The dose dependency of the over-dispersion of quartz OSL single grain dose distributions

The use of single grain quartz OSL dating has become widespread over the past decade, particularly with application to samples likely to have been incompletely bleached before burial. By reducing the aliquot size to a single grain the probability of identifying the grain population most likely to have been well-bleached at deposition is maximised and thus the accuracy with which the equivalent dose can be determined is – at least in principle – improved.However, analysis of single grain dose distributions requires knowledge of the dispersion of the well-bleached part of the dose distribution. This can be estimated by measurement of a suitable analogue, e.g. a well-bleached aeolian sample, but this requires such an analogue to be available, and in addition the assumptions that the sample is in fact a) well-bleached, and b) has a similar dose rate heterogeneity to the fossil deposit. Finally, it is an implicit assumption in such analysis that any over-dispersion is not significantly dose dependent.In this study we have undertaken laboratory investigations of the dose dependency of over-dispersion using a well-bleached modern sample with an average measured dose of 36 ± 3 mGy. This sample was prepared as heated (750 °C for 1 h), bleached and untreated portions which were then given uniform gamma doses ranging from 100 mGy to 208 Gy. We show that for these samples the relative laboratory over-dispersion is not constant as a function of dose and that the over-dispersion is smaller in heated samples. We also show that the dim grains in the distributions have a greater over-dispersion than the bright grains, implying that insensitive samples will have greater values of over-dispersion than sensitive samples.     

Distinguishing dose populations in sediment mixtures: a test of single-grain optical dating procedures using mixtures of laboratory-dosed quartz

Many natural deposits contain grains that have different burial histories, but reliable procedures to extract the component doses from mixed-dose samples have not been developed in optical dating. Here we present results for synthetic two- and three-component mixtures of data derived from laboratory-dosed sedimentary quartz using a single-aliquot regenerative-dose protocol and statistical models for finite mixtures. Composite sets of data were created from the doses measured for individual grains that had been bleached by sunlight and then given a beta dose of 5, 10 or 20 Gy. We found that the correct number of dose components, corresponding doses, and relative proportions of each component may be estimated if the within-component dispersion is small or is known. We also found, however, that recuperation of the OSL signal in sun-bleached quartz may produce artificially high dose estimates for some grains, an effect that should be taken into account when modelling natural mixtures.     

Identifying well-bleached quartz using the different bleaching rates of quartz and feldspar luminescence signals

When dating older sedimentary deposits using quartz, there are no unambiguous methods for identifying the presence of incomplete bleaching. Current statistical analysis of dose distributions depends entirely on the assumption that incomplete bleaching and mixing are the main causes of any excess dispersion in the distribution; the only existing way to test this assumption is using independent age control. Here we suggest a new approach to this question, based on the differential bleaching rates of quartz and feldspar luminescence signals. We first present data that confirm the differences in relative bleaching rates of quartz optically stimulated luminescence (OSL) and feldspar luminescence stimulated at 50 °C by infrared light (IR₅₀) and feldspar luminescence stimulated at 290 °C by infrared light after a stimulation at 50 °C (pIRIR₂₉₀), and use recently deposited samples to determine the likely significance of the difficult-to-bleach residual feldspar signals in non-aeolian samples. For a set of mainly Late Pleistocene non-aeolian sediments, large aliquot quartz doses are then used to predict feldspar doses (based on a knowledge of the sample dose rates). The differences between observed and predicted feldspar doses as a function of the quartz dose, combined with a conservative assumption concerning the relative feldspar and quartz residual signals after natural bleaching prior to deposition, are used to identify those samples for which the quartz is very likely to be well bleached (20 out of 24). Two of these apparently well-bleached samples are then examined using single-grain quartz dose distributions; one of these is consistent with the well-bleached hypothesis, and one indicates poor bleaching or a multi-component mixture. However, independent age control makes it clear that the large aliquot data are more likely to be correct. We conclude that a comparison of quartz and feldspar doses provides a useful independent method for identifying well-bleached quartz samples, and that it is unwise to apply statistical models to dose distributions without clear evidence for the physical origins of the distributions.     

