Single and multi-grain OSL investigations in the high dose range using coarse quartz

Abstract

There is evidence that optically stimulated luminescence (OSL) dating of quartz using the single-aliquot regenerative-dose (SAR) protocol underestimates the equivalent dose (De) for paleodoses above 100–200 Gy. Additionally, ‘infinitely’ old samples found not to be in laboratory saturation were reported. We present single and multi-grain SAR-OSL investigations for a coarse-grained (180–250 μm) quartz sample extracted from loess collected below the Brunhes/Matuyama transition at the Roksolany site (Ukraine). The sample was dated to more than 1000 ka by electron spin resonance using a multi center approach (Al and Ti signals), confirming that the De (∼2000 Gy) falls beyond the limit of standard OSL De measurement techniques. However, the natural signal measured using multi-grain aliquots of quartz was found to be below the laboratory saturation level. A comparison was made between synthetic dose response curves (DRCs) generated from single-grain and multi-grain aliquot data, respectively; the natural signal was found to be closer to the latoratory saturation level (92%) in the case of the single-grain synthetic DRC than for the multi-grain synthetic DRC where the signal was 86% of the saturation level. This difference could not be attributed to stimulation with different wavelengths, i.e. blue and green light stimulation for multi and single-grain measurements, respectively. By analysing synthetic data obtained by grouping grains according to their brightness, it was observed that brighter grains give a natural signal closer to the laboratory saturation level. This trend was confirmed for multi-grain aliquot data. Based on these findings we infer that variability in the contribution from populations of grains with different levels of brightness may represent a controlling factor in the closeness of the natural signal to laboratory saturation level for infinitely old samples.