Construction of a ‘global standardised growth curve’ (gSGC) for infrared stimulated luminescence dating of K-feldspar

We investigated the infrared stimulated luminescence (IRSL) and post-infrared IRSL (pIRIR) signals emitted by K-feldspars from sedimentary samples from Asia, Europe and Africa using a single-aliquot multiple elevated temperature (MET) stimulation procedure. For separate aliquots of the same sample, we show that variation among the dose response curves (DRCs), or growth curves, constructed from the regenerative dose signal (L_x), the test dose signal (T_x, an indicator of luminescence sensitivity) and the sensitivity-corrected signal (L_x/T_x) can be largely eliminated by normalising the DRCs using one of the regenerative dose signals; we call this procedure ‘regenerative-dose normalisation’ or re-normalisation. Furthermore, for the MET-pIRIR signals measured at 250 °C, we find that different samples have re-normalised DRCs that follow the same growth function, despite the samples differing significantly in terms of their geological provenance, sedimentary context, equivalent dose (D_e) and luminescence sensitivity. This common feature offers the potential to establish a ‘global standardised growth curve’ (gSGC) for different samples of K-feldspar, and thereby enable D_e values to be estimated for a large number of single aliquots by projecting the re-normalised natural signals on to the gSGC. For the 18 samples investigated in this study, we find that D_e estimates obtained from the gSGC are consistent with those obtained using full single-aliquot regenerative dose (SAR) procedures for doses of up to ∼1600 Gy. The establishment of a gSGC would greatly reduce the time required to date older samples using K-feldspar, as regenerative doses of several hundreds to a few thousands of Gy are typically delivered to each aliquot in each SAR cycle.     

Potential of establishing a ‘global standardised growth curve’ (gSGC) for optical dating of quartz from sediments

We report investigations of the optically stimulated luminescence (OSL) signals of sedimentary quartz from different regions of Asia, Africa, Europe and North America using a single-aliquot regenerative-dose (SAR) procedure. We show that variations in the shape of dose response curves (DRCs), or growth curves, of the test dose sensitivity-corrected OSL signals among single aliquots composed of multiple grains can be greatly reduced by normalising the DRCs using one of the regenerative dose OSL signals. We refer to this regenerative-dose normalisation procedure as ‘re-normalisation’. We find a common re-normalised DRC extends to doses of ∼250 Gy for samples that differ significantly in terms of geological provenance, sedimentary context and depositional age. This feature permits the development of a ‘global standardised growth curve’ (gSGC) for OSL signals from single aliquots of quartz. The equivalent dose (D_e) of an aliquot can be estimated from the natural signal, one regenerative dose signal and their corresponding test dose signals. For the variety of samples investigated, we find that D_e estimates obtained from the gSGC are consistent with those obtained using full SAR procedures for doses of up to ∼250 Gy. Use of the gSGC for single aliquots would greatly reduce the time required to estimate the D_e values of older samples and for a large number of aliquots.     

The effect of single grain luminescence characteristics on single aliquot equivalent dose estimates

We report an investigation of the effect of different single grain OSL behaviours on multiple-grain (single aliquot) D_e estimates. This was done by creating ‘synthetic aliquots’ in which signals are summed from collections of individual grains. The samples used were taken from aeolian linear dune sediments, from sites located in the Kalahari. We note this is a type of sample for which single multiple-grain aliquots are routinely used for dating. Several grain behavioural types were identified in our samples but the only grain type that systematically affected multiple-grain D_e estimates was the ‘over-saturating’ type (the case where the natural signal (L_n/T_n) is significantly greater than the regenerated L_x/T_x signal at saturation), the presence of which lead to higher estimates of D_e. The magnitude of this effect is controlled by the contribution of ‘over-saturating’ grains to the total light sum of the multiple-grain aliquot and the maximum increase observed in this study was ∼15 Gy. Of the four samples investigated here, inter-aliquot variations in the signal contribution of ‘over-saturating’ grains led to a significant increase in dispersion in just one of the samples. More generally, the presence/absence of these grains is considered to be a likely source of significant ‘intrinsic’ scatter in single aliquot dating and likely to broaden multiple-grain D_e distributions. We suggest all samples are screened for this behaviour before single aliquot dating is attempted.     

