Paleomagnetic measurements made during Leg 202 on cores from the Chile margin Sites 1233, 1234, and 1235 document that these sites may provide the highest-resolution long-term record of paleomagnetic secular variation (PSV) and excursional field behavior ever recovered. They also will provide some of the very first records that document high-resolution paleomagnetic field behavior in the Southern Hemisphere. Shipboard measurements indicate that reproducible records of directional PSV have been recovered from four independent holes at Site 1233 (Fig. F12) over the entire 137 mcd, from the uppermost 30 mcd of three independent holes at Site 1234 (Fig. F13), and from the uppermost 15 mcd of three independent holes at Site 1235. The prospect is good that even better final PSV records from these intervals extending to greater depths at Sites 1234 and 1235 will be developed with shore-based paleomagnetic studies.
Shipboard paleomagnetic measurements also clearly suggest that magnetic field paleointensity variations can be determined from these cores. Initial relative paleointensity estimates determined by normalizing the sediment NRM to magnetic susceptibility have been developed for all of Site 1233. The Site 1233 shipboard relative paleointensity and directional PSV records (Fig. F14) are thought to have the potential to provide the longest and highest-resolution "total-vector" PSV record ever observed. A close-up of the total-vector PSV record (Fig. F15) from Site 1233 shows that remarkable cyclicity, on the scale of ~3 m (equivalent to ~2 k.y. based on preliminary shipboard age models), exists in both the directional PSV and paleointensity records for more than 30 k.y. of the late Pleistocene. This total-vector PSV record, when finally developed, should provide important new insights into the working of the Earth's outer-core dynamo, which generates the Earth's magnetic field.
The directional PSV data from Sites 1233 and 1234 also document at least one and possibly two magnetic field excursions. The younger of these excursions probably occurred ~41 k.y. ago, based on an initial timescale (Fig. F14) determined by correlation of the paleointensity data with other sites around the world, and is almost certainly the Laschamp Excursion (Fig. F16). The PSV record of the Laschamp Excursion at Site 1233 is ~2 m in width (documented in three separate holes) and probably spans a time interval of <1500 yr. That makes this one among the highest-resolution paleomagnetic records of an excursion ever recovered. Moreover, a similar excursion is also recorded at Site 1234, more than 400 km away, and the patterns of more frequent directional PSV are comparable between the two sites (Fig. F16). Further shore-based paleomagnetic studies of these Laschamp Excursion records should provide valuable new insight into the workings of the Earth's magnetic field during times of anomalous behavior that may be related to the geomagnetic field reversal process, as well as magnetostratigraphic information in unprecedented detail that will likely provide reference stratigraphic sections for the region and for the world.