In an experiment in Hole 1235C, we deployed a nonmagnetic core barrel in place of one of the two standard steel core barrels. The nonmagnetic barrel was used every other core. The pattern of NRM (0 mT) and inclination (25 mT) for the first six cores of Hole 1235C are shown in Figure F2. Higher NRM (0 mT) intensities and more positive inclination (25 mT) values were obtained from the steel barrels in Cores 202-1235C-2H, 4H, and 6H. The steel core barrel results are consistent with results using steel core barrels without APCT tool deployment in Hole 1235A (Fig. F1). Significantly lower NRM (0 mT) intensities and more negative inclination (25 mT) values were obtained when using the nonmagnetic core barrel in Cores 202-1235C-1H, 3H, and 5H. The inclinations obtained with the nonmagnetic core barrel are much closer to the expected Site 1235 average (axial dipole) inclination of -55.6°. The reduction in overprint associated with the nonmagnetic barrel is significant in that it may mean the difference between a usable vs. an unusable paleomagnetic record. The nonmagnetic core barrel was alternated with a steel (magnetic) core barrel at all remaining sites during Leg 202 to further assess its effect on a variety of sediment types.
At Site 1237 on Nazca Ridge we cored open-ocean pelagic ooze. This site is located significantly closer to the equator (expected inclinations of about -30° during normal polarity). The sediments consist of a fine-grained nannofossil ooze with significantly lower concentrations of finer-grained siliciclastic components than the sediments on the Chile margin. The effects of the nonmagnetic core barrel on the upper 14 cores from Hole 1237B are shown in Figure F3. Because this interval represents approximately the last 6.5 m.y. of geomagnetic field behavior with both normal and reversed polarities, we show the mean values of only the negative inclination (normal polarity) intervals so that the contrast between the drill string overprint and the NRM becomes apparent. Even though the sediments at Site 1237 are significantly finer grained than those from the Chile margin and less susceptible to VRM, the reduced overprints associated with the nonmagnetic core barrel are easily observed. The sediments cored with the nonmagnetic core barrel have lower NRM (0 mT) intensities and more negative inclinations (after demagnetization at 25 mT) than those cored with the steel barrel. Additionally, the APCT tool was used in conjunction with the nonmagnetic core barrel on even-numbered Cores 202-1237B-4H through 14H (Fig. F3). Therefore, any VRM effect caused by the extra time that the core barrel sat on the ocean bottom (e.g., Fig. F1) was mitigated by the nonmagnetic core barrel's use.