Testing the accuracy of a Bayesian central-dose model for single-grain OSL,using known-age samples

While reviews of comparisons between multi-grain OSL ages and independent chronological information are available in the literature, there is hardly any such performance test for single-grain OSL ages. And yet, this is all the more needed as the interpretation of single-grain dose distributions remains a difficult task, given the typically considerable dispersion in equivalent dose values measured by OSL – and the numerous sources of such dispersion in measurements. Here, we present the study of 19 samples for which independent age control is available, and whose ages range from 2 to 46 ka. Based on multi-grain OSL age estimates, these samples are presumed to have been both well-bleached at burial, and unaffected by mixing after deposition. Two ways of estimating single-grain ages are then compared: the standard approach on the one hand, consisting of applying the Central Age Model to D_e values determined with the Analyst software; on the other hand, the central dose model recently proposed by Combès et al. (Combès, B., Philippe, A., Lanos, P., Mercier, N., Tribolo, C., Guerin, G., Guibert, P., Lahaye, C., in press. Quaternary Geochronology). The median of the relative discrepancy between single-grain OSL and reference ages is about twice as large for the standard approach (12%) as with the Bayesian model (7%). Statistical tests show that, based on our (limited) dataset, the difference between the two models seems to be significant for samples in the age range 4–46 ka. Finally, the influence of various factors on the (in-)accuracy of single grain OSL ages is discussed; it appears that the accuracy of ages estimated in a standard way decreases when age is increased, while the Bayesian model seems more robust. This study also shows that (i) there is no 20% limit on the CAM overdispersion parameter for well-bleached samples; (ii) dose recovery experiments do not seem to be a very reliable tool to estimate the accuracy of a SAR measurement protocol for age determination.     

Sources of uncertainties in OSL dating of archaeological mortars: The case study of the Roman amphitheatre “Palais-Gallien” in Bordeaux

Archaeological mortars are more convenient and much more representative for the chronology of buildings than brick or wood constructions that can be re-used from older buildings. Before dating unknown samples of mortars, further investigation of OSL from mortars is required and the most efficient methodology needs to be established. In this study we compared the ages obtained by OSL dating of quartz extracted from mortars of the Roman amphitheatre Palais-Gallien in Bordeaux with independent age information.Resetting of the OSL signal occurred during the preparation of mortar when grains of sand (quartz) were extracted and mixed with lime and water. The mortar was subsequently hidden from light by embedding within the structure which is the event to be dated.Various factors contribute to uncertainties in the age determination. The frequency of measured equivalent doses reveals a large scattering. Optical bleaching of certain grains can be partial due to the short duration of the exposure to light. We worked with the single grain technique in order to find and select the grains that were sufficiently exposed to daylight. To determine the average equivalent dose, we tried three different approaches: a calculation of an arithmetic mean and one following either the central age model or the 3-parameter minimum age model, the latter turned out to be the only relevant way to evaluate the experimental data. The proportion of grains included in the calculation of the average equivalent dose represents 2.7–4.7 % of the overall analysed grains. The results obtained for the three out of four samples are approaching the expected age, however, the minimum doses and the corresponding ages are significantly over-estimated in case of two samples.The studied material is very coarse, which causes heterogeneity of irradiation at the single grain scale, and contributes to a dispersion of equivalent doses. Different analytical methods (scanning electron microscopy with energy dispersive X-ray spectroscopy cartography, Beta-radiography imaging, inductively coupled plasma mass spectrometry) were employed to demonstrate the presence of this phenomenon.Despite the extremely large proportion of high equivalent doses in equivalent dose distributions, there is an apparent presence of well-bleached grains at the beginning of equivalent dose distributions. The study shows the potential of dating mortars by single grain OSL.     

Circumventing possible inaccuracies of the single aliquot regeneration method for the optical dating of quartz

A simple method of assessing the reliability of the single aliquot regeneration method is presented, based on the repeated measurement of identical regeneration doses. It is shown that for the majority of the samples measured the requirement of a constant repeat point in each cycle is not met. The ability to maintain a constant repeat point is shown to be dependent upon preheating conditions, the requirements for which were found to vary at both inter-sample and inter-aliquot levels. The expected effect on estimated values of D_e is calculated and shown to be significant. Three possible means of circumventing these inaccuracies are presented: (i) adjustment of the measurement procedure (specifically the preheat conditions); (ii) modelling and correcting for the effect, and (iii) stabilising the response prior to construction of the regenerated growth curve. It is suggested that much of the variation in D_e observed between individual aliquots and between individual grains from well-bleached sediments may possibly be attributed to differences in preheating requirements, and that such differences may also account for some of the spread in D_e for partially bleached sediments.     

