Detecting post-depositional sediment disturbance in sandy deposits using optical luminescence

Post-depositional mixing or exhumation is common in surficial sediments, yet may be unobservable from field evidence. However, any disturbance may have significant consquences in terms of establishing a reliable luminescence age determination. Optically stimulated luminescence (OSL) measurements, particularly measurements at the single grain level, can be used to gain an insight into both contemporary and past post-depositional processes.

This paper examines sites from Texas and Florida (USA) with independent chronological control to demonstrate the potential effects of varying degrees of bioturbation on OSL. Results show that contemporary soil forming processes clearly impact on the palaeodose (D_e) replicate distributions which are measured in order to derive an OSL age. Significant levels of scatter and apparently zero dose grains are observed in the upper-most sediments; declining with depth from the surface. D_e replicates from undisturbed and fully bleached sediments are unskewed, show low overdispersion (OD) and comparable single grain and single aliquot OSL ages. Bioturbated sediments, however, may show highly skewed multi-model D_e distributions with higher OD values, zero dose grains at depth, and significant diffences between single grain and single aliquot results. True burial ages may be derived from minimally bioturbated sediments through the application of statistical analysis such as finite mixture modelling to isolate D_e components. However, for significantly bioturbated sediments, the latter approach, even at the single grain level, produces inaccurate ages. In such cases we argue that additional evidence (both dating and contexual) may be required to identify with confidence the burial D_e population.     

Resetting of sediments mobilised by the LGM ice-sheet in southern Norway

Former geological field investigations in the Rondane area, east-central southern Norway, have proposed that the maximum Fennoscandian ice-sheet coverage occurred during the Late Weichselian Glacial Maximum (LGM, ca. 20 ka) and that subsequent glaciofluvial sediments were first deposited in the early Holocene (after 10 ka). However, recent field investigations with ages from three internally consistent quartz optically stimulated luminescence (OSL) age series show an apparent deglaciation of northern Rondane in the period 20.0–13.8 ka. We examine here the possibility that these ages are too old because the sediment was not completely zeroed prior to deposition. Our investigations of incomplete bleaching use modern analogues, small aliquots, and single grains of quartz. First, the symmetric shape of small aliquot equivalent dose distributions suggests that the sediment was probably well bleached at deposition. This is supported by 5 modern analogue equivalent doses (D_e) of 0.6 Gy, 1.5% of the typical D_e from the deglaciation sediments. Finally, from single grain studies on three samples, we conclude that there is no evidence for poor bleaching in these samples; thus the weighted mean gives the best estimates of D_e, and these are completely consistent with both large and small aliquot estimates for these samples. These comparisons between large aliquots, modern analogues, small aliquots and single-grain analyses help to validate the OSL ages and confirm the complete resetting of these sediments prior to deposition.     

ESR dating of sedimentary quartz: Possibilities and limitations of the single-grain approach

Usually, multiple-grain aliquots are used for electron spin resonance (ESR) dating of sediments. However, this approach excludes the ability of detecting insufficient bleaching, as would be the case when measuring several single aliquots or single grains. In this paper, we present preliminary results of single-grain ESR dating experiments on three different sedimentary deposits (fluvial, fluvio-aeolian and desert aeolian), ranging in age between 100 ka and 2 Ma. Titanium-related impurity centres (Ti–Li and Ti–H) were measured in a Q-band ESR spectrometer for estimating equivalent doses (D_e) of individual quartz grains (0.5–1 mm). Both additive and regenerative dose methods were used. The resulting D_e plots show a large scatter in D_e—from 100 Gy to more than 600 Gy—for the fluvial sample, probably reflecting different bleaching histories of the grains. On the contrary, a clear plateau can be observed in D_e for sands with a strong aeolian component. Preliminary single-grain ESR ages are in relatively good agreement with independent estimates, the best results being obtained for the desert aeolian sample (200 ka). It is concluded that ESR has a promising potential for estimating large naturally accumulated doses of well-bleached sediments. Nevertheless, the method is challenged by several experimental difficulties, such as long measurement times, excessive errors, and presumably a complex interrelationship between Ti–Li and Ti–H components.     