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

We simulated the variability in measured quartz optically stimulated luminescence (OSL) signals and dose response curves (DRCs) caused by measurement uncertainties, including counting statistics and instrumental irreproducibility. We find that these measurement errors can give rise to large variations in the observed luminescence signal and contribute to among-aliquot or among-grain scatter in DRCs and equivalent dose (D_e) values. Different measurement systems (i.e., luminescence readers) may have different counting statistics properties and, hence, may exhibit differing extents of variation in the observed OSL signal, even for the same sample. Our simulation shows that the random measurement uncertainties may result in some grains or aliquots being ‘saturated’ (that is, the measured natural signal is consistent with, or lies above, the saturation level of the measured DRC) and that the rejection of these ‘saturated’ grains may result in a truncated D_e distribution, with D_e underestimation for samples with natural doses close to saturation (e.g., twice the characteristic saturation dose, D₀). We propose a new method to deal with this underestimation problem, in which standardised growth curves (SGCs) are established and the weighted-mean natural signal (L_n/T_n) from all measured grains is projected on to the corresponding SGCs to determine D_e. Our simulation results show that this method can produce reliable D_e estimates up to 5D₀, which is far beyond the conventional limit of ∼2D₀ using the standard SAR procedure.     

The build-up of the laboratory-generated dose-response curve and underestimation of equivalent dose for quartz OSL in the high dose region: A critical modelling study

The application of a regeneration procedure for optically stimulated luminescence (OSL) dating requires that the dose-response curve (DRC) of a natural sample is the same as that of a laboratory-generated one. However, the build-up of the laboratory-generated DRCs of quartz has been widely reported in the literature, i.e., the laboratory-generated DRCs are significantly higher than the natural counterparts in the high dose region (above 150 or 200 Gy). This results in severe underestimation of equivalent dose (D_e) for quartz OSL in the high dose region during the application of a single-aliquot regenerative-dose (SAR) protocol. However, the potential mechanism governing the build-up of the laboratory-generated DRC is still unclear. In this study, we performed a comprehensive investigation of the natural and laboratory-generated OSL signals and DRCs using a kinetic model for quartz. We compared the differences in charge concentrations between natural and laboratory-irradiated aliquots following irradiation and monitored the competition for holes and electrons during preheat and stimulation, for the natural, regenerative, and test dose cycles. In the course of the modelling, we could see the build-up of laboratory-generated DRCs, the underestimation of D_e, and a double exponential saturation characteristic of the DRCs. We demonstrated a discrepancy in competition for electrons in the deep electron trap and recombination centres during stimulation between the natural, regenerative, and test dose cycles. The simulation results are directly relevant to quartz OSL D_e determination using the SAR protocol and reveal the mechanisms responsible for the experimentally observed different behaviours between natural and laboratory-generated DRCs.     

Testing the applicability of a standardized growth curve (SGC) for quartz OSL dating: Kalahari dunes,South African coastal dunes and Florida dune cordons

Standardized growth curves (SGCs) have recently been developed as a means to reduce measurement times when determining palaeodoses in the standardized luminescence (OSL) dating of quartz, but this approach is not without its critics. We test the performance of SGCs derived from samples from a range of environments in southern Africa and Florida (United States of America). Data presented here suggest that the method performed well when determining low (<50 Gy) equivalent dose (D_e) values from samples with different geological provenances within South Africa. However, the application of an SGC built from South African data performed poorly when used to estimate the palaeodoses of samples from Florida, with seven out of ten samples failing to replicate the single aliquot regenerative (SAR)-derived D_e value to within ±10%. We thus advocate the use only of regionally based SGCs for full D_e determination, although SGCs can also be utilized as a way of quickly generating SAR ‘range finders’ irrespective of sample origin. Furthermore, one sample from the Florida site did not show regenerative growth in accordance with any samples from either the South Africa or Florida data, resulting in significant D_e underestimation when using the SGC. Although the reasons for such behaviour are not yet understood, this re-iterates the recommendation of Burbidge et al. [2006. D_e determination for young samples using the standardised OSL response of coarse-grain quartz. Radiation Measurements 41(3), 278], to incorporate a single regenerative step in SGC protocols in order to check consistency.     

