Shipboard paleomagnetic measurements for Holes 1143A, 1143B, and 1143C consisted of long-core measurements of natural remanent magnetization (NRM) at intervals from 2 to 4 cm before and after alternating field (AF) demagnetization, usually up to 20 mT (see "Paleomagnetism" in the "Explanatory Notes" chapter). Measurements were carried out on the archive halves of all APC and a large number of XCB cores. In this last case, however, intervals affected by coring disturbances, clearly visible as reoriented and broken or crumbled biscuits, were not measured. In addition, discrete samples were also collected from the working halves of Hole 1143A and Cores 184-1143C-42X through 53X, at a spacing of one sample per section (1.5 m). Most of these samples were subjected to progressive AF demagnetization up to 60 mT and were kept for shore-based rock magnetic analysis. Cores 184-1143A-4H through 21H, 184-1143B-3H through 19H, and 184-1143C-3H through 19H were oriented using the Tensor tool.
Long-core measurements were carried out at 2-cm intervals with AF demagnetization steps at 10 and 20 mT. The NRM is on the order of 10-3 A/m in APC cores and 10-4 A/m in XCB cores. This difference probably results from interference of the magnetization of partially reoriented biscuits on a scale smaller than the measuring bandwidth of the instrument (~15 cm) and from interference between the magnetization of reconstituted mud and that of the biscuits.
Below the first five to six cores, the NRM is almost invariably oriented parallel to the +X direction in the ODP coordinate scheme, which is orthogonal to the scribed double line on the core liner. Since the core liner is arbitrarily oriented with respect to the direction of the Earth's magnetic field, this is certainly an artifact. This orientation could derive from a radially inward magnetization induced by the coring process (Fig. F12) (Feary et al., 2000; Stokking et al., 1993). Inclinations are sometimes very shallow, although more often unrealistically steep. Alternating field demagnetization to 20 mT removed a large fraction (>70%) of the NRM, mainly along the z-axis (i.e., the axis along which the overprint is maximum) but could not isolate realistic paleomagnetic directions, even when demagnetization steps up to 60 mT were used.
Stokking et al. (1993) observed that the overprint was not as strong in discrete samples taken from the center of working-half sections, suggesting that the intensity of the overprint decreases radially from the edge to the center of the core. For this reason, we have taken discrete cubic samples (one from the center of each section of the different cores). Alternating field demagnetization of these samples to 60 mT yielded very good quality demagnetization results (Fig. F13). The final direction, however, was still highly contaminated by the overprint. A possible explanation is that the coercivity spectrum of the overprint and of the primary magnetization largely overlap, so that AF demagnetization removes both components simultaneously.
This strong overprint hindered retrieval of reliable paleomagnetic directions from most of the core. Tensor-corrected declination and inclination values obtained from long-core measurements for 0-100 mcd after demagnetization at 20 mT are reported in Figure F14. The first three cores were not oriented, so declinations have no meaning. After reorientation using Tensor tool data, Core 184-1143A-4H has a declination of 260° and a normal inclination too steep for the latitude of the site, indicating some contamination from the overprint. Despite the contamination, we have interpreted this core as indicating normal (Brunhes) polarity. Core 184-1143A-5H has a positive inclination of ~15° (with an unexplained peak to very steep values) and declination of ~360°, except in the bottom ~1 m, where the declination starts pointing to nearly 180°. A transition to negative inclinations with south-seeking declination is clearly visible in Core 184-1143A-6H at ~42.5-43.8 mcd. We interpret this transition as the Brunhes/Matuyama polarity reversal.
Below this depth, the results are difficult to interpret. A short interval of normal declination occurs at the bottom of Core 184-1143A-6H and the top of Core 7H from 51.8 to 54.2 mcd. Core 184-1143A-7H has an average declination of ~150°, indicating continued reversed declinations. However, inclinations have unrealistically steep values (~70°), suggesting pervasive drill-string overprint. Core 184-1143A-8H has reversed declination (~220°) but a shallow positive inclination. Core 184-1143A-5H shows high scattering of both inclination and declination with a slightly lower intensity, possibly indicating another type of coring disturbance.
Farther downcore, the measurements produce unrealistically rapid changes in inclination and declination with both reverse and normal polarity in both APC and XCB cores. Figure F15 shows inclination and uncorrected declination from 110 to 180 mcd after AF demagnetization at 20 mT. The uncorrected declinations cluster around 0°, indicating a radial overprint as mentioned above. This ambiguity could not be resolved using data from discrete samples.
To mitigate the overprint observed in Hole 1143A, nonmagnetic core barrels and cutting shoes were used every second core in Hole 1143B (only one nonmagnetic assembly was available on board). This assembly was used during Leg 182 with some encouraging results (Feary et al., 2000).
At Hole 1143B, unfortunately, the nonmagnetic assembly did not produce any improvement in the quality of the data. The overprint appeared to be just as large in cores taken with the nonmagnetic assembly as in those taken with the standard assembly. Overall, the results are just as disappointing as in Hole 1143A.
Inclinations are ~15°-20° for the upper 43 m, followed by a transition to negative values. This could indicate the presence of the Brunhes/Matuyama boundary at this level. The declinations corrected with the Tensor tool data (starting at 28 mcd) are predominantly south seeking, in contrast to the inclination data. However, a clear declination change of nearly 180° at 43.2 mcd in the middle of Core 184-1143C-5H correlates with the inclination change to negative values and suggests that this could be the Brunhes/Matuyama reversal.
Farther downcore, the results are difficult to interpret. Numerous 180° changes occur in declination—sometimes between different cores and sometimes within a core, such as the one observed at 88 mcd. However, inclination data are very scattered and are predominantly positive. A polarity stratigraphy is therefore impossible to retrieve from these results.
In an additional attempt to lessen the overprint, the nonmagnetic shoe was used with a standard core barrel on every second core in Hole 1143C. This hybrid assembly produced good results during Leg 182 and was suggested to us by Mike Fuller (pers. comm., 1999). Unfortunately, it did not reduce the overprint. In fact, the nonmagnetic cutting shoe actually produced a steeper vertical overprint than that of the standard cutting shoe (Fig. F16). In the end, the lack of results from Hole 1143C was similar to that from Holes 1143A and 1143B.
No reliable magnetostratigraphy could be obtained from this hole.
Long-core measurement of both the archive and working halves, occasionally made at different depths, usually gave the same result, providing clear evidence for the presence of a radially inward horizontal magnetization component. If this component were the only overprint, it would correspond to inclinations close to 0°. This is not usually the case; in fact, inclinations are more often quite steep. Therefore, a vertical component related to drill-string remagnetization is also present in the overprint. Both components appear to be less intense in the upper part of each hole. Above 50 mcd the vertical overprint seems to be more transient, often occurring in the top several meters of the various cores. A strong radial overprint is observed in most cores below ~50 mbsf. The relative intensity of the two components varies along the cores, suggesting that some physical properties of the sediment itself may play a role in the acquisition of the overprint. The core barrel assembly also has a very strong effect on the overprint. The average inclination of the cores recovered using the standard assembly is clearly different from that of the cores where the nonmagnetic assembly was used. In either case, the overprints are pervasive and could not be removed by either AF or thermal demagnetization. Therefore, the primary magnetization proved impossible to recover below ~50 mcd.