Chronology of an oyster reef on the coast of Bohai Bay, China: Constraints from optical dating using different luminescence signals from fine quartz and polymineral fine grains of coastal sediments

Fossil oyster reefs are indicators of past sea levels, and their formation is usually dated by means of radiocarbon. However, radiocarbon dating of the shells from coastal areas may be complicated by the varying sources of carbon. Here we applied optical dating methods to date the samples from above and below a fossil oyster bed in a section on the coast of Bohai Bay, China. The optical ages of the sediments were used to constrain the oyster bed. Single-aliquot regenerative-dose procedures using the OSL signal from fine grain quartz, the IRSL and post-IR OSL signals from polymineral fine grains were employed to determine equivalent dose (D_e). The behaviors of the different luminescence signals from quartz and polymineral grains during D_e measurements were examined. The results showed that the quartz OSL signal is more reliable than the polymineral IRSL and post-IR OSL signals with respect to dating for these coastal samples. The optical ages indicated that the oyster reef formed between ca. 6.2 and 5.0 ka.     

Application of OSL dating to middle to late Holocene arroyo sediments in Kanab Creek,southern Utah,USA

Middle to late Holocene alluvium, identified as Quaternary alluvial unit 4 (Qa4), along Kanab Creek in southern Utah, USA was dated using optically stimulated luminescence (OSL) on quartz sand, and by radiocarbon dating of detrital charcoal. Entrenchment beginning in 1882 AD created arroyo walls that expose up to 35 m of the Qa4 alluvium. The stratigraphy and sedimentology suggest that fluvial aggradation along the study reach occurred rapidly. Due to the high sediment supply, short transport distances and semi-arid climate with flashy discharge, partial bleaching (zeroing) of the luminescence signal was expected to be a problem for OSL dating. We approached this problem by first using small-aliquot (∼20 grains) and single-grain dating of quartz sand to reduce the number of grains contributing to the OSL signal. Second, we used statistical parameters based on single-grain and small-aliquot equivalent dose (De) distributions of bleached sediment to help identify partial bleaching and to inform if a minimum age model (MAM) should be used for age calculation. Comparison of results with radiocarbon ages demonstrates the success of OSL dating on Kanab Creek arroyo-fill deposits, although careful attention should be paid to the sedimentary facies and stratigraphy of the targeted sample horizon to minimize the effects of partial bleaching. Thin, decimeter-scale plane-bedded and ripple cross-bedded sandy lithofacies were found to be the best target for OSL dating, as these sediments showed minimal evidence for incomplete solar resetting. Additionally, results generally indicate that better-bleached sediments are found in downstream reaches. Age control from these arroyo-fill deposits was acquired in order to fulfill larger research goals of understanding regional arroyo incision and aggradation cycles.     

Dating of lower terrace sediments from the Middle Rhine area, Germany

Modern and known-age Pleistocene fluvial sediments were investigated by optical dating of quartz to test the suitability of the approach for dating deposits from the deeply incised Middle Rhine Valley. Samples from modern flood sediments revealed skewed distributions indicating different residual levels of equivalent dose (D_e) within the different aliquots. Nevertheless, a substantial number of aliquots from the modern deposits reflect D_e values close to zero. For the Pleistocene samples, optical ages are in general consistent with age control given by the presence of the Laacher See Tephra and radiocarbon dating. However, some samples overestimate the known age by a few thousand years when using the arithmetic mean. This is apparently explained by including aliquots in the determination of mean D_e where the optical signal was incompletely bleached at deposition. The most difficult issue in this context is identifying a suitable approach that can distinguish between the variability of D_e due to partial bleaching and microdosimetry. However, even when considering these limitations it appears that optical dating will by a quite suitable method to date Pleistocene sediments from such a complex fluvial environment, especially when focusing on a precision scale beyond a few thousand years.     

OSL dating of sediments from deserts in northern China

Aeolian samples from deserts in northern China were studied using the optically stimulated luminescence (OSL) of quartz and potassium (K) feldspar separates. K-feldspar has shown advantages over quartz for dating samples younger than 100 years, due to its more homogeneous equivalent dose (D_e) values, higher internal dose rate and higher luminescence sensitivity to dose. Without additional measurements, the luminescence characteristics of quartz obtained in D_e determination procedures can be used to reveal the source material and/or quartz thermal history for the deserts in northern China. The results indicated that the duration of paleosol formation in desert areas reflects regional paleoenvironments.     

