Multi-grain Aliquots Research Articles

Luminescence dating of glacially sourced deposits from northern Switzerland: Comparing multigrain aliquots and single grains of quartz and feldspar

A complex picture of the Pleistocene glaciation history of northern Switzerland has been identified over the last three decades. To gain further insights into the long-term landscape evolution, numerical dating is required. In the absence of alternative dating techniques, luminescence dating is the key method for establishing chronological constraints of past glaciations. However, this is presented with complex challenges, especially in regard to the resetting of the luminescence signal prior to deposition, the components contributing to the signal as well as the signal intensity and stability. In this study, the luminescence properties of glacially sourced deposits from northern Switzerland are assessed using single aliquot (SA) and single grain (SG) measurements of feldspar (F) and quartz (Q). While no obvious connection between bleaching and distal or proximal deposition in relation to the proposed ice margin is observed, most samples seem to reveal a partially bleached signature in F SG De measurements. This appears to be masked in the respective F SA measurements even though only few grains emit luminescence signals. In addition, comparisons between fading corrected infrared stimulated luminescence (IR) and post-infrared infrared stimulated luminescence (pIR) De values appear to be unreliable indicators of bleaching, even though these signals bleach at different rates. Hence, it is recommended to conduct both IR and pIR investigations in combination with Q measurements on a SG level. The dating potential of the investigated deposits remains rather limited, yet, in the sedimentologic context the presented results reveal that several glacial advances occurred prior to the Last Interglacial in the study area.

Investigations on single and multi-grain optically stimulated luminescence (OSL) sensitivity and electron spin resonance (ESR) signals in quartz derived from sandstones: Insights on provenance of quartz in ancient depositional systems

Trapped charge techniques of luminescence and electron spin resonance (ESR) are classic tools for dating Quaternary deposits. Over the past decade, these techniques have been routinely applied to investigate provenance and/or the sedimentary history of quartz grains based on their different luminescence and ESR characteristics. Of these, optically stimulated luminescence (OSL) sensitivity is one of the most widely investigated parameter for luminescence-based provenance approach. A majority of studies on this parameter are based on evaluation of multi-grain quartz OSL sensitivity of the samples. This is particularly concerning because single-grain quartz luminescence studies have shown that the luminescence signal of a multi-grain aliquot is contributed by less than ∼1–10% of the total grains. Since the sole criteria for discrimination of sources based on luminescence sensitivity relies on its intensity, therefore the results based on multi-grain analysis will most likely be skewed depending on the ‘proportion’ and ‘brightness’ of a few grains. This demands a need to evaluate the potential of single-grain quartz OSL sensitivity in provenance studies. In this study, we investigate single and multi-grain quartz OSL sensitivity from compositionally different sandstones with well-characterised sources based on U–Pb zircon ages. We further complement this analysis with characterisation of ESR centres commonly used in quartz provenance, namely E′1 and [AlO4]0 centres. Our study shows that single-grain quartz OSL sensitivity can help distinguish between sediments that have a predominant input from a single source as compared to those with contribution from multiple sources, which otherwise cannot be inferred from multi-grain studies. Moreover, our results on characterisation of quartz-based ESR intensity of E′1 and saturated [AlO4]0 centres successfully differentiates between sandstones and further complements the luminescence-based characterisation.

Correlations between quartz OSL dose-response curve and TL glow curve characteristics, and their implications for equivalent dose estimation

