EXCURSIONS DURING THE BRUNHES CHRON

Our long-core paleomagnetic measurements (Lund et al., 1998; Keigwin, Rio, Acton, et al., 1998) indicated that at least 14 "plausible" magnetic field excursions have occurred over the past 780 k.y. during a time of apparently stable normal magnetic field polarity (Brunhes Chron). The "plausible" excursions were identified on the basis of three criteria: (1) presence of true excursional directions (virtual geomagnetic poles [VGPs] more than 45° away from the North Geographic Pole after reorienting core-segment average declinations to 0°), (2) occurrence in at least four different holes at two or more sites, and (3) location of excursional directions within a reproducible and correlatable pattern of more typical secular variation. (Two other potential excursions were identified that fail these criteria and were therefore not considered "plausible." Another one or two potentially "plausible" but unnumbered excursions are located just above the Brunhes/Matuyama Boundary and are considered elsewhere in the context of the transitional field behavior.)

We temporarily named the 14 "plausible" excursions by the SPECMAP oxygen isotope stage within which they occur (Stages 3-15) and their relative sequence within a single oxygen isotope stage ( = younger, = older; symbols "a" and "b" were used in Keigwin, Rio, Acton, et al. (1998) and Lund et al. (1998) but are changed herein to minimize conflict with SPECMAP substage names "a-e"). The stratigraphy of all identified Brunhes-aged excursions (>100 individual records) is shown in Figure F1. It is clear that individual excursions are not always noted in all holes. This is partially due to four limitations of long-core measurements and the coring process: (1) smearing of some sediment along the sides of the cores, (2) integration of paleomagnetic measurements over ~10 cm of sediment (a function of the magnetometer configuration), (3) limited demagnetization of the sediments (typically only up to 20 mT AF), and (4) coring gaps.

Previous paleomagnetic studies of piston cores from near the locations of Sites 1060-1063 (Lund et al., 1995, in press) identified three of the excursions noted in Figure F1: 3 (Laschamp Excursion), 5 (Blake Event), and 7. Discrete sample measurements made during Leg 172 on sediments that record excursions 8, 9, 9, and 11 confirm the long-core measurements but also illustrate the need for further detailed study of discrete and U-channel samples for all aspects of the observed field variability (Lund et al., 1998; Keigwin, Rio, Acton, et al., 1998).

We now have replicate U-channel records across all of the "plausible" excursions listed in Table T1. The U-channel measurements confirm the existence of 12 of the excursions and noted a new one (17) not identified in the long-core measurements. U-channel measurements contained no evidence of true excursional directions for excursions 3 and 5; we now consider them not to be true excursions but rather artifacts of our analysis using long-core measurements alone. A U-channel study of the 3 and 3 excursion sequence is presented as a companion paper in this volume (Lund et al., Chap. 11, this volume).

Selected U-channel records of excursions 13, 14, 15, 15, and 17 are shown in Figures F5, F6, and F7. Excursions 14 and 15 look very similar to the pattern noted in shipboard long-core measurements. They both contain strongly negative inclinations, significant declination variability, and excursional VGPs. By contrast, excursions 13 and 15 are more subdued in directional variability than was expected from the long-core measurements. Their overall variability is still distinctive, but further analysis of the replicate U-channel records plus discrete sample measurements will be needed to state certainly whether or not these are true excursions as opposed to exceptionally large-amplitude secular variations (as noted in Table T1).

One complication, which we noted in both the long-core and U-channel measurements, is the presence of intervals with very high positive inclinations (>80°), often with large declination variability, which have excursional VGPs. Several such intervals are noted in Figures F5, F6, and F7. We noticed this first aboard ship during long-core measurements, and in several instances we could ascribe the effect to narrow intervals with distinctive lithology and anomalously high or low NRM intensity. The same effect is apparent in the U-channels. Up to now, we have presumed that such features are all artifacts of long-core or U-channel measurement and do not reflect true geomagnetic field variability.

Nevertheless, it is important to note that the Mono Lake Excursion (Denham and Cox, 1971; Liddicoat and Coe, 1979) does clearly have excursional VGPs associated with very high (~84°) positive inclinations. Unfortunately, it is the only example we currently have of "true" excursional behavior with high inclinations; all other previously published excursions (almost all from sites in the Northern Hemisphere) have excursional VGPs associated with intervals of negative inclination. Up to now, we have not assigned any "plausible" excursions to intervals of unusually high positive inclination but we clearly do note such intervals (Fig. F5, F6, and F7). In most cases, we are reasonably certain they are artifacts of continuous paleomagnetic measurement. However, excursion 17 looks very similar in style to the Mono Lake Excursion and may indeed have excursional VGPs at times of high positive inclinations. Unfortunately, we currently do not have any unique way to confidently identify excursions associated with high positive inclinations on the basis of long-core or U-channel measurements. The only way we will ever confidently prove whether selected high-inclination intervals are truly excursional will be through discrete sample measurements that are planned for the future.