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1.
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 (De) 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 De, 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 De determination using the SAR protocol and reveal the mechanisms responsible for the experimentally observed different behaviours between natural and laboratory-generated DRCs. 相似文献
2.
A suite of samples from an extensive aeolian sandscarp near Victoria Falls, Zambia was used to explore several different methods of calculating optically stimulated luminescence (OSL) ages that account for the effects of saturated quartz grains. Beta dose rate heterogeneity and early OSL signal saturation of the samples exacerbate the impact that saturated grains have on the equivalent dose (De) values calculated. Saturated grains that cannot calculate De values are often rejected but the minimum burial dose information they contain can have a significant impact on a sample's average De value. This study compares multiple techniques for combining luminescence measurements that enables inclusion of this data and their sensitivity to a criterion that rejects grains with early OSL signal saturation. The methods tested are found to have different advantages and disadvantages, but reasonable agreement between the De values they calculate suggests that including data from saturated grains makes a more significant difference to De values calculated than the specific method used to combine the data. 相似文献
3.
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 least-squares 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 sensitivity-corrected 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. 相似文献
4.
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 (De) 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 De 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 De values of older samples and for a large number of aliquots. 相似文献
5.
Dating of quartz by optically stimulated luminescence (OSL) has been revolutionized with introduction of the test dose (TD) in development of a measurement sequence known as the single-aliquot regenerative-dose (SAR), whereby a valid sensitivity correction for the luminescence signal is provided in the measurement cycle. However, the size of the TD used in the SAR protocol remains controversial. Previous studies show that the TD has little effect on the equivalent dose (De) for young samples in luminescence dating in which the applicability of different deposits varies greatly in different regions. However, detailed studies are lacking on how TD size affects SAR–OSL results of samples with a relatively high De range. In this study, typical loess samples with high De values (∼60 Gy–∼250 Gy) from the eastern Tibetan Plateau were selected to investigate the effects of variation in TD size on the quartz SAR–OSL protocol. Dose recovery tests show that a known dose could be recovered successfully by applying different TDs. Test dose size has an effect on shapes of regenerated dose–response curves (DRCs) and has different influences on Des and characteristic saturation doses for quartz samples with a high dose range. A TD size of 20%–30% De is a good compromise for Tibetan loess with De of ∼60–120 Gy in the quartz SAR protocol, and a TD size larger than 30% should be considered for samples with a larger De. The results of this study highlight the importance of TD size in the SAR–OSL protocol for quartz samples with a high dose range. 相似文献
6.
Portable optically stimulated luminescence (OSL) readers are robust devices that allow rapid determination of luminescence signals of polymineralic coarse-grained samples. To date, however, the utility of portable OSL readers has largely been confined to the construction of luminescence profiles that depict variations of luminescence signal intensities with depth. Because the luminescence signals used to construct such profiles are uncalibrated, it is neither practical to quantitatively compare portable OSL data from different sites nor is it possible to approximate sample ages. Such comparisons could be facilitated by converting portable OSL signals into equivalent doses (De). However, determining De requires the construction of unique growth curves for each sample; a laborious procedure that negates the rapidity associated with portable measurements. To circumvent that limitation, we construct standardised growth curves (SGCs) using normalized regeneration dose signals obtained using a portable OSL reader from the feldspar component of polymineralic sands from five disparate eolian dune sites in Alberta, Canada. Comparison of SGCs from different samples indicates general congruence and we merge these to construct a regionally applicable curve which we test by comparing the De obtained from given samples with that obtained using conventional OSL dating protocols. Results show that, though the uncertainties associated with the portable OSL data are high, the De values from the two approaches are linearly proportional. This direct variation enables portable OSL SGC De values to be used independently in reconnaissance studies that aim to screen samples for more detailed analysis using standard OSL methods. The affordability of portable OSL readers suggests that the approach could be attractive to researchers who do not have ready access to conventional OSL readers. 相似文献
7.
