Repeating waveform initiated by a 180–190 ka geomagnetic excursion in western North America: Implications for field behavior during polarity transitions and subsequent secular variation

Abstract

New paleomagnetic, lithologic, and stratigraphic data are presented from the sediments of Lake Chewaucan in the Summer Lake Basin, Oregon. The new data place better age constraints on the sediments and improve the accuracy of the previously published paleomagnetic record from this locality. A complex, yet distinct, waveform is observed in all three components of the paleomagnetic vector. The waveform begins as the 180–190 ka Pringle Falls/Long Valley/Summer Lake II geomagnetic excursion and continues for two cycles after the excursion, until the record is interrupted by an unconformity that we correlate to the oxygen isotope stage 6/5e boundary. The waveform's directional morphology in virtual geomagnetic pole (VGP) space is defined by two clockwise loops followed by a distinctive counterclockwise, clockwise, counterclockwise looping sequence. The VGP paths of the two cycles after the excursion are rotated 180° about Earth's spin axis with respect to the VGP paths of the excursion cycle. The waveform also consists of a relative paleointensity variation which repeats during the two cycles after the excursion. The average paleointensity of the postexcursion waveform repetitions is high relative to the extremely low values that occur during the excursion. This observation indicates that excursion‐initiated secular variations can occur after the field fully recovers from the low intensities which commonly typify excursions. Because of the similarities noted previously between this excursion and full polarity transitions (Trie et al., 1991), our new observations constrain models for a wide range of field behavior including polarity transitions, excursions, and secular variation.