Optically stimulated luminescence (OSL) dating of quartz using the single-aliquot regenerative-dose (SAR) procedure requires construction of dose-response curves (DRCs). The shapes of DRCs and their characteristic saturation doses (D0 values) are well-known to be highly variable between samples and among individual grains from the same sample, but the factors that control DRC characteristics are imperfectly understood. Here we report correlations between OSL DRC shapes and thermoluminescence (TL) glow curve characteristics for a sample of quartz from northern Australia. Individual grains were measured using the SAR procedure and one of two preheat temperatures (160 °C and 260 °C), and grouped according to similarities in their DRC shapes. Grains from each DRC group were then physically transferred onto a separate disc for TL measurement as a multi-grain aliquot. A strong correlation was observed between DRC group and laboratory-irradiated TL glow curve shapes, along with some differences between the two preheats. Irrespective of the selected preheat, earlier saturating DRC groups are associated with the most intense 110 °C TL signal, and the least intense 325 °C TL signal, which corresponds to the main OSL trap. Later saturating DRC groups generally show the opposite trend. There are also trends within other regions of the glow curve and DRC group, such as prominent TL responses at 140–160 °C, 220–240 °C and 390–410 °C. We assessed the effect on the accuracy of equivalent dose (De) estimation using an associated radiocarbon age as an independent cross-check of the OSL ages for the DRC groups. De over- and under-estimates were obtained for some DRC groups. Most notably, grains preheated to 160 °C displayed a pattern of De values which decreased with an increase in D0. We relate this to variable concentrations of charge at the ∼230 °C TL trap prior to regenerative dose OSL measurements. OSL ages for most DRC groups preheated to 260 °C are close to the radiocarbon age, as are those of later saturating DRC groups preheated to 160 °C; the latter DRC groups have the smallest TL signals at ∼230 °C relative to the TL peak at 325 °C. The results of this study suggest that correlations between single-grain De and D0 values can occur due to underlying differences in the TL characteristics of grains with different DRC shapes. In the absence of independent age control and prior to measuring multi-grain aliquots of quartz, we recommend that OSL dating practitioners screen their samples for any single-grain patterns of De as a function of DRC shape, to determine the optimal SAR measurement conditions and data-analysis procedures for De estimation.

Rock and sediment luminescence dating of an ancient circular stone-walled enclosure at Sønnebøe, northern Scania, Sweden

Dating agricultural artefacts such as field walls and clearance cairns using radiocarbon can be challenging, especially since the association with datable material may be poor. Rock surface burial dating using luminescence offers an alternative. Here we report on the luminescence dating of a medieval circular stone-walled enclosure at Sønnebøe, northern Scania, Sweden, using both buried rocks and sediments. Luminescence burial profiles from IRSL signals measured at 50 °C (IR 50 ) indicated significant prior light exposure in 7 of the 8 samples tested (5 granite, 2 felsic gneiss), in some cases multiple exposure burial cycles were indicated. These rock surfaces had apparently been exposed for sufficient time to allow accurate IRSL ages for the most recent burial event. In contrasts, no useful post-IR IRSL profiles were obtained indicating that this signal was not sufficiently reset to allow accurate determination of the burial dose on any of these rocks. IR 50 fading corrections (typically ∼50%) were derived by comparing field saturation with that induced in the laboratory. Quartz extracted from sediments surrounding the rocks gave an average measured to given dose ratio of 1.03 ± 0.01 (n = 90), and these sediment samples were then dated using multigrain aliquots; the corresponding feldspar dose recovery ratio obtained using rock samples was 0.98 ± 0.05 (n = 28). A total of 15 ages were derived; 8 quartz OSL ages from the disturbed coarse grained sediments surrounding the structure, and 7 fading corrected IR 50 ages from the surfaces of rocks (2–3 mm chips, ∼1 mm thick) used in the construction of the structure itself. The exposure events preserved by the ring enclosure stones unequivocally show wall building taking place at the site between 800 and 300 years ago. • Direct dating of stone walled structures. • Rock surface burial dating using IR 50 luminescence bleaching profiles. • Comparison of dating results obtained from sand sized sedimentary grains and granitic rocks. • Application of improved modelling of infinite matrix beta and gamma dose rates in rocks.

