DISCUSSION

The paleomagnetic records from Leg 172 cores described above generally illustrate the ability of the Ocean Drilling Program to acquire state-of-the-art paleomagnetic field records from deep-sea sediments using the APC corer and, with definable limitations, to evaluate the sediment paleomagnetism using the shipboard cryogenic magnetometer, correlate secular variation on at least a site basis, and identify "plausible" magnetic field excursions for further analysis. The U-channel measurements indicate, however, that selected secular variation or excursional features noted in the shipboard long-core measurement processes may be artifacts of the measurement process and require careful analysis and corroboration. Specifically, excursion 3 appears to be a systematic artifact of shipboard measurements. The U-channel records provide a significantly better record of the secular variation around excursion 3 and indicate that the interval does not contain excursional VGPs.

U-channel measurements of excursion 3 verify its existence and provide good corroboration of its overall directional character. The U-channel records from Site 1061, however, still appear to display systematic differences with discrete sample paleomagnetic records from the same region that we interpret to indicate variable sediment smearing and systematic biases associated with the U-channel long-core measurement process. The U-channel record from Site 1063 displays a higher resolution view of excursion 3 that is generally consistent with discrete sample measurements from the same locality.

This study, in conjunction with the shipboard results described in Lund et al. (1998) and Keigwin, Rio, Acton, et al. (1998), represents the first successful attempt to use secular variation records from ODP cores to correlate between sites on a scale of more than 1000 km. It shows that excursion 3 is a real geomagnetic feature that resides within an extended interval of reproducible secular variation.