Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale: Paleomagnetism of radioisotopically dated tuffs from Laramide foreland basins

摘要

Age calibration of the early to middle Eocene geomagnetic polarity time scale remains highly uncertain due to conflicting magnetostratigraphic, radioisotopic, and astrochronologic results. In this study, new paleomagnetic polarity determinations of 29 ash-fall tuffs preserved in strata of five Laramide foreland basins were used in conjunction with previously published 40Ar/39Ar ages from the same tuffs to evaluate eight different calibration models for the early to middle Eocene part of the geomagnetic polarity time scale. Reliable paleomagnetic information was recovered from 23 tuffs, of which 17 showed normal polarity and six showed reversed polarity. After comparison of the models with the paleomagnetic and radioisotopic data from the tuffs and an array of independent chronostratigraphic observations, the new Willwood model is herein selected as the best alternative to the current geomagnetic polarity time scale calibration for the early to middle Eocene. Three important implications are apparent in our proposed model. First, the early Eocene is shortened by 0.6 m.y., and the middle Eocene is lengthened by 0.8 m.y. compared with the 2004 geomagnetic polarity time scale. Also, the early Eocene climatic optimum is estimated to have lasted from 52.9 to 50.7 Ma, ∼1 m.y. longer than previously suggested, and overlapping in time with the inferred age of the Wasatchian-Bridgerian faunal transition. Our new model agrees with a previous astronomical model when it is tied to the oldest proposed age for the Paleocene-Eocene Thermal Maximum at 56.33 Ma.