Paleomagnetism of Pleistocene Sediments From Drill Hole OL-92, Owens Lake, California Reevaluation of Magnetic Excursions Using Anisotropy of Magnetic Susceptibility

Magnetic Susceptibility) contains new anisotropy of magnetic susceptibility (AMS) data for selected samples from drill hole OL-92. The data were obtained from depth intervals that in earlier publications were interpreted to span several paleomagnetic excursions. Preliminary interpretation of the AMS results indicates that the high dispersion of the paleomagnetic directions within these intervals is due to core deformation rather than to rapid, high amplitude changes in the past geomagnetic field. Introduction Core OL-92 from Owens Lake, California, was obtained using rotary-core drilling into 323 m of lacustrine sediments, estimated to represent the past 800,000 years (Smith and Bischoff, 1997). Because deposition was rapid and was probably nearly continuous during this period, the core potentially contains a detailed record of geomagnetic field behavior for the late Quaternary. Glen and Coe (1997) presented paleomagnetic results that documented the Matuyama/Brunhes (M/B) polarity boundary near the bottom of the core. In addition, they interpreted the results to record detailed field behavior during the M/B transition as well as numerous (about 16) magnetic excursions during the Brunhes Chron. Although these paleomagnetic results were considered tentative, pending efforts to assess the validity of the record, many of the features were interpreted to correlate with magnetic excursions reported in the literature. These correlations were used to corroborate the depth/age model for the OL-92 core that was based on mass accumulation rates (Bischoff and others, 1997b). If the paleomagnetic record from core OL-92 is shown to record excursions, then this record will be important for age control of the sedimentary section as well as for a remarkable archive of geomagnetic field behavior through the Brunhes Chron. Use of a paleomagnetic record such as that from OL-92 to provide age control requires (1) that the measured characteristic directions of remanent magnetization are accurate enough representations of the ancient geomagnetic field that features of interest (reversals, excursions, or wave-forms due to secular variation) can be discerned, and (2) that features of interest can be unambiguously correlated with corresponding features from well-established, dated paleomagnetic records. Many factors can affect the quality of a sedimentary magnetic record, including sediment grain size, depositional environment, natural and coring-induced deformation, as well as authigenic growth and/or destruction of magnetic minerals. Perhaps the best way to evaluate the quality of sedimentary paleomagnetic records is to compare multiple records from a given sedimentary sequence. This approach is prohibitively expensive for the entire middle and late Quaternary sequence …