Abstract

The declination of the Earth's paleogeomagnetic field, determined from sediments in Lake Michigan deposited during the past 11,500 C14yr, exhibits fluctuations east and west of the mean declination with a period of about 2,090 C14 yr. Inclination and intensity measurements do not exhibit similar fluctuations. Each extreme in declination occurs in the same stratigraphic position in cores from different parts of the lake. The variations in declination in Lake Michigan are similar to those found in sediments from Lake Windermere, England, deposited during the same time span, but the period of the Windermere cycles is 2,800 C14yr. Plots of the paleogeomagnetic poles for inclination-declination pairs representing each east or west extreme for the two lakes are quite different, indicating that the geomagnetic effects were not a result of shifts of the main dipole field.

In a model of the standing nondipole field, the foci, represented by radial dipoles located one-fourth of the Earth's radius from the geocenter, are each allowed to oscillate with a characteristic period. This model gives a satisfactory explanation of the principal features exhibited by the declination and inclination records at the two lakes.

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