R.M. Bailey 《Quaternary Geochronology》2010,5(5):559-568
The usual practice in optical dating is to derive an equivalent dose (De) (and hence age) from integration of the initial part of the measured optically stimulated luminescence (OSL) signal. This ‘bulk’ OSL signal is known to comprise several semi-independent components, each of which decays at different rates during measurement, and thus contributes a different proportion to the bulk signal as measurement time progresses. Data are presented here which show a strong dependence of De on the bulk signal integration interval, with reduced De for later signal integration intervals resulting from lower medium component De values. This dependence leads to two problems: (i) deciding which signal integral to choose, and (ii) the possibility that all bulk signals will provide systematic age underestimation due to medium component signal contributions. Isolating the fast component of the bulk OSL signal provides a solution to both problems and several methods of achieving this are assessed; an efficient new method is described which is incorporated in to standard single-aliquot regenerative-dose measurement sequences. This method involves the direct measurement of the fast-component signal using infrared (830 nm) stimulation of quartz at 160 °C, prior to the standard bulk OSL measurement with 470 nm stimulation. It is shown that the measured quartz infrared stimulated luminescence signals resolve pure fast-component signals and provide De estimates consistent with those from signal deconvolution. This approach can only be applied to samples with relatively bright luminescence emissions, but in these cases is expected to provide a more robust estimate of palaeodose. 相似文献
8.
Relict periglacial wedge structures are widespread in mid-latitude and polar regions. The wedges have a high preservation potential and are often infilled with quartz-rich aeolian sand, making them potentially suitable for luminescence dating. This paper presents initial work from an anti-syngenetic sand wedge in the Tuktoyaktuk Coastlands, Arctic Canada. When samples were measured at the single grain level they showed poor palaeodose (De) reproducibility and consequent high age uncertainties. The aim of the study was to determine whether this intra-sample De scatter reflected methodological issues arising from OSL measurement or processes of wedge development.Initial single grain dose recovery tests show that De scatter was not easily explained by poor recycling, sensitivity changes, variable OSL components, recuperation problems, or large De uncertainties from dim grains. Single grain preheating tests did show that some De scatter might be attributable to individual grains requiring different preheat temperatures, whilst dose recovery tests revealed that highly sensitive grains showed less scatter than dim grains. However, selection of these bright grains from natural samples still resulted in scattered De values. An alternative explanation for the De scatter relates to the formation of anti-syngenetic sand wedges by thermal contraction cracking over thousands of years in an eroding landscape, which may result in sediment of very different ages being deposited in adjacent cracks. Finite mixture modelling was used to identify De components within each sample. Ages calculated from these components suggest sand wedge formation at ca. 5, 8.5, 12.5 and 18 ka which correlate well with known cooling events over the last 18 ka and support a multiple phase activity model for anti-syngenetic sand wedges. 相似文献
9.
《Quaternary Geochronology》2008,3(1-2):137-142
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 (De) 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 De value to within ±10%. We thus advocate the use only of regionally based SGCs for full De 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 De 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. De 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. 相似文献
10.
The standardised growth curve (SGC) technique has the potential to save instrument time for equivalent dose (De) 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 (SGCR), region with core-specific (SGCR + C) and core-specific (SGCC), 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 De 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 De estimates lower than ∼100 Gy while the LS-SGC procedure derives generally consistent De estimates of up to ∼230 Gy. Although LS-SGCC and LS-SGCR + C procedures replicate the most consistent De values, the LS-SGCR 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 De reliability on samples from similar sedimentary environments. 相似文献
11.
