Ar-40/Ar-39 dating links Albuquerque Volcanoes to the Pringle Falls excursion and the Geomagnetic Instability Time Scale

摘要

Incremental heating of basaltic lava groundmass from the Albuquerque Volcanoes, New Mexico, and plagioclase from Ash D at Pringle Falls, Oregon, yield Ar-40/Ar-39 isochron ages of 218 +/- 14 and 211 +/- 13 ka, respectively. Sediments in which Ash D was deposited and the eight lava flows of the Albuquerque Volcanoes display excursional paleomagnetic data with virtual geomagnetic poles (VGPs) in the southern hemisphere and together with the statistically indistinguishable Ar-40/Ar-39 dates, establish that both sites record the Pringle Falls excursion. This excursion is also recorded by the 227 +/- 8 ka Mamaku Ignimbrite, New Zealand, and by high deposition rate sediments at ODP site 919 in the North Atlantic Ocean that are dated astrochronologically at 209-207 ka. We propose that the names "Albuquerque" and "Jamaica" excursion be abandoned and that a radioisotopic age of 211 +/- 13 ka be adopted for the Pringle Falls excursion, which is one of five globally expressed, well-documented excursions in marine sediment cores or dated by Ar-40/Ar-39 methods that took place from 220 to 30 ka. Together with at least five other well-dated excursions between 730 and 520 ka, some ten excursions define the Geomagnetic Instability Time Scale (GITS) for the Bruhnes Chron. Eight of these excursions have been dated using Ar-40/Ar-39 methods. If the temporal clustering of excursions in the GITS is not a sampling artefact, this suggests that the geodynamo is not intrinsically unstable, but rather that instability in the flow pattern of the outer core fluid flow causes the main dipole field to be considerably weakened such that it becomes unstable with a 200 to 300 ka recurrence interval. Excursions are an important part of dynamo behavior over geologic time scales that, in addition to reversals of the main dipole field, need to be fully considered when assessing whether theoretical and numerical simulations of the dynamo produce earth-like results.