A revised burial dose estimation procedure for optical dating of youngand modern-age sediments

The presence of genuinely zero-age or near-zero-age grains in modern-age and very young samples poses a problem for many existing burial dose estimation procedures used in optical (optically stimulated luminescence, OSL) dating. This difficulty currently necessitates consideration of relatively simplistic and statistically inferior age models. In this study, we investigate the potential for using modified versions of the statistical age models of Galbraith et al. [Galbraith, R.F., Roberts, R.G., Laslett, G.M., Yoshida, H., Olley, J.M., 1999. Optical dating of single and multiple grains of quartz from Jinmium rock shelter, northern Australia: Part I, experimental design and statistical models. Archaeometry 41, 339–364.] to provide reliable equivalent dose (D_e) estimates for young and modern-age samples that display negative, zero or near-zero D_e estimates. For this purpose, we have revised the original versions of the central and minimum age models, which are based on log-transformed D_e values, so that they can be applied to un-logged D_e estimates and their associated absolute standard errors. The suitability of these ‘un-logged’ age models is tested using a series of known-age fluvial samples deposited within two arroyo systems from the American Southwest. The un-logged age models provide accurate burial doses and final OSL ages for roughly three-quarters of the total number of samples considered in this study. Sensitivity tests reveal that the un-logged versions of the central and minimum age models are capable of producing accurate burial dose estimates for modern-age and very young (<350 yr) fluvial samples that contain (i) more than 20% of well-bleached grains in their D_e distributions, or (ii) smaller sub-populations of well-bleached grains for which the D_e values are known with high precision. Our results indicate that the original (log-transformed) versions of the central and minimum age models are still preferable for most routine dating applications, since these age models are better suited to the statistical properties of typical single-grain and multi-grain single-aliquot D_e datasets. However, the unique error properties of modern-age samples, combined with the problems of calculating natural logarithms of negative or zero-Gy D_e values, mean that the un-logged versions of the central and minimum age models currently offer the most suitable means of deriving accurate burial dose estimates for very young and modern-age samples.     

OSL dating of mixed coastal sediment (Sylt,German Bight,North Sea)

As part of a study on coastal sedimentary processes this paper presents the OSL dating of mixed coastal sediment samples from the southern North Sea island of Sylt (German Bight). During coring of the swash-bar (beach) sediments, five samples were presumably contaminated by younger overwash and aeolian sediments because of the sampling method employed. To obtain reliable burial ages for these swash-bar sediments, single-grain and small aliquot measurements were used together with the Finite Mixture Model (FMM) proposed by Roberts et al. (2000) to identify the grain population containing the largest doses (from the deepest part of the core). Before the FMM was applied to dating, the parameters and performance of the FMM were first investigated by systematically comparing small aliquot (∼20 grains) and single-grain measurements of an undisturbed aeolian and swash-bar sample and a laboratory mixture of both sediments. This test case demonstrates the advantage of selecting the time interval immediately following the initial luminescence signals for background subtraction because unsuitable quartz grains were removed from the dose distribution. It is concluded that the measurement of small aliquots can be regarded as a reliable proxy for single-grain dose distribution if the sediment contains only a small proportion of quartz grains emitting a luminescence signal and that the FMM results are relatively insensitive to changes of the over-dispersion parameter between 5–40% for small aliquots and 10–40% for single-grains.We show that the burial ages of the contaminated swash-bar samples resulting from the maximum age populations from equivalent dose distributions measured using small aliquots are consistent with the stratigraphy and with ages obtained from uncontaminated samples.     

Single-grain quartz OSL dating of debris flow deposits from Men Tou Gou,south west Beijing,China

Debris flows in the mountainous regions south west of Beijing, China occur frequently and often result in considerable mass movements with disastrous consequences for human life, infrastructure and agriculture. Obtaining chronological information on such events is important for the prediction of the return frequency of these debris flows, risk assessment and climate change research. In this project, we use quartz single-grain optically stimulated luminescence (OSL) methods to determine the burial ages of five debris flow samples from the Zhai Tang region ∼60 km west of Beijing. OSL characteristics were found to be acceptable despite the low inherent brightness of quartz extracted from these samples. Single-grain thermal transfer was determined to be negligible and beta dose recovery experiments were satisfactory. The quartz single-grain dose distributions strongly indicate that the samples were poorly bleached prior to deposition; relative over-dispersions are larger than 60%. Minimum age modelling indicates that all five samples were deposited within the past few hundred years, indicating that catastrophic debris flows are occurring under the historically-recent land-use pattern.     