Yellow stimulated luminescence from potassium feldspar: Observations on its suitability for dating

Yellow stimulated luminescence (Y-OSL) is the light detected from potassium-rich feldspars at 410 nm under stimulation by a yellow light source emitting 590 nm. The investigation of this study aimed at understanding basic luminescence physics of Y-OSL in order to assess the suitability of the technique for dating. The Y-OSL signal properties tested were signal intensity, thermal assistance, thermal stability, sensitivity to daylight and the suitability of a single aliquot regenerative (SAR) protocol to be employed for equivalent dose (D_e) estimation. D_e measurements were conducted on samples of Holocene, last glacial and Tertiary age. The tests were undertaken on sedimentary feldspar separates extracted from aeolian, fluvial and coastal deposits.Results from experiments show that the signal intensity increases by measuring Y-OSL at elevated temperature suggesting thermal assistance characteristics similar to infrared stimulated luminescence (IRSL). The yellow stimulated signal remains unaffected by preheat temperatures up to ～200 °C suggesting higher thermal stability than the IRSL signal. The Y-OSL signal is less light sensitive than the IRSL signal and D_e residuals obtained from modern samples are up to 7 Gy indicating suitability of the technique for ‘older’ and well-bleached sediments. The dose recovery tests successfully recovered the given dose if the specific light sensitivity of Y-OSL is taken into account. For every sample Y-OSL D_e values obtained by a single aliquot regenerative dose protocol (SAR) are higher than those obtained by an IRSL SAR approach. From these results we infer high thermal stability and a minimal anomalous fading of the Y-OSL signal. We conclude that Y-OSL has a high potential to date Quaternary sediments that were sufficiently bleached in nature.     

Optically stimulated luminescence production in the single-aliquot regenerative dose protocol

When using a single-aliquot regenerative dose (SAR) protocol for luminescence dating of sedimentary quartz grains, the fundamental assumption is that the sensitivity of the optically stimulated luminescence (OSL) produced by the regenerative doses can be monitored by their following test dose OSL response. Using well-bleached coarse quartz grains, OSL production in a SAR protocol was studied in detail when dose response curves were constructed using both single and multiple aliquot regenerative dose procedures. During application of the SAR protocol, two preheats are applied, each ahead of an OSL measurement that is produced by the regenerative dose and test dose, respectively. It is shown that sensitivity changes caused by heating were well corrected for using the OSL response to the test dose. However, these preheats are shown to result in thermally stimulated OSL signals that contribute both to the OSL response from the regenerative doses used to construct the dose response curve and to the OSL response from the test dose used to monitor OSL sensitivity changes as the quartz grains are repeatedly measured. A simple test is proposed to identify the contribution of the latter signal.     

Quartz luminescence response to a mixed alpha-beta field: Investigations on Romanian loess

Previous SAR-OSL dating studies using quartz extracted from Romanian and Serbian loess samples report SAR-OSL dose–response curves on fine grained (4–11 μm) quartz that grow to much higher doses compared to those of coarse-grained (63–90, 90–125, 125–180 μm) quartz. Furthermore, quartz SAR-OSL laboratory dose response curves do not reflect the growth of the OSL signal in nature. A main difference in coarse- and fine-grained quartz dating lies in the alpha irradiation history, but the effect of mixed alpha-beta fields has so far received little attention. In the present study we investigate whether the alpha dose experienced by fine grains over geological cycles of irradiation and bleaching may have an effect on the saturation characteristics of the laboratory dose response. By applying time resolved optically stimulated luminescence we confirm that the OSL signals induced in quartz by alpha and beta radiation follow the same recombination path. We also show that a mixed alpha-beta dose response reproduces the beta dose response only up to about 800 Gy. Assuming an a-value of 0.04 we have shown that laboratory alpha and beta dose response curves overlap up to effective alpha doses of ∼50 Gy. Based on these results, we conclude that exposure of fine grains to alpha radiation during burial and transport cycles prior to deposition, as well exposure to the mixed radiation field experienced during burial are not responsible for the age discrepancies previously reported on fine and coarse grained quartz extracted from Romanian and Serbian loess.     

