U-CHANNEL PALEOMAGNETIC MEASUREMENTS OF EXCURSIONS 3 AND 3

We have now recovered replicate U-channel paleomagnetic records of excursions 3 and 3 from three holes: 1061B, 1061C, and 1063C. U-channel measurements (without deconvolution) were made at the University of California-Davis using the specialized U-channel cryogenic magnetometer in Professor Ken Verosub's laboratory. The U-channels were AF demagnetized sequentially in 10-mT steps up to 60 or 80 mT. The AF-demagnetization diagrams of individual horizons, both inside and outside of excursional intervals, routinely showed a linear decay toward the origin above 20 mT (Keigwin, Rio, Acton, et al., 1998). Subsequently, the U-channels were remagnetized with an anhysteretic remanent magnetization (ARM) and stepwise AF demagnetized up to 60 mT, then remagnetized with a saturation isothermal remanent magnetization (SIRM) acquired at 1 T; in some cases back-IRMs were sequentially applied at 0.1 and 0.3 T and measured. The U-channel rock-magnetic results are identical to those of Schwartz et al. (1997) from studies of piston cores collected during the site survey associated with Leg 172.

The shipboard long-core (half round) measurements and U-channel measurements for all three holes (after 20-mT AF demagnetization) are compared in Figures F5, F6, and F7. In all three cases, the anomalously low inclination associated with excursion 3 disappears in the U-channel measurements; the resulting U-channel directional variability does not contain excursional VGPs. The anomalous directional variability of excursion 3 is still present in all three holes, complete with excursional VGPs, but the detailed directional patterns display some significant differences between the shipboard and U-channel measurements and between Sites 1061 and 1063.

The inclination and declination records for excursions 3 and 3 in Holes 1061B and 1061C are overlain in Figure F8. Allowing for ~20 cm of offset between the two holes in the modified composite depth (mcd), it is clear that a reproducible pattern of secular variation is still present, although exact details in amplitude are not always the same. Both data sets contain similar records of excursion 3 with the same pattern of inclination and declination variability as well as a consistent phase relationship between them. However, excursion 3 in Hole 1061C is slightly more subdued (smeared) than in Hole 1061B.

These records can be compared with discrete sample paleomagnetic records of the Laschamp Excursion (= excursion 3) previously recovered from the same area by Lund et al. (in press). Paleomagnetic records from core JPC14 on the Blake Outer Ridge and core CH89-9P from the Bermuda Rise are shown in Figure F9. The two discrete sample paleomagnetic records are almost identical in their directional variability even though they are separated by more than 1200 km. They both show a clear directional phase relationship wherein the largest westerly to easterly declination swing occurs during a time of large negative inclinations. In the U-channel records from Site 1061, the westerly to easterly declination swing occurs significantly later within an interval of high positive inclinations. We think that the discrete sample paleomagnetic records are correct and that the apparent difference in phase relationship in the Site 1061 U-channels is an artifact of the integrative, continuous measurement process and sediment smearing. The differences in smearing may be related to overall sediment accumulation rates on the Blake Outer Ridge.

Figure F10 shows the average sediment accumulation rates across the Blake-Bahama Outer Ridge for the last glacial cycle. Site 1061 has an average rate of 30 cm/k.y., Site 1062 has a rate of 22 cm/k.y., and core JPC14 has a rate of 37 cm/k.y. The lower accumulation rate at Site 1061 may contribute to producing a more smeared record of excursion 3 relative to core JPC14 (or Site 1062). Corroboration for this can be noted by the results of Lund (1993; Lund et al., in press) where they note that the Laschamp excursion record in core CH88-10P from the Blake Outer Ridge (22 cm/k.y. average accumulation rate) (see Fig. F10) produced a strongly smeared record of the Laschamp Excursion with no excursional VGPs.

A comparison of the excursion 3 paleomagnetic record from Hole 1063C with these other records is also instructive. First, there is a more significant difference between the shipboard and U-channel measurements of the same core (Fig. F7). However, the U-channel record from Hole 1063C is much more consistent with the discrete sample records and U-channel records from Site 1061 than with its own shipboard measurements. The problem may be related to anomalous, and unexplainable, higher NRM intensity at ~18.6 mcd in the shipboard measurements that is not reproduced in the U-channel measurements. The Hole 1063C U-channel record comes closest of all the records to replicating the discrete sample paleomagnetic records. Hole 1063C displays almost complete reversal in declination with inclinations reaching almost -80°; this is nearly identical to the record from core CH89-9P (Fig. F9), located within a few kilometers of Hole 1063C. In Hole 1063C, the westerly to easterly declination change is associated with high positive inclinations that occur just before the large negative inclination interval, but the differences in phase with core CH89-9P are much more subtle than those noted at Site 1061. All in all, it appears that Hole 1063C faithfully records excursion 3 in a manner consistent with the discrete sample records described by Lund et al. (in press).

One final complication, which we have previously noted in both long-core and U-channel measurements, is the presence of narrow intervals with very high positive inclinations (>80°), often with large declination variability, which have excursional VGPs. An example (starred interval) is shown for Hole 1063C in Figure F7. We noticed this first aboard ship during long-core measurements, and, in several instances, could ascribe the effect to narrow intervals with distinctive lithology and anomalously high or low NRM intensity. The same effect is apparent in Figure F7. We presume that the feature in Figure F7 is an artifact of U-channel measurement in an interval of fast NRM intensity change and does not reflect true geomagnetic field variability.