Dose saturation represents a fundamental limitation for obtaining finite optically stimulated luminescence (OSL) quartz ages over Middle Pleistocene timescales. Single-grain OSL studies typically reveal significant intra- and inter-sample variability in quartz dose saturation properties at the individual grain level. This enhanced variability may offer the potential to obtain extended-range chronologies exceeding the traditional upper age limits of multiple-grain OSL dating. However, there have been relatively few detailed assessments of single-grain OSL properties over high dose ranges. In this study we investigate extended-range single-grain OSL dating potential at Cuesta de la Bajada, one of the most important Ancient Middle Palaeolithic sites in the Iberian Peninsula. The quartz samples from this site exhibit exceptional dose saturation properties and contain significant populations of individual ‘supergrains’ with bright OSL signals, very high characteristic saturation dose (D0) limits of 200 to >600 Gy, and dose-response curves that closely conform to single saturating exponential functions. Assessments of OSL signal composition and the ability to successfully recover a known laboratory dose of 470 Gy support the potential for obtaining high equivalent dose (De) values with reasonable fitting uncertainties. A series of novel sensitivity tests are used to assess potential biases in supergrain De estimation over high dose ranges related to (i) thermally transferred signal contributions, (ii) choice of dose-response curve fitting function, and (iii) insufficient dose saturation properties of individual grains. The single-grain De values obtained using the standard quality assurance criteria and novel sensitivity tests are consistent at 2σ, and are in agreement with quartz Ti-centre ages obtained on two of three paired electron spin resonance (ESR) samples collected from Cuesta de la Bajada. These comparisons support the suitability of our single-grain OSL results and suggest there may be good potential for using quartz supergrains to establish extended-range chronologies at some Middle Pleistocene sites. Comparisons with other single-grain OSL studies across the Iberian central plains suggest that favourable dose saturation properties may be influenced by regional-scale geological controls. The importance of undertaking single-grain OSL dating at Cuesta de la Bajada is also demonstrated by the results of synthetic aliquot experiments, which reveal multiple-grain age offsets of 60–170 ka when unsuitable grain types are inadvertently included in the final age calculations. The single-grain OSL results indicate that the main archaeological horizon at Cuesta de la Bajada (unit CB3) accumulated between 264 ± 22 and 293 ± 24 ka. These ages are consistent with the chronologies of other key early Middle Palaeolithic sites in the region (Ambrona AS6 and Gran Dolina TD10) and indicate that the Ancient Middle Palaeolithic techno-complex likely spread across Iberia from at least Marine Isotope Stage 9 onwards. 相似文献
12.
Sally E. Lowick Frank Preusser Roberta Pini Cesare Ravazzi 《Quaternary Geochronology》2010,5(5):583-590
Palynostratigraphical records have been used to understand the response of vegetation to climate change, and benefit from independent dating to ensure a robust correlation with global climate and sea-level change. In order to constrain the pollen chronology of a long sedimentary core taken at Azzano Decimo in the Friulian foreland of northeastern Italy, optically stimulated luminescence (OSL) has been applied to fine grain quartz. The samples meet all the standard performance criteria set to test the reliability of the single-aliquot regenerative-dose (SAR) protocol, and still show increasing dose response at 500 Gy. OSL ages are in good agreement with radiocarbon dating and the pollen interpretation down to 70 ka, and with De values of ～140 Gy, but below this point, they display an increasing age underestimation towards the Eemian and beyond. The comparison of De values measured using both a SAR and single-aliquot regeneration and added-dose (SARA) protocol, confirmed that both were successfully correcting for sensitivity changes in the quartz during measurement of the burial dose, and this was not the reason for the age underestimation. The quartz OSL dose response curve for all samples is best described by a saturating exponential plus linear (SEPL) function. Although all underestimated ages are derived from De values that fall on the high dose linear region of this curve, it is unclear if this is the cause of the underestimation. 相似文献
13.
All Quaternary dating methods involve the measurement of one or more variables to estimate the age of a sample. Each measured quantity has an associated error and uncertainty, and may also be subject to natural variation. We review the statistical estimation of such uncertainties and variation for comparing and interpreting age estimates, with specific reference to the estimation of equivalent dose (De) values in the optically stimulated luminescence (OSL) dating of sediments. We discuss statistical aspects of OSL signal and background estimation, the determination of De values for multi-grain aliquots and individual mineral grains from the same and different samples, and the extent of variation commonly observed among such estimates. Examples are drawn from geological and archaeological contexts. We discuss the strengths and weaknesses of various graphical methods of displaying multiple, independent estimates of De, along with statistical tests and models to compare and appropriately combine them. Many of our recommendations are applicable also to the clear presentation of data obtained using other Quaternary dating methods. We encourage the use of models and methods that are based on well established statistical principles and, ideally, are validated by appropriate numerical simulations; and we discourage the adoption of ad hoc methods developed using a particular set of measurement conditions and tested on a limited number of samples, as these may not be applicable more generally. We emphasise that the choice of statistical models should not be made solely on statistical grounds (or arbitrary rules) but should take into account the broader scientific context of each sample and any additional pertinent information. 相似文献
14.