Investigations of the post-IR IRSL protocol applied to single K-feldspar grains from fluvial sediment samples

The post-IR IRSL protocol with single K-feldspar grains was applied to three samples taken from a fluvial sedimentary sequence at the archaeological site of the Dali Man, Shaanxi Province, China. K-feldspar coarse grains were extracted for measurement. Approximately 30–40% of the grains were sufficiently bright to measure, and after application of rejection criteria based on signal strength, recuperation, recycling ratio and saturation dose, ～10–15% of the grains were used for D_e calculation. The relationship of signal decay rate and form of D_e(t) with the recovery dose were investigated. The dose recovery ratios of the samples after initial bleaching with the four different light sources were within uncertainties of unity. No anomalous fading was observed. The over-dispersion of the recovered dose and D_e values were similar, suggesting neither incomplete resetting of the post-IR IRSL signals nor spatially heterogeneous dose rates significantly affected the natural dose estimates. The values of D_e obtained with the single K-feldspar grain post-IR IRSL protocol were in the range ～400–490 Gy. Combining all of the measured single-grain signals for each of the individual samples (into a ‘synthetic single aliquot’) increased the D_e estimates to the range ～700–900 Gy, suggesting that the grains screened-out by the rejection criteria may have the potential to cause palaeodose over-estimation, although this finding requires a more extensive investigation. Thermally transferred signals were found in the single K-feldspar grains post-IR IRSL protocol, and the proportion of thermally transferred signal to test-dose OSL signal (stimulation at 290 °C) from the natural dose was higher than from regenerative doses, and the proportion was grain- and dose-dependent. As such, TT-post-IR IRSL signals at 290 °C have the potential to cause dose underestimation, although this may be reduced by using larger test-dose irradiations. Our study demonstrates considerable potential in the post-IR IRSL method in providing chronological control in studies relevant to human evolution in the later-Pleistocene.     

Paper II: The interpretation of measurement-time-dependent single-aliquot equivalent-dose estimates using predictions from a simple empirical model

Narrow spectrum (blue-green) laboratory partial bleaching of aliquots of natural sedimentary quartz has been shown to have a significant effect on equivalent dose (measured using a single aliquot procedure) calculated as a function of measurement time (D_e(t)). A simple model, based on empirical data, is presented in which the effects of partial bleaching on D_e(t) are predicted, taking into account the influences of pre- and post-burial doses. The model is applied to the case of heterogeneous populations of partially bleached single grains with various ranges of residual doses. Modelling shows that under realistic conditions, D_e histogram plots are unable to distinguish between samples having age overestimates and those with correct ages, whereas the proposed D_e–Z plot is able to make this distinction. Furthermore, modelling shows that D_e–Z plots can identify sub-populations of grains/aliquots that show most evidence of full bleaching, which can then be used to estimate a correct mean D_e value.     

On the dose-rate estimate of carbonate-rich sediments for trapped charge dating

In a wide range of environmental conditions sediments are subject to changing water content and carbonate cementation during burial. Trapped charge dating of these carbonate-rich deposits requires the determination of a dose rate which is not constant during burial because sediments were subject to post-depositional geochemical alterations. The dose-rate model established in this study assumes linear increase of carbonate mass and linear decrease of water mass in pores between sediment particles during burial. Numerical modelling assesses the effect of carbonate and water on the infinite-matrix dose rate as a function of time. Sensitivity testing of the system indicated that water and carbonate content have the greatest effect on the resulting dose rate, followed by the timing of onset and completion of carbonate formation. As a consequence, a comprehensive re-calculation of the water correction factors was undertaken. It revealed a 5% lower value for the annual beta dose and a 10% lower value for the annual gamma dose compared to values formulated by Zimmerman [1971. Thermoluminescence dating using fine grains from pottery. Archaeometry 13, 29–52].

