ESR ISOCHRON EXERCISES: HOW ACCURATELY DO MODERN DOSE RATE MEASUREMENTS REFLECT PALEODOSE RATES?

Abstract

In electron spin resonance (ESR) dating tooth enamel, after selecting the appropriate U uptake model, the most significant uncertainty lies in the external dose rate, Dext(t), which can vary with time. Unlike standard ESR which measures the external dose rate in the modern context, Dext(t0), assuming that it reflects the actual external dose rate over the millennia, the isochron method calculates the time-averaged dose rate, Dext(t), experienced by the tooth without such assumptions. In 45 teeth ranging from 30ka to 4.5Ma from 17 sites, Dext(t) determined by EU or LU isochrons only equalled Dext(t0) about 50% of the time. In several sites, geologic evidence indicates that secondary sedimentary processes have significantly altered sedimentary compositions and/or water concentrations, and hence, Dext(t), over time, accounting for 60-80% of Dext(t)-Dext(t0) disagreements. Simulated isochrons suggest that at least seven teeth, whose isochrons had negative ages or Dext(t), probably had lost U, while five, with accurate ages but very large Dext(t), have likely experienced a second U uptake event.