The nonmagnetic cutting shoe had a nonmagnetic flapper valve, but a nonmagnetic spacer was used initially in place of the usual 10-finger catcher. The spacer was later replaced with a standard 10-finger core catcher. The nonmagnetic tool was used for alternate cores starting at 3H and ending when the lithology precluded further use. Experiments were performed to investigate the effect of the nonmagnetic cutting shoe used with the experimental nonmagnetic barrels and with standard APC barrels.
The sediments sampled during the experimental coring are within Unit I of the sedimentary section and consist of nannofossil oozes with numerous small pelagic foraminiferal turbidites. A zone of debrites has been defined in Cores 7H and 8H in both Holes 1128B and 1128C. Beneath this are two more cores of nannofossil ooze that complete Unit I. Unit II consists of olive-green clay from which Cores 11H–14H of Holes 1128B and 1128C were taken.
The APC coring in Hole 1128B was performed using standard barrels and permits a comparison between sediments cored using these standard barrels and cores from the same depth recovered from Hole 1128C using nonmagnetic assemblies. The analysis here focuses upon the declination, although there are minor differences in inclination between control cores and those cored using the nonmagnetic assembly. The intensities are similar whether the cores are taken with nonmagnetic or standard APC assemblies. The interpretation of the effect of the nonmagnetic corer at this site is complicated because the section contains a sequence of reversals.
The declinations before and after 20 mT demagnetization for Cores 182-1128B-2H through 6H are shown in the top panels of Figure F3. The declinations of all cores are similar, with tightly grouped distributions with means close to zero. The means and standard errors are –0.8, 1.7 for Core 182-1128B-2H; 0.2, 2.3 for Core 3H; 3.0, 3.3 for Core 4H; 9.7, 4.3 for Core 5H; and 5.8,1.1 for Core 6H. This is the classic expression of the 0° declination phenomenon.
After AF demagnetization, the pattern in declination remains, but the means are not as close to the fiducial line on the core liners as in the NRM case. The means and standard errors after demagnetization are 11.6, 3.1 for Core 182-1128B-2H; –24.8, 33.3 for Core 3H; 8.3, 4.0 for Core 4H; 8.9, 4.6 for Core 5H; and 0.4, 2.0 for Core 6H.
The lower panels of Figure F3 show the declinations observed in Cores 182-1128C-2H through 6H. There is a systematic difference in declination of the NRM between the even cores, which were taken with standard assemblies, and the odd cores, which were taken with the nonmagnetic assembly. The former have declinations close to 0° with means of 7.7, –14.7, and 1.4, with standard errors of 3.5, 4.6, and 4.6, whereas the cores taken with the nonmagnetic assembly have means of 21.3 and 52.3, with standard errors of 4.0 and 4.3. There is no comparable systematic difference in inclination or in intensity.
The pattern observed in the NRM declination is again seen in the declination after 20 mT demagnetization, but with a larger systematic difference between the declinations of cores taken with the magnetic and nonmagnetic assemblies. Thus the means and standard errors of the declinations of cores taken with the standard assemblies were –5.0, 4.0 for Core 182-1128C-2H; –15.0, 5.1 for Core 4H; and –6.2, 6.4 for Core 6H; whereas with the nonmagnetic assemblies, the means and standard errors of the declinations were 39.0, 4.7 for Core 3H; and 87.3, 6.2 for Core 5H.
Anomalously steep inclinations are more evident in Cores 182-1128C-4H and 6H taken with the standard assemblies, but the means are close to the GAD inclination for the site, which is 52°. Core 182-1128C-3H has a slightly lower intensity compared with the neighboring Cores 2H and 4H. This intensity is also lower than that of Core 182-1128B-3H, which suggests that subtle differences in intensity may be induced by the different coring assemblies, with the cores obtained with the nonmagnetic assembly consistently having slightly lower intensities. However, it is the declination that shows the most distinctive pattern.
