Cores from Leg 202 sites off Chile (1232-1235) contain a drilling-induced magnetic overprint on the sediment natural remanent magnetization (NRM) prior to demagnetization. This overprint is easily quantified at all these sites because only sediments of Brunhes Chron age (0-0.78 Ma) were recovered, and therefore, the majority of the NRM should be restricted to normal polarity directions with negative inclination values near -55°. The overprint is generally along the positive z-axis, therefore inducing a steep positive inclination in the NRM.
A primary concern is that overprints may be either incompletely removed or, in some cases, not removed at all during routine shipboard demagnetization. This is critical, as the higher the field required to remove the overprint, the substantially greater the chance that the original NRM (the paleomagnetic component that we study) will be lost during the process.
Site 1235 was the shallowest site drilled on the Chile margin and contained the highest silt content. It was at this site that we discovered exceptionally strong overprints in cores taken when the APCT tool was deployed (Fig. F1). The APCT tool is an electronic device encased in the steel cutting shoe to measure in situ sediment temperatures. When a core barrel is shot into the sediment to cut a core, an instantaneous heat pulse is created that immediately starts to decay toward equilibrium with the surrounding sediments. The core barrel is left in place while the APCT tool records the time-temperature series of this decay for ~10 min. After the core is pulled from the bottom of the hole and raised to the seafloor, the ascent is interrupted for another ~5 min during which ocean-bottom temperature is measured. This procedure adds ~15 min to the time a core rests in the steel (magnetic) core barrel acquiring a magnetic overprint. The magnetic effect of the APCT tool is best illustrated in Hole 1235A by the presence of exceptionally high NRM (0 mT) intensities and positive inclinations even after 25-mT alternating-field (AF) demagnetization (Fig. F1). The observation that the inclinations are still positive after 25-mT AF demagnetization suggests that this overprint may have permanently compromised the NRM of these sediments.
Laboratory experiments were conducted on the APCT tool and its cutting shoe housing by placing a demagnetized sediment core inside the housing (with the electronics of the APCT tool active) and letting it sit for 30 min. The sediment core NRM measured after this experiment approached the intensities measured on regular cores (~0.5 A/m; Fig. F1). A second experiment was made using a normal cutting shoe without the APCT tool, and the same result was observed. For both cutting shoe experiments, we also measured the magnetic field inside the steel housing and found it to be 10-20 Gauss (1-2 mT) in both cases. This is a magnetic field more than 10 times the general strength of the Earth's magnetic field. We interpret the NRM, which was acquired inside the cutting shoe, to be a viscous remanent magnetization (VRM) related not to the APCT tool itself but rather to the time (~30 min) that the magnetic core barrel was in contact with the sediment.
It is well known that magnetic moments within any ferromagnetic material will align with an ambient magnetic field at a rate linearly proportional to log (time) (e.g., Collinson, 1983). This magnetization is termed a VRM. The rate at which a VRM is acquired depends on material properties, especially grain size. Coarse-grained titanomagnetites, which are common in marine sediments, acquire VRM relatively quickly, whereas fine-grained titanomagnetites may acquire VRM slowly or not at all. Thus, we might expect significant changes in the VRM acquired by relatively coarse grained sediments from the continental margin, even with relatively short changes in the time sediment spends in contact any applied magnetic field such as with a magnetic core barrel. This effect should be less pronounced in fine-grained pelagic sediment far from the continental margin.