The dose-rate model was tested using samples from geologically well-constrained coastal sites. The differences between onset and final dose rate were up to 30% resulting in differences between modelled and conventional optical ages between 2% and 15% depending on the final (today's) water and carbonate content. The divergence of dates may be greater under certain conditions. The dose-rate model can be applied to a wide range of contexts similar to those considered in this case study.     

Sources of overdispersion in a K-rich feldspar sample from north-central India: Insights from De,K content and IRSL age distributions for individual grains

Luminescence dating of individual sand-sized grains of quartz is a well-established technique in Quaternary geochronology, but the most ubiquitous mineral on the surface of the Earth—feldspar—has received much less attention at the single-grain level. In this study, we estimated single-grain equivalent dose values and infrared stimulated luminescence (IRSL) ages for K-rich feldspar (KF) grains from a fluvial sample underlying Youngest Toba Tuff (YTT) deposits in north-central India, and compared these ages (corrected for anomalous fading) with those obtained from individual grains of quartz from the same sample. Both minerals have broadly similar single-grain age distributions, but both are greatly overdispersed and most grains have ages substantially younger than the expected age of the YTT deposit (～74 ka). Almost half (45%) of KF grains used for age calculation have fading rates statistically consistent with zero, but the age distribution of these grains is as dispersed as that of the entire population. We obtained a similar distribution of ages calculated for 51 grains using their individually measured internal K contents, which exhibited only minor grain-to-grain variation. Given the lack of dependency of single-grain ages on the measured fading rates and internal K contents, and the overall adequacy of bleaching of grains collected from a sandbar in the modern river channel, we consider the spread in ages is most likely due to mixing, at the time of deposition and after the YTT event, of potentially well-bleached fluvially-transported sediments with older grains derived from slumping of riverbank deposits. Some spread may also be due to natural variations in the IRSL properties of individual KF grains.     

Use of the LM-OSL technique for the detection of partial bleaching in quartz

We present a study of the sensitivity to light (ease-of-bleaching) of the trapped charge in sedimentary quartz grains using an optically stimulated luminescence (OSL) technique in which the intensity of the stimulation light is linearly increased during the measurement period. The technique is known as linear modulation OSL (LM-OSL). In controlled laboratory conditions, this technique has been employed to study the ease-of-bleaching of the trapped charge in quartz by comparing the OSL curves of quartz aliquots which have been either: (1) fully bleached, followed by a laboratory dose of β-irradiation, or (2) partially bleached, followed by the laboratory β-dose. The ratio of the OSL signals due to the β-dose from the partly and fully bleached aliquots is illustrated to be a potential indicator of the degree of optical resetting of the OSL signal in dating material. The key parameter governing the ease-of-bleaching is the photoionization cross-section of the trap involved. The concept is also demonstrated in a model study from which very good agreement with the experimental observations has been found. Potential applications of the technique to dating are discussed.     

Optical dating of single sand-sized grains of quartz: sources of variability

Optically stimulated luminescence (OSL) measurements have been made of over 3000 sand-sized grains of quartz. Analysis at this scale highlights the variability in the luminescence sensitivity and the dose saturation characteristics of individual quartz grains. Using a new instrument capable of measuring single grains it is feasible to routinely measure the equivalent dose from many hundreds of grains from each sample. Analysis of such datasets requires assessment of the uncertainties on each equivalent dose since these may vary significantly. This paper assesses the significance of signal intensity, dose saturation characteristics and instrument uncertainty in equivalent dose calculation.     

Underestimation of equivalent dose in single-aliquot optical dating of feldspars caused by preheating

The ability of single-aliquot techniques to recover a known laboratory dose in coarse-grain potassium-rich feldspar separates is tested. It is shown that, for the samples under study, single-aliquot methods underestimate the known dose. It is deduced that this phenomenon is caused by an increase in electron trapping probability as a consequence of heating. The same phenomenon would lead to an underestimation in the equivalent dose in a natural sample. Multiple-aliquot techniques for equivalent dose determination may not suffer from this problem, but are not suitable for samples that might not have been completely bleached at the time of deposition. It is argued that testing for changes in electron trapping probability should become routine practice in the single-aliquot dating of feldspar.     

