The problem of magnetic overprints in ODP cores has a long history, which is reviewed in Chapter 6 of the Shipboard Paleomagnetist's Handbook (Stokking et al., 1993). All paleomagnetists who have sailed on the JOIDES Resolution have encountered these problems to some degree and many have made contributions toward a solution. In the discussion that follows, credit has been given where possible, but much of the information has been handed down without full documentation, so citing references is not always possible. For a number of legs, one of the authors (M. Fuller) has been investigating the magnetic overprint problem. Unfortunately, a recurrent feature of the work has been the failure to achieve consistently repeatable results in the various tests conducted. For example, the first time the nonmagnetic cutting shoe was deployed during Leg 182, there was a remarkable improvement in the magnetic record with the almost total elimination of the "0°" declination artifact, whereas in neighboring cores, the usual "0°" declination was seen. Yet, later the difference between the magnetic and nonmagnetic cutting shoes was not so clear.
The two fundamental factors in the generation of overprints, which all would probably recognize, are magnetic fields and deformation. In some cases the effect of one dominates, in others both are important. Thus, when adverse drilling conditions give rise to massive deformation in the form of core "suck in" as was experienced during Leg 189 at Site 1169, then deformation dominates and a hard vertical overprint is encountered as was also described earlier by Roberts et al. (1996). When the APC barrel with the core inside passes through the field of the bottom-hole assembly (BHA), there is no evidence of associated sediment deformation, but there is a strong magnetic field in which soft isothermal remanent magnetization (IRM) will be acquired. When moderate deformation is evident from visual inspection and measurements of anisotropy of susceptibility, or anhysteretic remanent magnetization (ARM), then the magnetization may be deflected as a passive marker in a similar way to that described by M. Okada (pers. comm., 1998). However, the configuration of the bedding planes as indicated by the maximum susceptibility plane appears to be in the form of a truncated cone with its apex upward (Herr et al., 1998). The deformation therefore gives a similar distortion of the recorded field as would a dome in a larger structural setting familiar to paleomagnetists who conduct tectonic studies. Corrections for this have been calculated by G. Acton (pers. comm., 2000).
In this paper, we describe the roles that deformation, magnetic fields, and other potentially important factors may play in the generation of overprints, as the recovered core passes through the various stages from the initial penetration of the sediment by the APC corer to final measurement with the 2G magnetometer. These are reviewed next and followed by the experimental work conducted during Leg 189 to investigate the various effects.