Optical resetting in large drainage basins: tests of zeroing assumptions using single-aliquot procedures

Optical resetting of previously accumulated charge populations at the time of transportation and burial of sediments remains a critical assumption in luminescence dating. This is particularly the case for fluvial sediments. We have tested the extent of bleaching in fluvial environments by applying the single-aliquot regeneration (SAR) equivalent dose (D-E) determination technique coupled with tests for partial bleaching. In the majority of cases, samples collected from a point bar complex on the Colorado River, Texas exhibit evidence of incomplete bleaching. Use of the minimum SAR D-E in these cases resulted in estimated residual ages of c. 100-300 years. In a similar examination of SAR D-E's for bed load samples collected along the Loire River we observe a monotonic decrease in effective equivalent dose (D-e) from values of 14, 10.2 and 3.2 (mean, median and minimum) Gy at source, down to 3.3, 3.6 and 1.6 Gy in the first kilometre of transportation, down to levels of 0.3, 0.2 and 0.0 Gy for transportation distances exceeding 300 km. We observed virtually no effect on the D-E in the bed load of the introduction of trunk tributary streams and conclude that the likelihood of resetting of OSL in large fluvial systems is somewhat more effective that previously considered when the recycling of sediments down the system are borne in mind. (C) 2000 Elsevier Science Ltd. All rights reserved.