AMERICAN WEST TEPHRAS – GEOMAGNETIC POLARITY EVENTS REDEFINED THROUGH CALIBRATION OF RADIO-ISOTOPIC AND ASTRONOMICAL TIME

The foundation of the EARTHTIME/GTSnext initiative seeks to construct an internally consistent geologic timescale based on astronomical and radio-isotopic geochronology. American west tephras offer a prime opportunity to integrate these two independent timescales with the geomagnetic timescale. Using an astronomically calibrated age for the monitor mineral Fish Canyon sanidine (FCs; 28.201 ± 0.046 Ma, Kuiper, et al., 2008), ages of Pleistocene geomagnetic polarity events are re-examined.

Of particular interest, the Quaternary mineral dating standard Alder Creek sandine (ACs) is the type locality for the Cobb Mountain geomagnetic event. New single-crystal analyses, performed on a Noblesse multi-collector noble gas mass spectrometer, suggest a refined ⁴⁰Ar/³⁹Ar age for ACs, with precision nearing the ambitious 0.1% goal of the EARTHTIME project. Moreover, this new ACs age is consistent with the astronomical age of the Cobb Mountain event, independently determined through correlation of oxygen isotopes in a piston core (Horng, et al. 2002).

Two other ash flows in the American west were analyzed: the Bishop and the Huckleberry Ridge Tuffs. Multi-crystal sanidine experiments of the Bishop Tuff provide an astronomically relative ⁴⁰Ar/³⁹Ar age for the eruption and associated Matuyama/Brunhes magnetic polarity transition. Single-crystal astronomically relative ⁴⁰Ar/³⁹Ar ages for the Huckleberry Ridge Tuff are indistinguishable from previously determined ages, and provide a degree of confidence in the astronomical calibration. Although this geomagnetic event is not part of the most recent geologic timescale, refined ages on short-lived excursions could hold importance to understanding time scales for the wavering nature of Earth’s magnetic field.

We propose a new ⁴⁰Ar/³⁹Ar age for the Quaternary mineral dating standard ACs that reflects the astronomical calibration of FCs and age of the Cobb Mountain polarity event. It is suggested that this ⁴⁰Ar/³⁹Ar age replace that of Renne, et al. (1998) when using ACs as the monitor in argon age determinations.

The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 215458.