Stochastic modelling of multi-grain equivalent dose (De) distributions: Implications for OSL dating of sediment mixtures

A number of recent optically stimulated luminescence (OSL) studies have cited post-depositional mixing as a dominant source of equivalent dose (D_e) scatter across a range of sedimentary environments, including those previously considered ‘best suited’ for OSL dating. The potentially insidious nature of sediment mixing means that this problem may often only be identifiable by careful statistical analysis of D_e data sets. This study aims to address some of the important issues associated with the characterisation and statistical treatment of mixed D_e distributions at the multi-grain scale of analysis, using simulated D_e data sets produced with a simple stochastic model. Using this Monte Carlo approach we were able to generate theoretical distributions of single-grain D_e values, which were then randomly mixed together to simulate multi-grain aliquot D_e distributions containing a known number of mixing components and known corresponding burial doses. A range of sensitivity tests were undertaken using sediment mixtures with different aged dose components, different numbers of mixing components, and different types of dose component distributions (fully bleached, heterogeneously bleached and significantly overdispersed D_e distributions). The results of our modelling simulations reveal the inherent problems encountered when dating mixed sedimentary samples with multi-grain D_e estimation techniques. ‘Phantom’ dose components (i.e. discrete dose populations that do not correspond to the original single-grain mixing components) are an inevitable consequence of the ‘averaging’ effects of multi-grain D_e analysis, and prevent the correct number of mixing components being identified with the finite mixture model (FMM) for all of the multi-grain mixtures tested. Our findings caution against use of the FMM for multi-grain aliquot D_e data sets, even when the aliquots consist of only a few grains.     

Maximum age limitation in luminescence dating of Chinese loess using the multiple-aliquot MET-pIRIR signals from K-feldspar

Numerical dating of loess is important for Quaternary studies. Recent progress in post-infrared infrared-stimulated luminescence (pIRIR) signals from potassium-rich feldspar has allowed successful dating of Chinese loess beyond the conventional dating limit based on quartz optically stimulated luminescence (OSL) signals. In this study we tested the multiple-aliquot regenerative-dose (MAR) pre-dose multiple-elevated-temperature post-IR IRSL (pMET-pIRIR) procedure on samples from the palaeosol S5 (∼480 ka) and S8 (∼780 ka) layers from the Luochuan and Jingbian sections, respectively. The results show that (1) compared to sensitivity-corrected signal (L_x/T_x), a higher saturation dose is observed for the sensitivity-uncorrected MET-pIRIR signals (L_x), suggesting that MAR is advantageous for dating old samples; (2) negligible fading component can be achieved using the pMET-pIRIR procedure; (3) for the sample from the top of palaeosol S5, D_e values (1360 + 226/-167 Gy) broadly consistent with expected D_e (1550 ± 72 Gy) can be obtained using the sensitivity-uncorrected 300 °C MET-pIRIR signal. Our study suggests that a D_e value of about 1800 Gy may be the maximum dating limit of Chinese loess using the MAR pMET-pIRIR procedure.     