Study of the effects of grain size on the mechanical properties of nanocrystalline copper using molecular dynamics simulation with initial realistic samples

Abstract

Molecular dynamics simulations have been performed to study the mechanical properties of a columnar nanocrystalline copper with a mean grain size between 9.0 and 24 nm. A melting–cooling method has been used to generate the initial samples: this method produces realistic samples that contain defects inside the grains such as dislocations and vacancies. The results of uniaxial tensile tests applied to these samples reveal the presence of a critical mean grain size between 16 and 20 nm, for which there is an inversion of the conventional Hall–Petch relation. The principal mechanisms of deformation present in the samples correspond to a combination of dislocations and grain boundary sliding. In addition, this analysis shows the presence of sliding planes generated by the motion of perfect edge dislocations that are absorbed by grain boundaries. It is the initial defects present inside the grains that lead to this mechanism of deformation. An analysis of the atomic configurations further shows that nucleation and propagation of cracks are localised on the grain boundaries especially on the triple grains junctions.     

Suppression of superconductivity in granular metals

We investigate the suppression of the superconducting transition temperature due to Coulomb repulsion in granular metallic systems at large tunneling conductance between the grains, g(T)>1. We find the correction to the superconducting transition temperature for 3D granular samples and films. We demonstrate that, depending on the parameters of superconducting grains, the corresponding granular samples can be divided into two groups: (i). the granular samples that belong to the first group may have only insulating or superconducting states at zero temperature depending on the bare intergranular tunneling conductance g(T), while (ii). the granular samples that belong to the second group in addition have an intermediate metallic phase where superconductivity is suppressed while the effects of the Coulomb blockade are not yet strong.     

Inflation and inflation uncertainty: A dynamic framework

This paper aims to investigate the direct relationship between inflation and inflation uncertainty by employing a dynamic method for the monthly country–region–place United States data for the time period 1976–2007. While the bulk of previous studies has employed GARCH models in investigating the link between inflation and inflation uncertainty, in this study Stochastic Volatility in Mean models are used to capture the shocks to inflation uncertainty within a dynamic framework. These models allow researchers to assess the dynamic effects of innovations in inflation as well as inflation volatility on inflation and inflation volatility over time, by incorporating the unobserved volatility as an explanatory variable in the mean (inflation) equation. Empirical findings suggest that innovations in inflation volatility increases inflation. This evidence is robust across various definitions of inflation and different sub-periods.     

Empirical insights into multi-grain averaging effects from ‘pseudo’ single-grain OSL measurements

In this study we assess the signatures of multi-grain averaging effects for a series of sedimentary samples taken from the archaeological site of Hotel California, Atapuerca, Spain. We focus on the special case of equivalent dose (D_e) measurements made on single-grain discs that contain more than one quartz grain in each of the individual grain-hole positions with the aims of (i) providing insight into the nature and extent of averaging effects in very small multi-grain aliquots of sedimentary quartz, and (ii) assessing the suitability of ‘pseudo’ single-grain D_e measurements for this particular dating application. Pseudo single-grain OSL measurements made on standard discs loaded with 90–100 μm grains (equivalent to ～30 grains per hole) yield significantly different D_e distribution characteristics and finite mixture model (FMM) burial dose estimates compared with single-grain OSL measurements. Grains with aberrant luminescence behaviours, which are routinely rejected during single-grain analysis, exert strong averaging effects on the pseudo single-grain and multi-grain aliquot D_e distributions. Grain-hole averaging effects arising from pseudo single-grain measurements also give rise to ‘phantom’ dose components and are apt to provide bias assessments of quartz signal characteristics and grain type classifications. Though this is a site-specific study, it serves as a cautionary note for interpretations of other pseudo single-grain OSL and D_e datasets – particularly those obtained from measurements of discs containing several tens of grains per hole and those derived from complex depositional environments. The use of custom single-grain discs drilled with smaller sized grain holes is recommended as a means of limiting grain-hole averaging effects when dealing with very fine (<180 μm) sediments.