Reassessment of the Mono Lake Excursion

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The Mono Lake Excursion occurred around 35,000 years ago. It represents the most recent time when the Earth's magnetic field has gone through an anomalous, large-scale change in its space/time pattern, perhaps even changing globally into a profoundly different configuration, an excursional state. The Mono Lake Excursion was defined from paleomagnetic studies of lacustrine sediments at Mono Lake, California. This study addresses several unresolved issues surrounding the Mono Lake Excursion. There is currently no record of the complete directional waveform or paleointensity pattern associated with the Mono Lake Excursion outside its type locality at Mono Lake. There is still legitimate uncertainty about the absolute age of the Mono Lake Excursion. There is no high-resolution record of the Mono Lake Excursion in deep-sea sediments from western North America (near Mono Lake), which have an oxygen-isotope chronostratigraphy that can tie the excursion to the global GISP2 ice-core chronology, Heinrich Events, or Dansgaard/Oeschger cyclicity. Assuming (as we do) that the Mono Lake Excursion is ~6000 years younger than the Laschamp Excursion (the next older excursion about 41,000 years ago), what is the relationship between them (if any)? The current study is recovering paleomagnetic records of secular variation and paleointensity from deep-sea sediments of the eastern North Pacific Ocean, off the Oregon coast, and lacustrine sediments from Baldwin Lake, Southern California. The deep-sea sediments have a high-resolution oxygen-isotope record associated with them and the lake record has more than 20 radiocarbon dates that bracket the Mono Lake/Laschamp Excursion interval. Preliminary paleomagnetic results indicate that the Mono Lake Excursion and the older Laschamp Excursion are present in both records with ages that are consistent with previous estimates. The new paleomagnetic records will be combined with previously published far-field paleomagnetic records of the Mono-Lake/Laschamp Excursion intervals from the Arctic Ocean, North Atlantic Ocean, Philippine Islands, and Chile coastal margin to define the space-time pattern of global field variability associated with the Mono Lake Excursion. These data will also be used to build a series of low-degree spherical harmonic models that quantitatively describe that pattern of variability on a global scale. Such observations are required to test concepts of geomagnetic field timescales of variability, the origin and nature of excursions, and their relationship to dynamo behavior.This study is using a high-school student, USC graduate student, and post-doctoral researcher to make measurements and help in the data analysis. The final paleomagnetic data sets and spherical harmonic analyses will be available at the PI's website (http://earth.usc.edu/~slund) and forwarded to other database initiatives.