Small aliquot and single grain IRSL and post-IR IRSL dating of fluvial and alluvial sediments from the Pativilca valley,Peru

Infrared stimulated luminescence (IRSL) and post-IR IRSL are applied to small aliquots and single grains to determine the equivalent dose (D_e) of eleven alluvial and fluvial sediment samples collected in the Pativilca valley, Central Peru at ca. 10°S latitude. Small aliquot D_e distributions are rather symmetric and display over-dispersion values between 15 and 46%. Small aliquot g-values range between 4 and 8% per decade for the IRSL and 1 and 2% per decade for the post-IR IRSL signal. The single grain D_e distributions are highly over-dispersed with some of them skewed to higher doses, implying partial bleaching; this is especially true for the post-IR IRSL. Measurements of a modern analog reveal that residuals due to partial bleaching are present in both the IRSL as well as the post-IR IRSL signal. The g-values of individual grains exhibit a wide range with high individual uncertainties and might contribute significantly to the spread of the single grain D_e values, at least for the IRSL data. Electron Microprobe Analysis performed on single grains reveal that a varying K-content can be excluded as the origin of over-dispersion. Final ages for the different approaches are calculated using the Central Age Model and the Minimum Age Model (MAM). The samples are grouped into well-beached, potentially well-bleached and partially bleached according to the evaluation of the single grain distributions and the agreement of age estimates between methods. The application of the MAM to the single grain data resulted in consistent age estimates for both the fading corrected IRSL and the post-IR IRSL ages, and suggests that both approaches are suitable for dating these samples.     

Applicability of a quartz OSL standardised growth curve for De determination up to 400 Gy for lacustrine sediments from the Qaidam Basin of the Qinghai-Tibetan Plateau

The standardised growth curve (SGC) for quartz OSL has recently been developed as a practical means to reduce measurement times when determining palaeodoses using quartz of aeolian sediments, especially loess and desert sand from the same section or the same geographical area. In the present study, we test the performance of SGCs for lacustrine sediments of three cores in the Qaidam Basin of the Qinghai-Tibetan Plateau (QTP) in China. A total of nine samples were collected (three samples from each of the three cores), and silt-sized (38–63 μm) quartz was extracted for the experiment. The results demonstrated that: (a) Nine samples display similar dose–response curves up to a regeneration dose of 600 Gy using single aliquot regenerative-dose (SAR) protocol, suggesting the existence of a standardised growth curve for lacustrine sediments in the Qaidam Basin; (b) For samples with D_es of up to ～400 Gy, the D_es determined by the SGC are in agreement with the D_es by the SAR protocol, suggesting that the SGC approach could be used for D_e determination up to a dose of ～400 Gy for lacustrine samples from the Qaidam Basin of the Qinghai-Tibetan Plateau in China; (c) The saturation dose for these samples is more than 600 Gy, and in the growth curve a linear growth part was observed in the high dose range of >200 Gy.     

OSL dating of loess deposits bracketing Sheep Creek tephra beds,northwest Canada: Dim and problematic single-grain OSL characteristics and their effect on multi-grain age estimates

Geochemically-fingerprinted tephra beds provide unique chronostratigraphic markers for comparing Quaternary sedimentary records across eastern Beringia (Alaska and Yukon Territory). Establishing reliable numerical age control on these tephra horizons enables them to be placed within firm temporal frameworks and increases their potential as correlative tools for regional palaeoenvironmental reconstructions. To this end we present new single-grain and multi-grain quartz optically stimulated luminescence (OSL) chronologies for loess deposits bracketing three well-documented and regionally significant variants of the Sheep Creek tephra (SCt) at two sites in west-central Yukon Territory (Ash Bend and Quartz Creek). Single-grain OSL ages bracketing the SCt-A and SCt-K reveal that these tephras were deposited during late Marine Isotope Stage (MIS) 5 or early MIS 4. The SCt-C variant and associated organic-rich bed at Ash Bend were likely deposited during late MIS 5, based on a single-grain OSL age of ∼81 ka for the overlying sediments. The single-grain OSL ages obtained for these deposits are in stratigraphic order and in broad agreement with a fission track age estimate of ∼77 ka for the SCt-K. In contrast, comparative chronologies obtained using multi-grain aliquots are stratigraphically inconsistent and unexpectedly young when compared with the independent SCt-K age. Detailed examination of the single-grain OSL datasets reveal a range of unfavourable luminescent properties that could have contributed to the multi-grain aliquot age discrepancies; including, very low yields of luminescent grains, weak OSL signal sensitivities and large populations of aberrant grains (particularly 0 Gy grains and ‘dim’ grains with a tendency to sensitise during the equivalent dose (D_e) measurement sequence) that have similarly sized OSL signals as grains used for D_e analysis. Synthetic aliquot D_e datasets constructed from single-grain OSL measurements reveal that the large proportional light sum contributions of 0 Gy and dim grains could possibly account for multi-grain age underestimations in some of the Ash Bend samples. In light of these potentially problematic averaging effects, we do not consider the multi-grain OSL ages to be reliable and suggest that single-grain approaches may be preferable for dating sediments with similar quartz luminescence behaviours across this region.     

