DISCUSSION

This overview of new U-channel paleomagnetic results from Leg 172 further supports our new view of geomagnetic field variability during the Brunhes Chron. It also illustrates the ability of ODP to acquire state-of-the-art paleomagnetic field records from deep-sea sediments using the APC and, within definable limitations, to initially evaluate the sediment paleomagnetism using the shipboard cryogenic magnetometer. U-channel measurements indicate, however, that selected secular variation or excursional features noted in the long-core measurement processes may be artifacts of the measurement process and require careful analysis and corroboration.

The number and ages of the Brunhes excursions listed in Table T1 suggest that excursions can no longer be viewed as simply or even primarily regional anomalies of the geomagnetic field. Within one small region, we have identified more Brunhes-aged excursions than all previous good-quality paleomagnetic studies worldwide put together. Moreover, for almost any previously identified excursion anywhere in the world, we can find an excursion record in Sites 1060-1063 that is not significantly different in age. This suggests that excursion intervals are broadly synchronous on a global scale, which in turn implies that the geomagnetic field is globally in an "excursional state" during these intervals.

Most of the excursions tend to occur in "bundles" of two or three close together with intervening intervals of distinctive (large amplitude) magnetic field secular variation. Altogether, the bundles tend to last 20-50 k.y. It is possible that these bundles of closely spaced excursions indicate a continuing "excursional state" or pattern in the core dynamo process that spans the duration of the bundles. Although each individual excursion probably contains only 1-2 k.y. of true excursional directions, the bundle time intervals, including the intervening distinctive secular variation, may all together represent more than 20% of Brunhes time.

All of these observations suggest that excursions are not rare, random perturbations of the stable geomagnetic field, but are rather an important systematic and distinct component of the Earth's magnetic field variability between field reversals. Based on our new results and previous studies, such an "excursional state" of the Earth's magnetic field may have both a strongly multipolar spatial pattern of variability and a complicated temporal pattern of variability. If this perspective is true, identification and correlation of individual excursion records on a global scale must depend on independent high-resolution age estimates; also, the detailed behavior of individual excursions may be quite different in different parts of the Earth. However, the exact space-time pattern of geomagnetic field behavior during excursional states and the relationships between individual excursions worldwide in any one narrow time interval must await further work and comparison with other global records.

NEXT