During Ocean Drilling Program (ODP) Leg 178, nine sites were drilled off the western coast of the Antarctic Peninsula with the aim of deducing the history of the peninsula's ice sheet over the last 10 m.y. (Fig. F1) (Barker, Camerlenghi, Acton, et al., 1999). Three sites were drilled into sediment drifts on the continental rise to provide a continuous but distal record of glacier output. This record is also influenced by the ocean current system. Two successful sites were drilled into the shelf to provide a proximal but discontinuous and poorly dated record of glacial activity. Two of the rise sites (Sites 1095 and 1096) and one of the shelf sites (Site 1103) were logged with downhole logging tools.
The geological high-resolution magnetic tool (GHMT) (Roperch et al., 1993) was deployed at all three of the logged sites. It comprises two sondes, the susceptibility measurement sonde (SUMS) and the nuclear resonance magnetometer sonde (NMRS). The SUMS measures the magnetic susceptibility in the formation surrounding the borehole. The susceptibility measurement is primarily dependent on the concentration of ferrimagnetic minerals (particularly magnetite) in the formation. In this environment, it can be interpreted in terms of magnetite input from terrestrial sources and magnetite dissolution. The NMRS measures the total magnetic field in the borehole using a proton-precession magnetometer (it does not measure the direction of the field). The major contribution to the magnetic field measurement is the field generated in the Earth's core, with lesser contributions from local seafloor anomalies, the drill string, and the remanent and induced magnetization of the sediment surrounding the borehole. The time-dependant magnetic field of the magnetosphere also contributes. We determine the polarity of the remanent magnetization by two methods: (1) taking the sign of the remanent anomaly (a positive anomaly corresponds to normal polarity and a negative anomaly corresponds to reversed polarity) and (2) the correlation analysis method (Vibert-Charbonnel, 1996), based on the fact that in an interval of normal polarity sediment the remanent anomaly will correlate with the induced anomaly, whereas in reversed polarity sediment they will anticorrelate.
The GHMT data from Leg 178 and the other ten ODP legs during which the GHMT was run are available at: http://www.ldeo.columbia.edu/BRG/ODP/DATABASE.