The effect of weathering on optically stimulated luminescence dating

Optically stimulated luminescence (OSL) dating was applied to quartz samples recovered from the gravel sediments of deformed marine terrace at the Suryum site, southeastern Korea, but has yielded stratigraphically inconsistent ages. Micromorphological, chemical, and mineralogical analyses of the gravels indicate the possible influence of chemical weathering on the apparent OSL ages. In the course of weathering after emerging from beach environments, originally permeable gravels underwent infilling by halloysite-rich clays in their voids, export and import of radioactive elements, and loss of mechanical strength with the creation of a porosity within pebbles. Calculation based on the evolution of the weathering profile showed a severe fluctuation of dose rate caused by the change of water content and radioactive element concentrations. Samples recovered from the weathering profile inevitably include quartz grains derived from mechanically weakened pebbles or illuviated from the upper layer, making it difficult to determine the equivalent dose (D_e) values of exclusively depositional quartz sands. Quantitative evaluation of the change of fabric and chemistry of the sediments on the basis of pedologic insight significantly aids in the derivation of OSL ages consistent with geomorphology and other independent age controls.     

Developing a 150 ka luminescence chronology for the barrier dunes of the southern Cape, South Africa

The southern coastline of South Africa exhibits extensive aeolianites and spectacular barrier dunes; the ages and palaeoenvironmental significance of which have, until recently, received limited attention. We present preliminary results for samples taken from a single 6.5 m section in the seaward barrier dune near to the town of Wilderness. The reliability of these ages has been investigated using a series of dose recovery and pulsed irradiation experiments, as well as through analyses of single aliquot and single grain D_e distributions. Results indicate that the SAR protocol is providing accurate and precise D_e determinations and that adequate sensitivity correction is being made. The application of pulsed irradiations does not result in significant changes in the dose–response curves or averaged D_e's. Single grain D_e distributions are broad, with significant levels of overdispersion, which may reflect beta dose heterogeneity effects.

Peaks in aeolian activity date to 157–154 and 140–125 ka, with a slower rate of deposition recorded from 115 to 85 ka. Brief phases at 40 and 21 ka are also identified. Some ages, notably those from OIS 3 and OIS 2, are surprising from a geomorphological perspective, given the likely distance of the dune from the shoreline at such times, although they are not inconsistent with evidence from the east coast of South Africa. The results reveal multiple phases of Late Quaternary barrier dune accumulation, and suggest that aeolian activity in this region has been episodic and potentially more complex than previously thought.     

Application of single-grain OSL dating to ice-proximal deposits,glacial Lake Benson,west-central Minnesota,USA

Glacial Lake Benson formed in west-central Minnesota as the Des Moines lobe of the Laurentide ice sheet retreated north of a small moraine in the Minnesota River lowland. Although previous research has constrained the timing of glacial Lake Agassiz immediately to the north, little age control is available for the formation of glacial Lake Benson and ice-marginal positions to the south. In order to constrain the age of glacial Lake Benson and test the application of single-grain optically stimulated luminescence (OSL) dating to ice-marginal deposits, seven OSL samples were collected from a variety of depositional settings. These included deltaic deposits linked to specific lake levels, pro-glacial fluvial, ice-contact and supra-glacial deposits. Single-grain OSL results indicate evidence for incomplete resetting (partial bleaching) of the luminescence signal, as expected for glacial environments, and therefore ages were calculated using a minimum age model. OSL results constrain the timing of ice-margin retreat and lake formation to 14.4–14.8 ka. Analysis of single-grain equivalent dose distributions indicates that deposits created by glacial-dominated processes typically had higher over-dispersion (>50%) and greater positive skew (>0.9) than deposits originating from fluvial processes. These results suggest that water-lain deposits should be targeted for OSL sampling over those created by glacial processes when dating ice-proximal settings.     

