Site 1254. Additionally, pronounced high variability of the magnetic susceptibilities observed below 275 mbsf at the previous site was not detected in the sedimentary cores at Site 1254. These results indicate that the concentration and distribution of the susceptible magnetic particles around the décollement zone at Site 1254 are different from Site 1040.
For analysis of the paleomagnetic investigations, all discrete samples were demagnetized up to 80 mT in increments of 5 mT. Some samples show discontinuous demagnetization curves for both inclination and declination on the Zijderveld diagram (Fig. F40A), which may indicate drilling disturbance of sediments. Samples obtained from sedimentary sections corresponding to relatively low magnetic intensity zones (Fig. F38) often show rapid decrease of their magnetization in the early steps of AF demagnetization (Fig. F40B). Anomalous demagnetization curves were observed in most samples taken from the sediment exhibiting high magnetic intensity peaks (Fig. F40C, F40D). Their declination and inclination curves do not converge to the origin of the Zijderveld diagram, and magnetic intensity increases progressively with an increase in the AF demagnetization levels. This odd behavior is similar to demagnetization curves of greigite on stepwise AF demagnetization. Sediments in lithostratigraphic Subunit U1A show a clear and smooth demagnetization trend during the AF demagnetization (Fig. F40E). This suggests that the magnetization of sediments in Subunit U1A is significantly more stable than that of prism sediments.