Testing the applicability of standardised growth curves (SGCs) for OSL signals of quartz grains from the Yangtze Delta,China

The standardised growth curve (SGC) technique has the potential to save instrument time for equivalent dose (D_e) determination when applying single-aliquot regenerative-dose (SAR) protocol during optically stimulated luminescence (OSL) measurements. In this study, we test the applicability of two commonly used SGC procedures for OSL signals of quartz grains from aqueous deposits of the Yangtze Delta in China, which have been reported for weak luminescence signals and suffering from partial bleaching. Multiple silt-sized and sand-sized fractions of quartz samples from eight cores are used to construct SGCs by test dose standardisation (TD-SGC) and least-squares normalisation (LS-SGC), respectively. Three strategies, i.e. region-specific (SGC_R), region with core-specific (SGC_R + C) and core-specific (SGC_C), are adopted to categorise these normalised data into different SGC datasets. The large variability of dose response signals is substantially reduced by the SGC procedures for most of these datasets. Hence, common SGCs for a variety of samples from the Yangtze Delta can be established, irrespective of their distinctive particle sizes and luminescence characteristics. The D_e values are then estimated using both TD-SGC and LS-SGC procedures for samples from a specific core. Comparing to the full SAR protocol, the TD-SGC procedure roughly gives reproducible D_e estimates lower than ∼100 Gy while the LS-SGC procedure derives generally consistent D_e estimates of up to ∼230 Gy. Although LS-SGC_C and LS-SGC_R + C procedures replicate the most consistent D_e values, the LS-SGC_R procedure performs better in efficiency with a slightly less accuracy. In addition to careful comparison of LS-SGC and full SAR procedures, we suggest that a synchronous ratio derived by the chosen regenerative dose and its response signal for re-normalisation can be used to predict the LS-SGC D_e reliability on samples from similar sedimentary environments.     

Investigating the potential of HCl-only treated samples using range-finder OSL dating

Rapid sample preparation and measurement protocols for optically stimulated luminescence (OSL) dating have been investigated as a method of increasing the throughput of samples (e.g. Roberts et al. 2009). Here, we investigate the potential of dating samples treated using only hydrochloric acid (HCl) for providing accurate range-finder ages for quartz. The equivalent dose (D_e) is underestimated for older samples using a standard single-aliquot regenerative-dose (SAR) protocol, but measurement of the post-IR OSL signal (e.g. Roberts and Wintle, 2003) provides D_e estimates within 15% of the ‘target fully prepared D_e’ for two-thirds of samples. The application of a standardised growth curve (SGC) is also investigated. For these dune samples, the most efficient preparation and measurement procedure from which accurate D_e estimates can be obtained is treatment with HCl-only, measurement of the natural and regenerative post-IR OSL signal, and calculation using a region-independent SGC. A slightly longer protocol incorporating an additional preparation stage of heavy liquid density separation can be used to improve D_e estimates in samples where feldspar-contamination is thought to be particularly problematic. In practice, these time savings of days to weeks in the preparation and measurement protocols mean that a large number of samples can first be rapidly treated with HCl, the post-IR blue OSL signal measured using a shortened double-SAR protocol, and D_e calculated by applying an independent SGC before then deciding which samples require full preparation and measurement, rather than the inefficiency of committing time and resources to all samples if this is unnecessary for the context of a particular suite of samples. The potential of this rapid preparation and measurement protocol is discussed in relation to sand dune dating studies.     

