INTRODUCTION

During Ocean Drilling Program (ODP) Leg 172, almost 6 km of Pliocene/Pleistocene deep-sea sediments was recovered from three sediment drifts (Keigwin and Jones, 1989) of the western North Atlantic Ocean: the Blake Outer Ridge (Sites 1054-1061), the Bahama Outer Ridge (Site 1062), and the Bermuda Rise (Site 1063) (Keigwin, Rio, Acton, et al., 1998). These sediment drifts (see Fig. F1) are regions of anomalously high sediment accumulation rates (typically 10-40 cm/k.y.), and they contain perhaps the highest resolution record of geomagnetic field variability ever recovered from deep-sea sediments. Sediments in the uppermost 150-220 m of each site were collected using the advanced piston corer (APC), which is capable of recovering almost pristine cores of soft sediment. Aboard ship, we measured the archive halves of all sediment cores using a new long-core cryogenic magnetometer (Model 760 from 2G Enterprises) with an in-line alternating magnetic-field (AF) demagnetizer, which was installed after ODP Leg 169. From the long-core measurements, we were able to estimate the pattern of geomagnetic field secular variation (both directions and intensity) for the Brunhes Chron and identify 14 "plausible" Brunhes-aged magnetic field excursions (Keigwin, Rio, Acton, et al., 1998; Lund et al., 1998).

This paper considers in more detail excursions labeled 3 and 3 by Lund et al. (1998; Chap. 10, this volume). We reconsider the shipboard long-core paleomagnetic results that led us to suggest these excursions existed, we present new U-channel paleomagnetic records for three replicate sediment sections containing excursions 3 and 3 (= Laschamp Excursion), and we compare these results with independent discrete sample paleomagnetic results from the same time interval (Schwartz et al., 1997; Lund et al., in press). These results confirm the existence of excursion 3 but not 3. They also show that long-core and U-channel measurements in excursion intervals, characterized by fast changes in field direction and/or intensity, may not be able to routinely recover the detailed pattern of directional field variability.