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

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

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

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

OSL Dating of Earthen Mortars from a Medieval Building in Northwestern Spain: Crypt of Basílica da AscensiÓn (Allariz, Ourense)

ABSTRACTThere are few papers that focus on optically stimulated luminescence (OSL) dating of earthen mortars. These mortars are abundant in historical buildings in northwestern Spain. The Basílica da AscensiónyForno da Santa building is an unfinished church built on a previous structure that was transformed into a crypt (Allariz, Ourense, NW Spain). Previous archaeological studies established a sequence of phases of construction, the first dating back to the Iron Age, with significant changes occurring in the Early and Late Medieval ages. The only datable material in the crypt is earthen mortar. Thus, eight mortar samples (seven joint mortars and one wall infill) were taken, seven of them dated by OSL. The dose rate was assessed, and the expected equivalent doses estimated based on the established archaeological age. Several grain sizes (from fine to coarse) were used in small multigrain aliquots to assess the equivalent doses and ages. No evidence of partial bleaching was observed in most samples and grain sizes. The resulting ages are younger than expected for most samples. This is explained by the fact that joints were repaired with new mortar from the 16th century onwards.

Exploring complexity in luminescence dating of quartz and feldspars at the Middle Stone Age site of Mwulu's cave (Limpopo, South Africa)

Luminescence dating is applied to sediments from Mwulu's Cave, whose lithics have been typologically considered Middle Stone Age (MSA) and have been previously assigned to what is called the Pietersburg industry. This industry is, however, poorly defined both chronologically and typologically. Luminescence methods were applied to both quartz and potassium feldspars by two different laboratories. Age results from single-grain quartz are much younger than those from multi-grain aliquots and this is attributed to the use of different fits to the growth curves and the effect of this on samples near saturation. Single-grain potassium feldspar ages are much older than the single-grain quartz ages. Application of various methods to address anomalous fading produced broadly consistent results among samples. Most K-feldspar samples appear to be about 90 ka. These are compared to other South African MSA sequences and industries.

Bombardment history of asteroid 4 Vesta recorded by brecciated eucrites: Large impact event clusters at 4.50 Ga and discreet bombardment until 3.47 Ga

Abstract The thermal/impact histories of sixteen eucrite meteorites were investigated: three monomict eucrites (NWA 999, A-87272,87, and Stannern), five polymict eucrites (NWA 1000, NWA 1666, NWA 5601, Y-980066,100, and Y-980255,100), three quench-textured, eucrite melt rocks (Y-981646,21, Y-981651,105, and MIL 0766214), one eucrite dominantly comprised of quench-textured clasts (QUE 99005,11), three unclassified eucrite breccias (LAP 031316,9, LAR 06870,5, QUE 99799,4) and one unbrecciated eucrite (EET 92004,17), included here due to its shock features. We have measured fifteen high-precision new 40 Ar/ 39 Ar plateau ages on plagioclase and matrix for ten of these meteorites with a tight cluster of nine ages obtained from three different polymict breccias. These ages range from 4534 ± 56 Ma to 4491 ± 16 Ma resulting in a concordant age population ( P = 0.16). The fact that such a cluster of ages is recorded in unrelated breccias which are made of a priori unrelated components, leads us to propose that those ages recorded a single heating event on a large scale, and is interpreted here as a high-energy impact event, early in the history of Vesta at 4500 ± 4 Ma. We propose that the debris was ejected and isolated from subsequent large impacts in a secondary rubble pile asteroid where the energy of the outgoing shock wave from an impact is significantly reduced as it compacts the target material ( Holsapple et al., 2002 , and references therein) . The other analyses define a spread of five plateau ages ranging from 3851± 21 Ma to 3469 ± 35 Ma, over ∼380 Ma. An additional apparent plateau age of 4288 ± 38 Ma, but with a diffusion profile of cumulative 39 Ar release, along with published U-Pb apatite age of ∼4.14 Ga, suggests that the data might either define a true continuum (normal background bombardment) from 4.5 Ga to 3.47 Ga or cluster between ∼3.85 Ga and ∼3.47 Ga (excavation of a fresh surface at 3.85 Ga continuously bombarded until 3.47 Ga). Both scenarios are compatible with a final ejection age of 3.47 Ga when a major impact liberates the bulk of the brecciated meteorites into another secondary rubble pile asteroid, where the brecciated eucrites stayed relatively well protected from subsequent major impacts. Based on these results and recent crater counting measurements, we propose that the excavation and bulk ejection were caused by the Rheasilvia (ca. 3.47 Ga) basin-forming impact. Diffusion models on plagioclase crystals with different Ar age spectrum signatures, from a single breccia (NWA1666; 4501 ± 7 Ma), suggest that either: (1) the formation of the breccia is very young, or (2) different plagioclase crystals have different diffusion characteristics, and/or (3) the porosity caused heterogeneous temperatures during an impact heating event, particularly likely if the 4.5 Ga brecciated eucrites were stored in a rubble pile asteroid. Many or possibly most large asteroids being re-accumulated rubble piles with potential large porosity ( Holsapple et al., 2002 ). Scenarios (2) and (3) preclude the usage of multi-grain aliquots to decipher the time-temperature history of most impact breccia using 40 Ar/ 39 Ar thermochronology.

