Speaker: Brad S. Singer, UW Department of Geoscience
Abstract: During the past 2.6 million years Earth's outer core geodynamo has produced at least 18 geomagnetic excursions and 5 full polarity reversals. This record has been compiled from terrestrial volcanic rocks, including mainly basaltic lava flow sequences, but also two silicic ash beds, that have been analyzed using modern paleomagnetic techniques and dated using the 40Ar/39Ar variant of the K-Ar radio-isotopic clock. Several brief periods of field instability associated with excursions correlate with lows in paleointensity or directional changes recorded globally in marine sediments that are dated using astronomically-forced oxygen isotope signals or ice layer-counting. However, the lack of correlation of several excursions between marine and terrestrial records indicates that neither sediments, nor lava flows, are ideal recording media. Another factor complicating correlation is that some excursions may be geographically localized and not expressed globally. Despite decades of observation, these records remain fragmentary, especially when periods of millions of years are considered. Recent 40Ar/39Ar dating in our laboratory, that includes age determinations for the Mono Lake, Laschamp, Blake, Pringle Falls, Big Lost, West Eifel excursions, as well as the Halawa (C2r.2r-1) cryptochron, prompt us to critically review the terrestrial record of geodynamo instability and propose a Geomagnetic Instability Time Scale (GITS) for the Quaternary period. Both the ca. 4:1 ratio of excursions to reversals during the past 2.6 Ma as well as the temporal pattern of occurrence of these events provide fundamental input as to the long‑term behavior and, possibly, the structure of the core dynamo. On the one hand, intervals of significant temporal clustering of excursions have highlighted a relatively stable period of high field strength lasting >250,000 years in the middle of the Brunhes chron during which time few, or no, excursions took place. On the other hand, successive paleomagnetic excursion records may be critical in regard to understanding the behaviors and interactions between the mantle‑influenced field source in the shallow core (the hypothesized SCOR‑field) and the deeper‑held source of the axial dipole. If in fact a successful reversal attempt requires the axial dipole field source to be weakened below some threshold strength for substantial duration (ca. 10,000 ‑20,000 years), times of grouped excursions may also be the most probable times for a change in polarity.