Search

Single-grain OSL dating of quartz is a popular approach to OSL dating, even when incomplete bleaching is not likely to be significant. However, little testing of the accuracy of single-grain dating has been published; particularly for samples older than 50 ka. In this study, we investigate the accuracy of single-grain quartz OSL dating, when a significant number of individual grains are no longer able to accurately measure the burial dose because of saturation effects. We compare standard multi-grain OSL results with those obtained from single-grain OSL measurements for five OIS substage 5e (Eemian) samples (∼128 ka). We show that for these samples, standard multi-grain quartz dose estimation results in dose estimates in good agreement with the predicted doses (four of the five samples recover age control), but that standard frequentist single-grain dating procedures significantly underestimate the age controls, i.e. the measured to predicted dose range between 0.42 ± 0.03 and 0.84 ± 0.06, where the underestimation increases with increasing relative number of grains in saturation. Attempting to remove the inevitable bias in the dose estimation resulting from a significant number of saturated grains (by using the D c criterion) reduced the underestimation, i.e. the measured to predicted dose ratio range between 0.63 ± 0.05 and 0.94 ± 0.08, but only the sample with the smallest absorbed dose is consistent with the age control. Using Bayesian analysis ("BayLum") the ratio of measured to predicted dose range between 0.75 ± 0.07 and 1.14 ± 0.08, but only two of the five samples agree with the independent age control. Our results have implications for the evaluation of single-grain OSL dating of quartz in the 100–200 Gy natural dose range. [ABSTRACT FROM AUTHOR]

The main purpose of this study is to investigate the accuracy of single-grain quartz OSL SAR dating for samples with known absorbed natural doses in the 150-200 Gy range. There exists some evidence that at least multi-grain quartz OSL ages tend to underestimate the true burial age in this dose interval by ~10- 15% (e.g. [1,2]). It has been hypothesized that single-grain data are superior to multi-grain data, because the latter must contain signals from `aberrant' grains (e.g. grains failing SAR procedural tests and/or grains in saturation), which can be excluded in single-grain analysis. Here we test this hypothesis using five known-age Eemian (MIS 5e, ~125-130 ka) sediment samples collected in northern Russia (Sula [1]) and in Denmark (Gammelmark [2]); conventional multi-grain methods using quartz gave results consistent with expectations for both sites, although there appeared to have been a small systematic age underestimate. Preliminary single-grain measurements show that ages are in poor agreement with the age control when using the conventional frequentist approach; the expected dose is underestimated by ~50%. Applying commonly used rejection criteria (e.g. recycling, OSL IR depletion and recuperation) does not improve this significant underestimation. However, a significant fraction (~40- 50%) of the detectable grains have natural sensitivity corrected OSL signals which cannot be interpolated on to the individually constructed laboratory dose response curves to give finite dose values, i.e. the grains are said to be in saturation. Previously, it has been argued that discarding such grains has the potential to significantly bias the resulting dose distributions towards low doses. In an attempt to circumvent this bias, it has been suggested that all grains with dose response curves with low characteristic doses (Dc values) compared to the natural dose ought to be rejected. Here we test if the so-called Dc criterion suggested by [3] in combination with Bayesian modelling using the specifically designed software tool BayLum [4] can help overcome this bias. [ABSTRACT FROM AUTHOR]

