This preliminary analysis of the experimental coring on Leg 182 clearly shows that the nonmagnetic assembly consisting of cutting shoe, flapper valve, spacer for 10-finger core catcher, and APC barrel does reduce the 0° declination phenomenon in some cores. However, this is not a simple invariable result. There are occasions when it appears to have no effect. There are even occasions when cores taken with the standard assemblies give better results than similar cores obtained using the nonmagnetic assembly. Two disadvantages of the nonmagnetic barrel assembly are (1) an anomalous field at the joint between the nonmagnetic barrels with the standard barrel (i.e., this assembly is not entirely nonmagnetic) and (2) the failure of nonmagnetic barrels in general to shield the sediment from stray fields as do standard barrels during passage through the bottom-hole assembly (BHA) and up the drill string. The effect of the nonmagnetic shoe alone appears to be similar to that of the whole nonmagnetic assembly, although additional relevant data remain to be analyzed. The variability of the effects of the nonmagnetic cutting shoe and assembly is a major puzzle. Moreover, to understand this variability in the effect is a major part of the study because the phenomenon of coring contamination itself is so variable. If the observed effects are a combination of magnetic field plus sediment deformation during piston coring, then it is natural to think that the physical properties of the sediments may be a key factor. It was in the region of partial lithification and increased vane strength in Hole 1128C that the effect of the nonmagnetic cutting shoe appears to decrease and excellent paleomagnetic records are obtained whether the nonmagnetic or the standard shoe is used.
These experiments have also drawn attention to the between-hole variability at a single site. We have found repeatedly that the quality of the paleomagnetic record is different in two holes from a single site, although nothing has changed in the BHA or in the APC barrel assembly.
The study will be continued on shore with measurement of the magnetization of discrete samples and U-channel samples as a means of orienting the cores and of investigating the distribution of the coring contamination as a function of position in the cross section of the core.
As a result of these preliminary results, it is recommended that further trials of the nonmagnetic shoe and APC assembly be planned. The results from this leg were of considerable interest, but it was far from ideal as a test of the nonmagnetic cutting shoe and APC assembly because the signal was in many cases so weak that comparisons between nonmagnetic and standard elements were compromised. It remains possible that the nonmagnetic shoe alone may provide a relatively cheap and quick fix for a significant part of the 0° declination problem.
The explanation of the declination effect remains elusive. A particularly puzzling feature, noted since the early work of Jean-Pierre Valet and David A. Schneider on Leg 154 (Curry, Shackleton, Richter, et al., 1995), is its intermittent occurrence. The experiments conducted during Leg 182 demonstrate that the declination effect can sometimes be mitigated by the use of a nonmagnetic cutting shoe and also suggest that the deformation of the sediment during coring plays a role in the effect.