Sedimentation rates on embanked floodplains determined through quartz optical dating

We investigate the use of quartz optically stimulated luminescence (OSL) dating for determining fluvial overbank sedimentation rates over decades to centuries. For the study we took 11 samples from three cores from an embanked floodplain along the River Waal (Rhine) near Neerijnen (The Netherlands). We propose a measurement protocol for young fluvial quartz based on the single-aliquot regenerative dose procedure. Parameters for the protocol are chosen to isolate the fast OSL component, eliminate an ultrafast OSL component and avoid thermal transfer. The protocol shows excellent dose recovery and recycling ratios. For each sample, a Gaussian is fitted to the lower part of the equivalent dose distribution to obtain an estimate of the burial dose. We discuss the validity of the OSL ages using internal and external controls, and conclude that there is no evidence for large systematic offsets in the OSL ages. OSL based sedimentation rates are between ～3 and 8 mm/a, in line with previous estimates.     

Quartz and polymineral luminescence dating of Japanese loess over the last 0.6 Ma: Comparison with an independent chronology

There are only a few luminescence dating studies of loess sediments in Japan, but interleaved with these deposits are many well-described tephras of known age based on ¹⁴C and fission track analysis; these independent age controls provide an opportunity to test the reliability of loess luminescence ages. This study provides such a comparison at two sites in central Honshu, the largest island in the Japanese archipelago. Samples were collected from sequences of interleaved volcanic tephra and loess deposited on a Middle Pleistocene river terrace in the Niigata Prefecture and on an Early to Middle Pleistocene dissected fluvial surface at in the Tochigi Prefecture, Honshu. Equivalent doses (D_e) were estimated from fine grains (4–11 μm) using both polymineral IR-OSL and (post-IR) blue-OSL, and quartz blue-OSL. The blue-stimulated luminescence signals could be represented by up to three exponentially decaying components; only the most light sensitive of these components was used in the final D_e estimation. Almost all the estimates of D_e from polymineral IR-OSL are smaller than those from polymineral (post-IR) blue-OSL and quartz blue-OSL, whereas the latter two are in good agreement. The blue-light stimulated luminescence ages using the most light sensitive component are shown to be in good agreement with the independent control, up to 500 ka. Although the IR-OSL signals underestimate the known age, a simple laboratory fading test is found empirically to correct for this underestimation. We conclude that the most rapidly blue-stimulated luminescence signals from quartz extracted from our loess samples give reliable ages, and that future loess dating studies should concentrate on these signals.     

Problems associated with luminescence dating of Late Quaternary glacial sediments in the NW Scottish Highlands

Linking the timing of glacial episodes and behaviour to climatic shifts that are documented in ice and marine sedimentary archives is key to understanding ocean-land interactions. In the NW Scottish Highlands a large number of closely spaced (‘hummocky’) moraines formed at retreating glacier margins. Independent age control on one palaeo-glacier limit is consistent with the timing of Younger Dryas (YD) glaciation in the area, but adjacent glacier lobes have remained undated due to the lack of sites and material for ¹⁴C dating. Direct dating of ice-marginal moraines using optically stimulated luminescence (OSL) techniques has never been attempted before in Scotland, but if successful, they may be the most appropriate methods for constraining the age of sediment deposition in the absence of organic material. Coarse-grained quartz and K-feldspar minerals from supraglacial sheet flow deposits and glacilacustrine sediments within ice-marginal moraines were analysed using the single-aliquot regenerative-dose (SAR) protocol. Independent age control and clear geomorphological relationships indicate that all samples should yield YD or post-Last Glacial Maximum (LGM) ages. Quartz OSL shine down curves showed low luminescence sensitivity, significant medium-to-slow components, a weak fast component, and scattered SAR data; Linearly Modulated-OSL (LM-OSL) measurements confirmed that the fast component was weak or absent. In contrast, feldspar infrared stimulated luminescence (IRSL) was highly sensitive with excellent SAR data. However, SAR data from 3 mm diameter aliquots of feldspar (200 grains) give higher than expected equivalent doses (D_e) by an order of magnitude. SAR measurements of small clusters of feldspar grains (ranging from 1–8) considerably broaden the apparent D_e distribution, but even the lowest value is about 2–3 times the expected D_e. Two possibilities arise to explain the quartz and feldspar data: (1) that glacial sequences in the NW Highlands re-work inherited (older) glacial deposits and that some of the pre-Devensian existing glacial landforms have only been modified during Devensian glaciation; or (2) that the sedimentary processes operating in these ice-marginal environments are not conducive to adequate bleaching of quartz and feldspar grains. Our study implies that ice-proximal supraglacial sediments from this region in NW Scotland reflect older ages of deposition, but dating YD sediments has not been possible.     