In this study, we use optically stimulated luminescence (OSL) to establish a robust absolute chronology for the lower part of the stratigraphic record at "Les Roches d'Abilly", an important Palaeolithic site in Central France. There, lithic assemblages recording the Middle-to-Upper Palaeolithic transition and the behaviour of Neanderthal and Anatomically Modern Human populations are preserved. At the Bordes-Fitte rock shelter, part of "Les Roches d'Abilly", the archaeo-stratigraphic record has been sealed and preserved by the collapse of the shelter roof and this site has provided important data on the timing of human occupations and Middle-to-Upper Palaeolithic transition in this region. Here, we first obtain burial ages using standard multi-grain quartz techniques on sand-sized sediment grains and then investigate the information available in blue-stimulated luminescence-depth profiles into the surfaces of four quartz-rich cobbles found in the sedimentary record below the various collapses of the shelter roof. These profiles show qualitative evidence for past daylight exposure and a single burial event, allowing the data to be analysed quantitatively using a multiple-event mathematical model. Based on the results of this modelling, it is concluded that at least part of the cobble surfaces were well-bleached at burial and thus that estimated rock surface burial ages are unlikely to be significantly affected by incomplete bleaching. However, it appears that not all cobble surfaces were well-bleached, demonstrating the importance of only using surface doses derived from those cobble layers that can be shown to have been well-bleached at burial. The rock-surface burial ages of the most recent burial event are consistent with the quartz OSL ages derived from the surrounding sediments, giving burial ages of ∼ 50 ka. The OSL ages are consistent with previously published 14C age control, although a single 14C age (uncalibrated age of 41 , 900 ± 1 , 500 yr BP) of a lower unit may underestimate the OSL ages. This study provides a reliable absolute OSL chronology for the Bordes-Fitte rock shelter constraining the Middle-to-Upper Palaeolithic transition to occur between 44±2 ka and 48±3 ka ago (at the 68% confidence level). [ABSTRACT FROM AUTHOR]

Single grain OSL has become a widely used approach in Quaternary geochronology. However, the origins of D e 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 D e 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 D e estimates, by (i) using appropriate statistical tools to characterize D e 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 D e values rely mainly on unknown parameters; this ignorance may lead to an inadvertent bias in D e 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. [ABSTRACT FROM AUTHOR]

Two fluvial sediment cores taken from a floodplain of the Hawkesbury- Nepean River system in the Sydney region, eastern Australia are dated using Optically Stimulated Luminescence ( OSL) to provide a reliable chronology essential for the management and planning of water resources. Nine charcoal 14 C ( AMS) dates constrain these OSL ages. Quartz extracted from seven OSL samples from each of the cores was measured using both single-grain and multi-grain OSL techniques. Three of the single-grain natural dose distributions appear to be well bleached, but the others appear to be incompletely bleached to various degrees. Three minimum-age models ( MAM, MAMUL and IEU) are applied to the single-grain dose distributions. We conclude that these models give consistent age estimates. For one of the cores it appears to be necessary to use a minimum-age model to obtain accurate ages, but in the other core incomplete bleaching is probably less important than postdepositional mixing and mixing during sampling. As a result, the burial age is probably best estimated using the weighted average of the individual single-grain dose estimates. The application of multi-grain OSL techniques to these samples results in an average apparent age overestimation of ∼200 years, which is significant for these samples, but negligible for sediments older than a few thousand years. The intention is that the chronology obtained in this study will be used in conjunction with a proxy flood record, derived from floodplain sediments, to gain an understanding of the long-term variability in periods of high and low rainfall in eastern Australia. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Luminescence dating is used extensively to provide absolute chronologies for Late Pleistocene sediments. Nowadays, most optical dates are based on quartz optically stimulated luminescence ( OSL). However, the application of this signal is usually limited to the last ∼100 ka because of saturation of the quartz luminescence signal with dose. In contrast, the feldspar infrared stimulated luminescence ( IRSL) dose-response curve grows to much higher doses; this has the potential to extend the datable age range by a factor of 4-5 compared with quartz OSL. However, it has been known for several decades that this IRSL signal is unstable, and this instability often gives rise to significant age underestimation. Here we test against independent age control the recently developed feldspar post- IR IRSL approach to the dating of sediments, which appears to avoid signal instability. A physical model explaining our observations is discussed, and the method is shown to be accurate back to 600 ka. The post- IR IRSL signal is reduced by exposure to daylight more slowly than that from quartz and low-temperature IRSL, preventing its general application to young (e.g. Holocene) sediments. Nevertheless, this new approach is widely applicable (feldspar of appropriate luminescence behaviour is even more ubiquitous than quartz). These characteristics make this a method of great importance for the dating of Middle and Late Pleistocene deposits. [ABSTRACT FROM AUTHOR]