The origin of dose distributions in fluvial sediments, and the prospect ofdating single grains from fluvial deposits using optically stimulated luminescence

We examine the causes of the asymmetric distributions of dose observed from measurements of the optically stimulated luminescence emitted by small ali quots of fluvial quartz, and deduce that the asymmetry arises as a result o f samples being composed of a mix of mainly well bleached grains with grain s that were effectively unbleached at the time of deposition. We demonstrat e that the shapes of the dose distributions can be used to assess the likel ihood that aliquots consist only of grains that were well-bleached at the t ime of deposition. The more asymmetric the distribution, the greater the pr obability that the aliquots with the lowest dose most closely represent the true burial dose. Single grains with differing doses are present in each o f the samples examined, and the population with the lowest dose gives an op tical age consistent with the expected burial age. This result implies that the beta-dose heterogeneity in these deposits is small, and that the effec ts of micro-dosimetric variations on optical dating of individual grains ar e not significant for these samples. We demonstrate that single-grain datin g of fluvial material is possible and practicable using standard Rise optic al dating equipment, and we conclude that application of a new regenerative -dose protocol to single grains of quartz, using the lowest dose population to estimate the burial dose, is the best available means of obtaining reli able luminescence ages for heterogeneously bleached fluvial sediments. (C) 1999 Elsevier Science Ltd. All rights reserved.