Optical dating of young deltaic deposits on a decadal time scale

Modern Mississippi Delta sediments were analyzed to investigate quartz OSL signal resetting in large river deltas and test the accuracy of OSL dating on a decadal time scale with the early background subtraction and a recently proposed burial dose estimation procedure. Both fine silt-sized and sand-sized quartz were measured with a modified single-aliquot regenerative dose (SAR) protocol and equivalent dose (D_e) was calculated using different background subtraction methods. Evidence of insufficient bleaching was observed, but the residual signal is equivalent to ∼100 a on average for both sandy quartz and fine silt-sized quartz. It is shown that dose distributions of sandy quartz are affected by the background subtraction. The proportion of aliquots that have D_e in agreement with expectation is significantly larger when an early background is subtracted compared to the late background subtraction. This is, in contrast, not observed for fine silt-sized quartz. Accurate OSL ages were obtained by employing the unlogged minimum age model to D_es of sandy quartz obtained with the early background subtraction method.     

Luminescence dating of quartz from ironstones of the Xingu River,Eastern Amazonia

This study reports on the first investigation into the potential of quartz luminescence dating to establish formation ages of ferruginous duricrust deposits (ironstones) of the Xingu River in Eastern Amazonia, Brazil. The studied ironstones comprise sand and gravel cemented by goethite (FeO(OH)), occurring as sandstones and conglomerates in the riverbed of the Xingu River, a major tributary of the Amazon River. The Xingu ironstones have a cavernous morphology and give origin to particular habitat for benthic biota in an area that hosts the largest rapids in Amazonia. So far, the Xingu ironstones have uncertain formation ages and their sedimentary origin is still poorly understood. In this way, seven samples of ironstones distributed along the lower Xingu River were collected for optically stimulated luminescence (OSL) dating of their detrital quartz sand grains. Additionally, the organic content of some samples was dated by radiocarbon (¹⁴C) for comparison with quartz OSL ages. The luminescence ages of the sand-sized quartz grains extracted from the ironstones were obtained from medium (100–300 grains) and small (10–20 grains) aliquots using the single aliquot regenerative-dose (SAR) protocol. Equivalent doses (D_e) distributions have varied overdispersion (OD) both for medium size aliquots (OD = 19–58 %) and small size aliquots (OD = 29–76 %). No significant trend was observed between D_e and aliquot size. The studied ironstones grow over the riverbed, but stay below or above water throughout the year due to the seasonal water level variation of the Xingu River. However, the effect of water saturation in dose rates is reduced due to relatively low porosity of ironstones. Water saturated dose rates (dry sample dose rates) range from 2.70 ± 0.21 (2.79 ± 0.22) Gy/ka to 12.34 ± 0.97 (13.26 ± 1.12) Gy/ka, which are relatively high when compared to values reported for Brazilian sandy sediments elsewhere (∼1 Gy/ka). Samples with high overdispersion (>40 %) are mainly attributed to mixing of grains trapped in different time periods by goethite cementation. The obtained OSL ages for water saturated (dry) samples range from 3.4 ± 0.3 (3.3 ± 0.4) ka to 59.6 ± 6.0 (58.1 ± 6.4) ka, using D_e determined from medium size aliquots and dose response curves fitted by an exponential plus linear function. Radiocarbon ages of the bulk organic matter extracted from selected ironstone varied from ca. 4 cal ka BP to ca. 23 cal ka BP. Significant differences were observed between OSL and radiocarbon ages, suggesting asynchronous trapping of organic matter and detrital quartz within the ironstone matrix. These late Pleistocene to Holocene ages indicate that ironstones of the Xingu River result from an active surface geochemical system able to precipitate goethite and cement detrital sediments under transport. The obtained ages and differences between OSL and radiocarbon ages point out that the ironstones have multiphase and spatially heterogeneous growth across the Xingu riverbed. Our results also expand the application of luminescence dating to different sedimentary deposits.     