Single grain OSL has become a widely used approach in Quaternary geochronology. However, the origins of De distributions and the sources of variation in individual dose estimates are still poorly understood. The amount of scatter in these distributions on top of the known uncertainties in measurement and analysis is defined by overdispersion and this quantity is generally used for weighting individual De values to calculate a central equivalent dose. In this study, we address the nature and amount of different sources of dispersion in quartz single grain De estimates, by (i) using appropriate statistical tools to characterize De populations and (ii) modelling, with a specifically designed Geant4 code, dose rate distributions arising from the presence of potassium feldspar grains in well-sorted sands. The model uses Monte Carlo simulations of beta emissions and interactions in a random close packing of quartz and feldspar spheres representing a sand sample. Based on the simulation results, we explain the discrepancy between intrinsic and natural overdispersion values in a well-bleached sample, thus validating the model. The three parameters having the most influence on dispersion in dose rate distributions, and modelled in this study, appear to be grain size, potassium content and total dose rate.Finally an analysis of measurement uncertainties and other sources of variations in equivalent dose estimates lead us to conclude that all age models (both logged and unlogged) which include an overdispersion value to weight individual De values rely mainly on unknown parameters; this ignorance may lead to an inadvertent bias in De estimates. Assuming counting statistics make a small contribution to dispersion (as is often the case), we suggest that in some cases it is most appropriate to use unweighted averages of equivalent doses when dividing by commonly measured average dose rates. 相似文献
15.
The prehistoric site of Ifri n'Ammar is situated in northeastern Morocco, in the northern prolongation of the Middle Atlas Mountains. It is a key location in unravelling the history of anatomically modern humans (AMH) in northern Africa as it reveals Middle and Late Palaeolithic occupation phases since ∼170 ka. Whilst the archaeological sequence within the rock shelter has been well studied, the timing of landscape dynamics around Ifri n'Ammar is still poorly understood. This study therefore aims to establish a detailed chronology of the Wadi Selloum profile at the apron of the shelter, based on optically stimulated luminescence (OSL) dating of ephemeral stream deposits. Coarse-grain quartz was used for single-grain and multiple-grain dating procedures to investigate the luminescence properties of these deposits and to get more accurate age information concerning the phases of human occupation. Continuous wave OSL (CW-OSL) revealed a dominant fast component for all quartz samples. The dose distribution of the uppermost samples showed overdispersion values >25% and significant positive skewness. We identified partial bleaching as the main source of scatter in the equivalent dose (De) distribution. The lowermost sample appeared to be close to signal saturation. The shapes of the dose response curve varied widely between aliquots and coarse quartz grains exhibited therefore very different dose saturation behaviours among aliquots. With fully saturated dose response curves (DRCs), meaningful D0 values were assumed for De estimation.The eight OSL samples yielded stratigraphically consistent ages ranging from 1.3 ± 0.2 ka to 76 ± 5 ka, thus reaching the Middle Palaeolithic period. Moreover, a pottery shard dated to 7.4 ± 0.6 ka (Early Neolithic period) by thermoluminescence (TL), perfectly matched the Holocene OSL samples extracted at the same depth of the profile. In summary, our results point to fluvial aggradation during OIS 5.1, the late glacial period, and the Holocene. 相似文献
16.
