The magnetic remanence of all archive halves of APC cores from Holes 1151C and 1151D (0.0-97.2 mbsf and 0.0-93.0 mbsf, respectively) and all RCB cores from Hole 1151A (78.0-1113.6) were measured at 5-cm intervals. Following measurement of the natural remanent magnetization, each section was subjected to stepwise alternating-field (AF) demagnetization. Sections from Hole 1151A were demagnetized at 10, 20, and 30 mT, with the exception that the 10-mT step was skipped for Cores 186-1151A-16R through 77R (Fig. F19). Because of time limitations, sections from Holes 1151C and 1151D were AF demagnetized at 20 mT only (Figs. F20, F21). Magnetic susceptibility was measured on whole-core sections every 2 cm for Hole 1151A and every 5 cm for Holes 1151C and 1151D (Figs. F19, F22). Both remanence and susceptibility data for these sections are available from the ODP (Janus) database.
The lithology and sedimentation rates at Site 1151 were very similar to those at Site 1150, so it is not surprising that the paleomagnetic and rock magnetic properties also appear to mimic those at Site 1150. As at the previous site, before demagnetization, the inclinations display a very strong tendency toward large positive values (~60°-90°) over the entire cored interval, indicating the presence of a steep downward-directed drill-string overprint. The radial bias is not as evident at this site, but we have not evaluated its presence to the same degree as for Site 1150 (see "Paleomagnetism" in the "Site 1150" chapter).
The magnetization remaining after demagnetization at 20 or 30 mT reflects the characteristic remanent magnetization (ChRM), a term we use to refer to the high coercivity or high unblocking temperature component that records the ancient magnetization of the sediments. As at Site 1150, in some cases the ChRM at Site 1151 is not a primary magnetization that was acquired at or near the time of deposition but is instead a secondary magnetization probably acquired during subsequent chemical and mechanical remagnetization.
Examination of inclinations after 20 or 30 mT AF demagnetization (Figs. F19, F20) illustrates that both normal and reversed polarities are recorded in at least the upper 700 m, though with a considerable amount of noise. Below 700 m, however, virtually the entire section has very stable positive inclinations, with mean inclinations of 56° to 60°. This would indicate that the sediments are of normal polarity, with the exception of a few short intervals. This conflicts with biostratigraphic information, which instead indicates that about half of the section should be reversed polarity. Given the abundance and good preservation of diatoms, and the resulting high quality of the biostratigraphy (see "Biostratigraphy"), it seems unlikely that the sedimentary section below 700 mbsf has a primary remanence. Remagnetization appears to be required and is supported by other evidence (see "Paleomagnetism" in the "Site 1150" chapter).
The remagnetization process that has replaced all or nearly all of the primary remanence in the lower 700 mbsf has probably also partially replaced the magnetization in the section above. The noise in the interval below ~80 mbsf we attribute to the low intensities and to partial remagnetization of the sedimentary section. Assuming that either chemical remagnetization, mechanical remagnetization, or both increase progressively downhole, then it is not surprising that below 700 mbsf there is relatively little evidence of reversed polarity and that in the interval from 700 to 80 mbsf polarity identification is difficult. Given that the field has had a normal polarity for the past 780,000 yr, there would be ample opportunity of the net bias of the sedimentary section to be in a normal polarity direction.
In contrast, the sediments above 80 mbsf likely contain a primary magnetization acquired at or near the time of deposition. The magnetization of this interval appears to be unaffected by reduction diagenesis as evidenced by the intensity and susceptibility data, which again are about an order of magnitude higher than sediment lower in the section. The presence of pyrite in the upper cores may indicate that some level of reduction takes place shortly after deposition, but not enough to consume completely magnetic minerals carrying a primary remanence. Unlike the lower part of the sedimentary section, there has been little time for mechanical deformation to remagnetize the sediment in the upper 80 mbsf, and evidence from ash layers and burrows suggests deformation is absent. Finally, inclinations within this interval are very stable and consistent with a Brunhes normal polarity direction (Fig. F20). The stability of these sediments is further confirmed by consistent directions, intensities, and susceptibilities obtained for Holes 1151C and 1151D (Figs. F20, F21, F22).
Discrete samples measured with the NP2 (Fig. F23; Table T10, also available in ASCII format) agree well with the split-core results (Fig. F19). On average, the intensities from the NP2 are slightly higher than those from the cryogenic magnetometer, which mainly is a result of using the 20-mT AF demagnetization results for the NP2 magnetometer and the 30-mT AF demagnetization results for the cryogenic magnetometer.
Magnetostratigraphic interpretation is hampered by the large amount of overprinting caused by drilling, reduction diagenesis, coring disturbance, and structural deformation. The location of the Brunhes/Matuyama reversal would appear to be well resolved by the abrupt downhole change to negative inclinations at about 78.2 mbsf in Hole 1151C, 80.1 mbsf in Hole 1151D, and at 82-84 mbsf (between Cores 186-1151A-2R and 3R) in Hole 1151A. However, the reversals lower in the section are not easily correlated with the geomagnetic polarity time scale. As at Site 1150, diatom datums provide the main age control for the sediments at Site 1151. Given the overall age constraints provided by the diatoms, the few prominent reversals can be used to make slight adjustments to the age estimates for the sediments, particularly between diatom datums (see "Sedimentation Rates").