Paleomagnetists have found that the natural remanent magnetization (NRM) measured in advanced piston coring (APC) sediments is commonly oriented parallel to the +X direction in the Ocean Drilling Program (ODP) coordinate scheme, which is defined to be parallel to the scribed double line on the core liner (i.e., 0° declination) (e.g., Curry, Shackleton, Richter, et al., 1995). Because the core liner is oriented arbitrarily with respect to Earth’s magnetic field, this must be an artifact. In very weakly magnetized rocks, a machine error may cause the observed declinations, although no clear explanation has been presented. In more strongly magnetized rocks, the declination anomaly can arise from a radially inward magnetization, as illustrated in Figure F1. The magnetization in discrete samples taken from the cores has also been determined, and the horizontal component has been found to be oriented radially inward (Curry, Shackleton, Richter, et al., 1995; Herr et al., 1998). The degree of remagnetization has been shown to decrease from the margin of the core inward (Stokking et al., 1993; Curry, Shackleton, Richter, et al., 1995).
A natural explanation for the magnetic contamination observed in APC cores is that the magnetic fields of the various components used in the coring process have given rise to magnetic fields in which remagnetization has taken place. Numerous investigations of the magnetic fields of the various components used in coring have indeed demonstrated that APC barrels and shoes can have relatively strong magnetic fields, as much as two to three orders greater than the geomagnetic field, and that there can be very strong local fields in the bits and in the pipe (e.g., Stokking et al., 1993; Fuller et al., 1998). Another possibility is that in the process of coring, particles are mechanically realigned. Sediment deformation during coring with a circular cutting shoe could give such a realignment a radial symmetry, as required to explain the radially inward moments. To test these models and to investigate possible mitigation of the observed contamination, a program of coring with experimental nonmagnetic barrels and cutting shoes was performed during this leg and during Leg 174B (Shipboard Scientific Party, in press).
A comparison between the paleomagnetic records of cores obtained with nonmagnetic APC barrels and standard barrels was conducted during Leg 174B (Fuller and Garrett, 1998). The nonmagnetic barrels were made with 15-15-LC steel, which is a nonmagnetic material commonly used in the petroleum industry. This did not solve the 0° declination problem. However, additional experiments demonstrated that the magnetization acquired as the core travels up the drill string was predominantly vertically downward and magnetically soft and could be removed by alternating field (AF) demagnetization. The experiments also demonstrated that relatively hard magnetization can be acquired by the core as the APC barrel comes to rest in the sediment. However, the problem of the 0° declination remained.
The use of half cores for the measurement in the magnetometer may contribute to the 0° declination problem because of the off-centered position of the sample in the pick-up coil array; studies are under way to investigate this effect (J. Gee, pers. comm., 1998). However, it seems unlikely that measuring off center should introduce major error. If a radial moment is generated by the coring process, then measuring the whole core should eliminate the integration effect discussed above. We therefore compared whole-core and half-core measurements of cores taken with standard and nonmagnetic APC assemblies and shoes.
The analysis of the data generated during Leg 182 is under way, and only preliminary results are available. Here we describe the results from Site 1128, the first site at which experimental coring was performed, and from Site 1131, the first site at which the comparisons of whole-core and half-core measurements were made in conjunction with coring using the experimental nonmagnetic cutting shoe.
1Examples
of how to reference the whole or part of this volume can be found under "Citations"
in the preliminary pages of the volume.
2HIGP-SOEST,
University of Hawaii, 2525 Correa Road, Honolulu, HI 96822, U.S.A. mfuller@soest.hawaii.edu.
3Dept.
of Earth and Planetary Sciences, University of New Mexico, Northrop Hall,
Albuquerque, NM 87131-1116, U.S.A.
4Shipboard Scientific Party addresses
can be found under "Shipboard
Scientific Party" in the preliminary pages of the volume.
Ms 182IR-APP