Studies of modern sediments, their sedimentology and depositional processes are important for understanding the behaviour of the luminescence characteristics of quartz and feldspar in fluvial settings. Previous studies have shown large variations in OSL characteristics of quartz from different fluvial systems, while the IRSL and pIRIR signals from K-feldspar have been understudied. We test the effects of fluvial setting on luminescence characteristics by collecting modern (<1 year old) bedload sediments down the courses of three river systems with very different hydrological characteristics, geologic contexts, and catchment lithologies. The single grain (SG) and multi-grain aliquot (MGA) OSL (quartz) and IRSL and pIRIR (K-feldspar) properties of samples were measured and compared to better understand intra- and inter-fluvial system patterns in sensitivity, bleaching, and equivalent dose (De) distribution skewness and kurtosis. The quartz OSL and K-feldspar IRSL and pIRIR signal sensitivities increase with downstream transport distance of sediments, confirming previous studies (quartz) and showing that IRSL signals from K-feldspar also increase in response to reworking cycles. Increasing transport distance also results in better bleaching of the OSL signal from quartz samples (MGA and SG) due to more grains being exposed to sunlight. By contrast, the IRSL and pIRIR signals retain significant residuals in all samples, though 5–15% of grains yield zero-dose De values and age modelling of SG data yields accurate burial dose estimates. Additionally, the skewness and kurtosis of SG OSL De datasets from one river increase with transport distance, with the best bleached samples exhibiting the highest skewness, thereby questioning the applicability of the skewness-value of a De dataset as an accurate indicator for partial-bleaching. Our data shows marked variability between (i) different river systems and (ii) the measured minerals, however consistent use of statistical models allows accurate De estimation in all contexts. Age modelling of SG data from K-feldspar, thus, provides a valuable tool for future fluvial research in regions where poor OSL characteristics prevent the use of quartz as a dosimeter. 相似文献
17.
The Qinghai-Tibetan Plateau is an important area for the study of Quaternary glaciation. Optically stimulated luminescence (OSL) dating has the potential to contribute to the chronology of glaciation in this region, but it is important to assess the accuracy of OSL dating of these glacial sediments. In this study, single grain quartz OSL signals are examined for five glacial samples collected from the moraines outside the Baiyu Valley, southeastern Tibet. The quartz grains exhibit poor luminescence characteristics, with a small proportion of grains passing the screening criteria. Grains which pass the screening criteria have relatively low signal intensity, leading to De values with large uncertainties. MAM and CAM were used to determine De values for these samples. The OSL ages are consistent with the sequence of events derived from the geomorphological relationship of the samples, and also with previous published radiocarbon ages. However, it is more difficult to reconcile the OSL ages and the terrestrial cosmogenic nuclide (TCN) 10Be ages. Analysis of both single grain quartz OSL data and TCN 10Be data is complex in this area. Further work is required to increase confidence in the OSL ages generated for the glacial sediments from this region. 相似文献
18.
In optical dating, the last time that a sample of sediment was exposed to sunlight is determined by dividing its equivalent dose (De) by the dose rate. For single-grain dating, the sample De is based on the statistical analysis of the distribution of De 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 De 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 De 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 De values and dose rates, the associated measurement uncertainties, the effect of natural variability among grains, and the outlier probabilities of De 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 LnTn methods to integrate information on the full distribution of single-grain De 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. 相似文献
19.
IR laser stimulated luminescence was used to determine the equivalent dose (De) for about 150 luminescent K-feldspar grains of two well-bleached aeolian sand samples, previously dated using quartz optically stimulated luminescence (OSL) and thermally-transferred OSL (TT-OSL). Fading tests were performed both on the entire single grain disc, using stimulation by IR LEDs, and on each grain individually, by stimulation with an IR laser. The single grain De distributions are highly over dispersed, even after applying rejection criteria, such as recycling ratio and recuperation. Fading rates determined with the IR LED stimulation do not represent the fading behavior of the IR laser stimulated signal and are therefore considered inappropriate for the fading correction of single grain De values. Fading rates of individual grains exhibit a wide range of values (0%–10% per decade) and plotting the single grain fading rates against the single grain De values reveals a negative relationship. Different approaches were taken to correct for fading, but the resulting ages tend to underestimate the reference ages. 相似文献
20.
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 (Lx), the test dose signal (Tx, an indicator of luminescence sensitivity) and the sensitivity-corrected signal (Lx/Tx) 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 (De) 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 De 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 De 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. 相似文献