Optical dating of Holocene lake sediments: Elimination of the feldspar component in fine silt quartz samples

Optically stimulated luminescence of fine silt quartz was investigated to establish chronologies for lake sediments from the British Isles. Feldspar contamination and its potential effects on equivalent dose (D_e) determination were examined. A new feldspar contamination test based on the thermal quenching of quartz OSL and thermal assistance of feldspar OSL identified feldspar contamination in samples being treated for 20 min in 20% hydrofluoric acid. Feldspar component monitoring based on the post-IR OSL/OSL ratio was not sufficiently sensitive to monitor a feldspar component, but is a good indicator whether a standard SAR procedure can be applied successfully, despite the presence of feldspar. Based on these results we used a modified SAR protocol including a 100 s IRSL stimulation at 50 °C before OSL readout. This protocol could overcome any malign effects of feldspar even for a feldspar bearing polymineral sub-sample.     

OSL dating of individual quartz ‘supergrains’ from the Ancient Middle Palaeolithic site of Cuesta de la Bajada,Spain

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 (D_e) values with reasonable fitting uncertainties. A series of novel sensitivity tests are used to assess potential biases in supergrain D_e 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 D_e 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.     

Testing single-grain quartz OSL methods using sediment samples with independent age control from the Bordes-Fitte rockshelter (Roches d'Abilly site,Central France)

We present quartz single-grain dose distributions for four well-bleached and unmixed sediment samples with independent age control (22–48 ka), from the archaeologically important Bordes-Fitte rockshelter at Roches d'Abilly, France. This site has previously been dated using ¹⁴C AMS dating and standard multi-grain OSL dating using both quartz and feldspar. The effect of rejection criteria usually employed in single-grain dating on dose and over-dispersion is tested using both laboratory irradiated samples and natural samples. It is shown that had these samples been analysed in the absence of other age control, standard modelling decisions based on the shape of single-grain dose distributions would have led to significant misinterpretation of results and a corresponding >40% underestimation in age. If we instead ignore this standard decision process and apply weighted average and mixing models then the most likely results deviate from the expected ages by >10%. Finally, we show that by careful consideration of the luminescence characteristics of individual grains, we are able to obtain good agreement with the independent age control by applying alternative rejection criteria but this is at the cost of reducing the accepted grain population by more than an order of magnitude, with the corresponding inevitable decrease in precision.     

Optically stimulated luminescence dating of young (<500 years old) sediments: Testing estimates of burial dose

The reliability of single-grain optically stimulated luminescence (OSL) dating of young (<500 years old) samples is assessed by examining the results of 166 single-grain OSL analyses of Australian alluvial, lacustrine, estuarine, coastal and marine sediment samples. All samples have been analysed using the modified SAR protocol of Olley et al. [2004b. Optical dating of Holocene sediments from a variety of geomorphic settings using single grains of quartz. Geomorphology 60, 337–358], with burial doses (D_b) calculated by fitting a single Gaussian curve to the peak of a multi-Gaussian summed probability distribution (PDF_Gaussian). The near-equivalence of this approach to currently accepted methods of palaeodose estimation – namely the minimum age and central age models of Galbraith et al. [1999. Optical dating of single and multiple grains of quartz from Jinmium rock shelter, northern Australia: part I, experimental design and statistical models. Archaeometry 41, 339–364] – is demonstrated. Considered as a whole, OSL ages calculated using the PDF_Gaussian D_b determination method are consistent with other chronometric, geomorphological, botanical and historical indicators of depositional age, while four alternative (previously published) methods are generally less reliable when applied to the samples analysed here. The single-grain data from 12 samples of known age (reported by Olley et al. [2004b. Optical dating of Holocene sediments from a variety of geomorphic settings using single grains of quartz. Geomorphology 60, 337–358]) are re-analysed using the PDF_Gaussian approach, with all but one yielding a D_b consistent with the independently known age. Examples of luminescence data (dose recovery tests, preheat plateaux, continuous-wave and linear-modulated OSL, growth curves, single-grain dose distributions) are provided that together indicate the suitability of Australian quartz for dating young sediments.