Investigation of the applicability of standardised growth curves for OSL dating of quartz from Haua Fteah cave, Libya

Abstract

The establishment of standardised growth curve (SGC) for equivalent dose (De) determination can substantially reduce the amount of instrument time required for OSL measurements. In this study, we investigated the applicability of SGC for the optically stimulated luminescence (OSL) signal from single grains and small aliquots of sedimentary quartz from Haua Fteah cave, in Libya. The samples exhibit large inter-grain and inter-aliquot variation in the shape of their single grain and small aliquot dose response curves (DRCs) constructed from a range of sensitivity-corrected regenerative dose signals (Lx/ Tx), which prevents the establishment of a single common SGC among different grains or small aliquots. Instead, the DRCs for the small aliquots can be divided into a minimum of three groups using the Finite Mixture Model, with the DRC saturating at a different dose level for each group. In order to establish a common DRC, or SGC, for each group, we propose a new normalisation method, the so-called leastsquares normalisation procedure ('LS-normalisation'), which largely reduces the variation between aliquots within the same group and allows the establishment of a common DRC, or SGC, for each group. In order to apply the SGC method for these samples, two regenerative dose points are needed for each aliquot to attribute it to one of the groups based on the ratio of the Lx/Tx signals for two sensitivitycorrected regenerative dose points. Equivalent dose (De) values for each aliquot can be estimated using the fitting parameters calculated for the SGC of the relevant group to which it belongs, together with measurements of the natural signal (Ln), one regenerative dose signal (Lx1) and their corresponding test dose signals (Tn, Tx1). For the samples investigated from Haua Fteah, we found that De estimates obtained from the SGCs are consistent with those obtained using a full SAR procedure. Our results suggest that small single-aliquot and single-grain De values obtained from application of the SGC may be underestimated if there is a significant proportion of early-saturating grains present in the sample; such grains or aliquots are mostly rejected due to saturation when analysed using the full SAR procedure. In this case, it is necessary to calculate the De values based only on those grains or aliquots that have relatively high saturation levels.

Variability in quartz OSL signals caused by measurement uncertainties: Problems and solutions

“…Douka et al (2014) made some pertinent observations about the OSL behaviour of the grains, including the following: (a) among the wide range of OSL decay curve shapes, some had much slower M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 5 rates of decay than others, but carry-over of OSL signal between successive measurement cycles was not problematic; (b) the majority of DRCs could be fitted with a single saturating exponential function; (c) grains show a large range of DRC shapes; and (d) some of the natural OSL signals are close to, or in, dose saturation: 6.4% of the multi-grain aliquots and 13.1% of the single grains have L n /T n ratios that lie at, or above, the saturation level of the corresponding DRC and can be classified as either saturated grains or as Class 3 ('oversaturated') grains . Li et al (2016) re-analysed the multi-grain data for HF11. Based on the observation that fewer than 5% of the measured single grains contributed >80% of the total OSL signal, they deduced that the measured OSL signal from the multi-grain aliquots arises from only one or two grains, thereby effectively making these measurements surrogate single-grain measurements.…”

“…Understanding differences in single-grain dose response curves (DRCs) is important since some studies have shown that D e estimation can be dependent on the observed variation in the shape of the DRC (or characteristic saturation dose, D 0 ) (e.g., Gliganic et al, 2012;Duller, 2012;Li et al, 2016;Thomsen et al, 2016;Guo et al, 2017). Characterising the intrinsic variability of experimentally observed optically stimulated luminescence (OSL) signals from individual grains of quartz is, therefore, imperative to assess the reliability of DRCs and the resulting equivalent dose (D e ) values and ages.…”

“…We also investigate how these measurement uncertainties may cause difficulties with D e estimation for samples with natural doses close to the saturation level of the DRC when using a conventional SAR or standardised growth curve (SGC) procedure Murray and Wintle, 2000;Li et al, 2015a). To potentially overcome this problem, we propose a new method, based on construction of a SGC (Roberts and Duller, 2004;Li et al, 2015a;2015b) and test the validity of this method using experimental data for a sample collected from an archaeological site in North Africa (Douka et al, 2014;Li et al, 2016).…”

New ages for the Upper Palaeolithic site of Xibaimaying in the Nihewan Basin, northern China: implications for small-tool and microblade industries in north-east Asia during Marine Isotope Stages 2 and 3

“…Li et al . ( ) have suggested that if a significant proportion of grains in a sample yield infinite D e values (i.e. L n / T n values in the saturated region of the DRC), then this could result in a truncated D e distribution and a corresponding underestimation of true D e and age.…”

“…The latter value is slightly less than unity and indicates that the given dose was not recovered fully using the measurement conditions and/or the data selection criteria. Li et al (2016) have suggested that if a significant proportion of grains in a sample yield infinite D e values (i.e. L n /T n values in the saturated region of the DRC), then this could result in a truncated D e distribution and a corresponding underestimation of true D e and age.…”