Distribution in SAR palaeodoses due to spatial heterogeniety of natural beta dose

Abstract

Abstract:In luminescence dating of sediments, Mayya et al. (2006) pointed out that at single grain level, the beta dose for quartz grains is heterogeneous. This heterogeneity arises due the fact that the total potassium in sediment is contributed by few feldspar grains with up to 11-14% stoichiometric potassium (Huntley and Baril, 1997). Beta particles have a range of ~2 mm, which is comparable to grain sizes and inter-grain distances. This fact implies that the spatial fluctuation of beta emitters (Kfeldspars) around individual quartz grains results in heterogeneous dose deposition. These fluctuations therefore, lead to an inherent spread in palaeodoses received by individual quartz grains. In this study, we compute the spread in single aliquot palaeodoses that arises exclusively due to heterogeneity in beta radiation dose received by individual grains. We thus postulate that 'single aliquots' (comprising several -typically 100 -heterogeneously irradiated single grains) would have an inherent spread in the palaeodose. In this work, we used Monte Carlo simulations to quantify the extent of spread in palaeodoses arising due to heterogeneity of beta dose and hence put a limit on the precision of age estimation. Simulations results indicated, that, 1) the average of the single aliquot palaeodoses provides the closest approximation to the true palaeodose, 2) the minimum number of aliquots that are needed to obtain a robust estimate of average palaeodose value depend upon desired precision and the concentration of K, and 3) the ratio of maximum to minimum single aliquot palaeodose values for a given K concentration provides a measure of inherent spread arising due to beta dose heterogeneity. Any spread over and above this range, can be ascribed to other sources such as heterogeneous bleaching and sensitivity changes. Radiation dose from other uniformly distributed sources of beta particles (U, Th and Rb) however would reduce this spread.

Optically stimulated luminescence ages from the Lake Agassiz basin in Manitoba

“…Ages were computed using the mean values and also the leading edge method (Lepper and Mckeever, 2002) (Table 1). The computation of final ages was in accordance with the work of Chauhan and Singhvi (2011), which indicated that most dispersion in these samples was attributable to heterogeneity of the beta dose. In the present case where the K concentration ranges from 0.7% to 1.6%, the anticipated dispersion because of heterogeneity of the beta dose distribution is given by the minimum to maximum paleodose ratio of <10, and hence, we used the mean dose for the computation of ages.…”

Luminescence dating of sediments from a Palaeolithic site associated with a solution feature on the North Downs of Kent, UK

“…A reduction in average dose rate and increased dispersion (Fig. 12) with a higher degree of heterogeneity is consistent with modelling simulations and calculations by Nathan et al (2003) and Chauhan and Singhvi (2011). In both sets of results for 316-2 there is one outlying (higher) dose rate measurement that may be associated with a grain source and, if so, its presence in the powdered sample is surprising but may be the result of ball-milling that was insufficient to disperse smaller grains such as zircons of the size observed in the SEM examination.…”

Late Quaternary coastal evolution and aeolian sedimentation in the tectonically-active southern Atacama Desert, Chile

“…As the samples were from well-bedded sediments it is unlikely that this is due to post-depositional disturbance (Bateman et al, 2007). It could instead be caused by (i) microdosimetry heterogeneity within the dose rate (Chauhan and Singhvi, 2011), which is relatively low at these sites, (ii) the poor luminescence characteristics of the quartz in these samples, or (iii) poor bleaching. Given all samples are thought to have come from the same source and had the same potential mixing of sediments prior to deposition, it is unclear why only some samples would appear multimodal/skewed if the poor luminescence characteristics were the dominant cause of this.…”