) and another in MIS 5 (Excursion 5) should no longer be considered true excursions. The U-channel measurements also resulted in the identification of one new excursion in MIS 17.
One of the striking results of Leg 172 was the discovery of numerous geomagnetic excursions that occur within the Brunhes Chron (0-780 ka). Lund et al. (1998) documented 14 plausible excursions from shipboard results. The term plausible was used to denote excursions that gave virtual geomagnetic poles (VGPs) deviating by more than 45° from Earth's spin axis and that occurred in at least four different holes at two or more sites. They noted that not only was the number of excursions much larger than previously thought, the percentage of time that the Brunhes paleomagnetic field was in an excursional state was much longer than previous observations indicated or models predicted. The total time the geomagnetic field was in an excursional state possibly represented more than 20% of the past 780 k.y. Lund et al. (Chap.
10, this volume) reiterated these basic findings, although they also further probed the paleomagnetic signal of the sediments with detailed U-channel and discrete sample measurements. They confirmed the directional changes for 12 of the 14 plausible excursions, suggesting that one of the directional deviations that occurred in MIS 3 (Excursion 3) and another in MIS 5 (Excursion 5) should no longer be considered true excursions. The U-channel measurements also resulted in the identification of one new excursion in MIS 17.
More detailed analyses of individual excursions have also been conducted. Lund et al. (Chap.
11, this volume) explored the variability of the geomagnetic field in MIS 3 at Sites 1061, 1062, and 1063. The younger of the two MIS 3 excursions, referred to as Excursion 3, was shown to have less directional variability than was apparent in the shipboard split-core paleomagnetic measurements. The older MIS 3 excursion, referred to as Excursion 3
, was interpreted to be the Laschamp Excursion. They also showed that the secular variation of the geomagnetic field before and after this excursion could be correlated between sites >1000 km apart. Williams et al. (submitted [N3]) examined three excursions in MIS 7 and 8. One, referred to as Excursion 7, occurs at ~190 ka and correlates well with known events on land in North America and in the Pacific region. The two other excursions are associated with changes in sediment characteristics such as carbonate content. Although there is no obvious relationship between the sediment changes and the magnetic changes, these authors are less certain that the magnetic changes in those samples reflect real magnetic field behavior than they are for the ~190-ka event.
Acton et al. (submitted [N4]) evaluated magnetic artifacts in ODP piston cores. Besides the ubiquitous isothermal remanent magnetizations imparted by the drill string, which are easy to recognize and remove, they noted the existence of more subtle overprints that result from shearing of the sediment as the corer penetrates. They developed a model to predict the size and sense of the deflection of the paleomagnetic remanence. This model successfully accounts for the difference in remanence between split-core data, which contain shear biases, and U-channel data from the center of a core where there is no shear.
Finally, Clement et al. (submitted [N5]) took advantage of high sedimentation rates to examine the reversals associated with the Cobb Mountain Subchron on the Bermuda Rise. U-channel samples were analyzed from two holes at Site 1063 to identify and remove artifacts. The stacked results compare favorably with the path taken by the virtual geomagnetic pole as measured in other sites, and they indicate that the transitional magnetic field was probably dipolar throughout the reversal.
