Disequilibria in the uranium decay series in sedimentary deposits at Allen's Cave, Nullarbor Plain, Australia; implications for dose rate determinations

Disequilibrium in the uranium decay series can cause a time-dependent variation in the radiation dose received by deposited sediments. In luminescence and ESR dating studies, the observation of disequilibrium usually requires modelling of the variation in dose rate as a function of burial time. In this paper, we report on radionuclide data collected from Allen's Cave on the Nullarbor Plain, in South Australia. This important archaeological site contains sediments in which concentrations of 230Th exceed those of its parent 238U by up to 400%, and lesser levels of disequilibrium exist between 230Th and 226Ra, and between 236Ra and 210Pb. Thorium isotope ratios (230Th:232Th) are used in assessing whether all the sediments are derived from a common source (and are therefore likely to have had the same initial levels of disequilibrium). Radionuclide chemistry is used to determine whether the disequilibria have developed in situ or were present on the sediments at the time of deposition. From these observations, we develop a model for the evolution of the dose rate through time, and determine the effects of dose rate variations on the calculated luminescence ages of various depositional units. We conclude that a precise knowledge of the state of disequilibrium in the uranium decay series is necessary at this site to provide accurate luminescence and ESR dates. (author)