Test of high sampling density OSL dating of aeolian samples from the south margin of the Tengger Desert using the global standardised growth curve (gSGC) method

Chronostratigraphic records in the drylands of north China provide basic archives to reveal the dynamic connections between climate changes and the behaviour of aeolian systems. However, the interpretation of aeolian chronostratigraphy is not straightforward and may be associated with significant uncertainty due to a number of external and localised forces. Taking into account this complexity of aeolian systems, excluding preservation and sampling bias from palaeoclimatic signals requires that the interpretation of chronostratigraphic records be based on as many dates from as many sites as possible. Optically stimulated luminescence (OSL) dating provides a direct way to date aeolian sediments. However, the determination of equivalent dose (D_e) using the commonly applied single-aliquot regenerative dose (SAR) protocol is relatively time-consuming for samples with burial doses exceeding 50 or 100 Gy.In this study, the global standardised growth curve (gSGC) method is applied to multi-grain aliquots of coarse quartz from various sites covering a wide region of the south margin of the Tengger Desert in north China to rapidly determine D_e for a large number of aliquots and samples. The large between-aliquot variability in the shape of the dose response curve (DRC) is significantly reduced using a least-squares normalisation procedure. The results of a leave-one-out cross-validation (LOOCV) protocol demonstrates that D_e values determined using the gSGC and SAR methods are very consistent with each other up to at least 200 Gy and 400 Gy, for the 90–150 μm and 63–90 μm fractions, respectively. It suggests that the gSGC protocol can be used as an efficient procedure for D_e determination of a large number of aeolian samples from multiple sites. This in turn provides a better constraint for the interpretation of Late Quaternary aeolian chronostratigraphic records in drylands of north China.     

A Bayesian hierarchical age model for optical dating of single grains of quartz

In optical dating, the last time that a sample of sediment was exposed to sunlight is determined by dividing its equivalent dose (D_e) by the dose rate. For single-grain dating, the sample D_e is based on the statistical analysis of the distribution of D_e values estimated for individual grains, whereas the dose rate is usually determined from measurements of the environmental radioactivity of the bulk sample, together with allowances for radiation sources internal to the grains and cosmic rays. Conventionally, the D_e and dose rate are measured and analysed separately to produce an estimate of the depositional age of a sample, but this approach may result in loss of information because distributions of single-grain D_e values are influenced by several factors. Existing statistical models do not incorporate all the key information contributing to age estimation, such as the pattern and scale of dispersion of single-grain D_e values and dose rates, the associated measurement uncertainties, the effect of natural variability among grains, and the outlier probabilities of D_e and dose rate estimates. Here we propose an empirical Bayesian hierarchical age model (BHAM) for optical dating of quartz samples that incorporates the above information to estimate their depositional ages. The BHAM is based on the implementation of standardised growth curve and L_nT_n methods to integrate information on the full distribution of single-grain D_e values, sources of measurement uncertainty, beta-dose heterogeneity (observed or modelled), and detection of outliers. We present the results of validation tests using data sets of optically stimulated luminescence measurements and dose rates obtained for quartz samples dated previously from Denisova Cave (Russia) and for simulated samples. We conclude that the BHAM represents a robust and flexible approach to dealing with data for single grains of quartz within a Bayesian hierarchical framework and is suitable for application to sediments deposited in a variety of depositional settings.     

