Initial Paleomagnetic Results from a New Drill Core from Clear Lake, California

Abstract:

We report the initial paleomagnetic results from a new core from Clear Lake, California. Drilling progressed to 160 meters sediment depth at a site close to the location of Core 4 collected by John Sims and his USGS colleagues in 1973. Throughout the core, the NRM has only moderate intensity but directions appear to be stable after demagnetization to 20 mT. A relatively large fluctuation of the geomagnetic field is observed at a depth of about 48.5 meters. The pattern of changes in inclination and declination can be interpreted as a subdued version of the Mono Lake excursion, whose age is between 32,000 and 35,000 years BP. A chronology for the upper third of the Clear Lake core derived from 20 radiocarbon determinations on fossil pollen samples gives an age of about 34,000 years BP for the feature, consistent with the age of the Mono Lake excursion. The observation of the Mono Lake excursion in the Clear Lake core implies that the paleomagnetic directions provide a reliable record of geomagnetic field behavior and that it should be possible to correlate the secular variation record from Clear Lake with secular variation curves from western North America spanning the last hundred thousand years or more. In the lower portion of the Clear Lake core are several 10-20 cm intervals where the NRM intensity increases about an order of magnitude. In some cases, these intervals are associated with rather anomalous directions. Investigations are underway to determine if these high-intensity intervals arise from coring disturbances, large-scale variations in lithology and/or magnetic mineral concentration, or unusual geomagnetic field behavior. These features do not correspond to tephra layers observed in the core.