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 Alien's Cave on the Nullarbor Plain, in South Australia. This important archaeological site contains sediments in which concentrations of Th-230 exceed those of its parent U-238 by up to 400%, and lesser levels of disequilibrium exist between Th-230 and Ra-226, and between Ra-226 and Pb-210. Thorium isotope ratios (Th-230:Th-232) 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. If the state of disequilibrium at the time of excavation is well known, then the maximum error in the dose rate, from the use of present-day individual nuclide concentrations, is < 2% (the uranium decay series accounts for similar to 15% of the total beta and gamma dose). However, if only the parent nuclide concentrations are determined, this error increases to similar to 8%. 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. (C) 1997 Elsevier Science Ltd.