Investigating the applicability of a standardised growth curve approach on Middle Pleistocene sediments from northern Switzerland

The introduction of the Single Aliquot Regenerative Dose (SAR) protocol established luminescence dating as an indispensable tool in Quaternary research. A major impediment of this technique is the time required for measurements, since the protocol is repeated for various aliquots of each sample to establish a sound statistical basis. To reduce the demand on machine time, Standardised Growth Curve (SGC) approaches have been developed and successfully applied for samples from some regions. However, differences in luminescence properties require careful testing of this techniques when applied to samples with other geological background.In this study, the application of the SGC approach of Li et al. (2016) is successfully verified for multi-grain aliquots of coarse-grained quartz and feldspar samples from three sites in northern Switzerland. In-depth quality control measures ensure the reproducibility of equivalent dose (D_e) values obtained by the common SAR protocol and sample-specific SGCs. For both minerals little sensitivity was found to the re-normalisation dose and the sample-specific SGCs performed well. In contrast to other studies, no different types of dose response curve shape were observed for quartz. A minimum number of full SAR measurements of eight and six aliquots per sample has been found appropriate for quartz and feldspar, respectively. For the fading corrected feldspar signals, site-specific SGC worked well and D_e values of up to 800 Gy were consistently replicated.In summary, sample-specific SGCs for samples from northern Switzerland perform well and their application reduces measurement times by up to 70%. The construction of a regional SGC may well be beneficial, however, caution regarding the choice of given doses and curve fitting is recommended and a thorough verification of SGC results is needed before the technique is widely applied.     

Optically stimulated luminescence dating of single and multiple grains of quartz from perennially frozen loess in western Yukon Territory, Canada: Comparison with radiocarbon chronologies for the late Pleistocene Dawson tephra

Optically stimulated luminescence (OSL) dating of perennially frozen loess was tested on quartz grains extracted from deposits associated with the late Pleistocene Dawson tephra in western Yukon Territory, Canada. OSL samples were obtained from ice-rich loess bracketing the Dawson tephra, while radiocarbon (¹⁴C) samples were collected from the bulk sediments directly underlying the tephra and from a ground-squirrel burrow 2.7 m below the tephra. Here we report the OSL characteristics and ages of the extracted quartz grains, as well as additional radiocarbon ages for samples described in Froese [2002. Age and significance of the late Pleistocene Dawson tephra in eastern Beringia. Quaternary Science Reviews 21, 2137–2142; 2006. Seasonality of the late Pleistocene Dawson tephra and exceptional preservation of a buried riparian surface in central Yukon Territory, Canada. Quaternary Science Reviews 25, 1542–1551]. We refine the time of Dawson tephra deposition to between 25,420±70 and 25,290±80 ¹⁴C a BP. Bayesian analysis of constraining radiocarbon ages places the deposition of the Dawson tephra at between 30,433 and 30,032 cal a BP. Linear modulation (LM) OSL analysis of multi-grain aliquots of quartz showed that the initial part of the decay curve is dominated by a rapidly bleached (‘fast’) component; these samples, however, had relatively dim continuous wave (CW) OSL signals at the multi-grain aliquot (each composed of 80 grains) and single-grain scales of analysis. The single-aliquot regenerative-dose protocol was applied to multi-grain aliquots and single grains to obtain equivalent dose (D_e) values for samples collected from below and above the Dawson tephra. The D_e values were examined graphically and numerically, the latter using the central age, minimum age, and finite mixture models. For multi-grain aliquots, the central age model gave weighted mean D_e values between 30 and 50 Gy, which greatly underestimated the expected D_e of 74–81 Gy for both samples studied. Possible reasons for these underestimations are discussed, and a solution proposed based on single-grain analysis. Measurements of single grains produced D_e values in agreement with the expected D_e, and yielded OSL ages of 28±5 and 30±4 ka for the samples taken from above and below the Dawson tephra, respectively. Examination of individual grains with differing luminescence behaviors showed that a significant number of the measured quartz grains exhibited anomalous luminescence properties that would have compromised the results obtained from multi-grain aliquots. We therefore recommend analysis of individual grains to overcome the age-shortfall from multi-grain analysis of these and similar samples of quartz.