The distribution of apparent dose as determined by optically stimulated luminescence in small aliquots of fluvial quartz: Implications for dating young sediments

We noted recently (Murray et al., 1995) that quartz samples taken from in-channel fluvial deposits have apparent doses ranging over several orders of magnitude. We suggested that these dose distributions are best explained by a mixture of a small number of poorly bleached grains and a larger number of well-bleached grains. In this paper we examine the distribution of apparent dose in small aliquots teach composed of similar to 2000 grains) of different particle size fractions (63-90 mu m, 90-125 mu m, 125-180 mu m, 180-212 mu m, 212-250 mu m diameter) from two samples collected from the Murrumbidgee River, New South Wales, Australia. The samples are of known burial age, the first being 70 years old, and the second being its modern equivalent (< 1 year old). In each of the size fractions from both samples we observe a spread in the apparent dose (measured using the ratio of the natural optically stimulated luminescence (OSL) to the regenerated OSL). The average dose decreases significantly; with increasing particle size, indicating that the coarser fractions are generally better bleached than the finer fractions; the 'SARA' protocol is used to demonstrate that at least for large aliquots (10 mg), the difference in apparent dose between the various size fractions is not the result of a change in sensitivity. Approximately 12% of the aliquots from the different size fractions from the modern channel sediment have apparent doses consistent with zero, indicating that they contain only well-bleached grains. In contrast, the different size fractions from the 70 yr-old sample all give finite apparent doses of > 0.2 Gy. The difference in the lowest apparent dose in the two samples is considered to be the result of their different burial times. We suggest a method for dating recently deposited (< 500 years), poorly bleached fluvial sediments, and apply this method to a sediment core from fluvial deposits of unknown age from the Namoi River, NSW. We also compare the dose distribution observed in modern fluvial samples with that observed in a well bleached modern aeolian sand (<5 yr old). (C) 1998 Elsevier Science Ltd. All rights reserved.