As we noted above, it is important to check this estimation of the effect of the coring on the magnetization against an independent determination of the orientation of the field that the sediments should be recording. Taken at face value, the results of the tensor tool are again consistent with a reduction of coring contamination and better recording of the geomagnetic field in Cores 182-1128C-3H and 5H compared with Cores 2H, 4H, and 6H. Thus, Core 182-1128C-3H gives an estimate of the field direction of ~70°, whereas the tensor tool gives 95°. Core 182-1128C-5H gives 95° where the tensor tool gives 197°. Although these can hardly be regarded as in good agreement, the agreement is better than for Cores 182-1128C-4H and 6H, which disagree with the tensor tool by 178° and 166°, approaching the maximum possible disagreement of 180°.
The NRM of Hole 1128C presents a similar picture in which Cores 182-1128C-3H and 5H depart from the 0° direction in the appropriate sense as defined by the tensor tool, whereas the magnetization of Cores 182-1128C-2H, 4H, and 6H is closely aligned parallel to the fiducial line. The controls from Hole 1128B yield results aligned with the fiducial line again, as do the 20 mT values, with the exception of Core 182-1128B-5H, which departs in the correct sense and gives a result in agreement within 60° of the tensor tool.
Unfortunately, Hole 1128C was not sampled on the ship, so checking the tensor tool results by the magnetization of discrete samples must await analysis on shore. However, samples taken from Section 182-1128C-3H-2 yielded inconsistent declinations close to zero. Core 182-1128C-3H has remanence oriented close to zero and therefore should give 0° declinations in the ODP core axis convention, which it does before demagnetization. However, neither the demagnetized remanence nor the tensor tool orientation gives this direction. Two results from Core 182-1128B-4H gave declinations of 45° and 75°. The tensor tool gave 105°, but the half-core NRM direction was 6.7°, and after demagnetization to 20 mT it remained essentially unchanged at 8.3°. In this case, the tensor tool and the discrete sample results are in some degree of agreement and the values suggest that the half core is again giving a false near-zero declination. These results show the possibility of using discrete samples as another check of core orientation and tensor tool performance, but much more work is required on shore.
To summarize the results from this first sequence of cores, it is evident that the nonmagnetic APC assembly has significantly decreased the tendency for the direction of the magnetization of the half cores to be aligned with the fiducial line, or double-line scribed on the core liner. This implies a reduction of the radially inward moment discussed above. In both Cores 182-1128C-3H and 5H, the field appears to be recorded better by these cores and one type of systematic noise has been reduced. Curiously, there appears to be increased scatter of the NRM declination in all of the cores from Hole 1128C compared with those from Hole 1128B. This pattern is also maintained after AF demagnetization.
The disturbed zone in Cores 182-1128B-7H and 182-1128C-7H was not used in the analysis because of the difficulty in interpreting the magnetization in this zone. However, the declinations were strongly dispersed in Core 182-1128C-7H, which was cored with the nonmagnetic assembly, whereas Core 182-1128B-7H, taken using the standard assembly, again gave declinations near 0°.
Beneath the disturbed zone, Cores 182-1128C-9H, 11H, and 13H were again taken with the nonmagnetic assembly, whereas the even numbered cores were taken with a standard assembly. Plots of declination from the equivalent cores from Hole 1128B, which again act as controls, are shown in Figure F4 in the top panels, with NRM on the left and the 20 mT value on the right. The NRM declination is close to 0°, but after demagnetization, the declination of Cores 182-1128B-8H through 10H all move substantially away from zero, whereas Cores 182-1128B-12H through 14H move much less.
The declination for the NRM and 20 mT demagnetization value for Cores 182-1128C-8H through 14H are shown in Figure F4 in the lower panels. The NRM in the top row again shows the directions clustered around 0° declination, with the possible exception of Core 182-1128C-9H. Inclinations are, for the most part, steeply downward. The intensity of magnetization falls to a minimum at depths of 90 meters below sea-floor. With demagnetization to 20 mT, the declination and inclination patterns change radically, but the intensity changes less and becomes almost uniform throughout the sequence.
The results from this second group of cores are not as dramatic as in the shallower cores, but again those cored with the nonmagnetic assembly move away from the 0° declination and are in broad agreement with the tensor tool results. However, Core 182-1128C-10H gives an excellent paleomagnetic record in agreement with the tensor tool, but was collected with the standard assembly. As in the shallower cores, the overall scatter in declination is greater in Hole 1128C than in Hole 1128B.