Changes in the optically stimulated luminescence (OSL) sensitivity of single grains of quartz during the measurement of natural OSL: Implications for the reliability of optical ages

Abstract Single Aliquot Regeneration (SAR) protocol has been routinely used to estimate palaeodose for sediment samples. However, it was observed that significant changes in the optically stimulated luminescence (OSL) sensitivity occur during read out of the natural OSL and that the current SAR protocol does not take into account these changes (Singhvi et al., 2011). It was suggested that if not corrected, this change of OSL sensitivity would lead to substantive systematic offsets in ages based on the conventional SAR protocol. To circumvent this issue of sensitivity change of OSL, a natural correction factor based SAR (NCF-SAR) measurement protocol and a correction procedure was suggested by Singhvi et al. (2011) with an implicit but a reasonable assumption that the sensitivity of 110 °C TL peak of quartz correlates with the OSL sensitivity. Use of this correction resulted in reduced over-dispersion in paleodoses and additionally provided a satisfactory way to deal with samples, where natural signal was significantly higher than the saturation value of the regenerated signal. As a logical extension of this work, similar sensitivity changes in the OSL from single grains of quartz were examined and, their impact on the accuracy and reliability of single grain based SAR ages is discussed in this contribution. About ∼200 single grains each from 9 samples of diverse depositional environments were analysed and the NCF of each grain was measured. The single grain NCF’ [=(1/NCF)] values varied from 0.7 to 20. Of these ∼50% grains had sensitivity changes in the range of 0.7–1.3. The results indicated that, 1) the brighter grains (photon counts > 1000 c/s in a standard Risoe reader) had NCF′ closer to 1, though significant outliers did exist, 2) the distribution of NCF′ was positively skewed with significant number of values greater than one and, 3) the averaged single grain and multigrain NCF’ values were different, suggesting that in the case of multigrain aliquots, sensitivity changes of dim grains, on account of their larger numbers, collectively contributed significantly to the light sum in OSL.

Single and multi-grain OSL investigations in the high dose range using coarse quartz

There is evidence that optically stimulated luminescence (OSL) dating of quartz using the single-aliquot regenerative-dose (SAR) protocol underestimates the equivalent dose (De) for paleodoses above 100–200 Gy. Additionally, ‘infinitely’ old samples found not to be in laboratory saturation were reported. We present single and multi-grain SAR-OSL investigations for a coarse-grained (180–250 μm) quartz sample extracted from loess collected below the Brunhes/Matuyama transition at the Roksolany site (Ukraine). The sample was dated to more than 1000 ka by electron spin resonance using a multi center approach (Al and Ti signals), confirming that the De (∼2000 Gy) falls beyond the limit of standard OSL De measurement techniques. However, the natural signal measured using multi-grain aliquots of quartz was found to be below the laboratory saturation level. A comparison was made between synthetic dose response curves (DRCs) generated from single-grain and multi-grain aliquot data, respectively; the natural signal was found to be closer to the latoratory saturation level (92%) in the case of the single-grain synthetic DRC than for the multi-grain synthetic DRC where the signal was 86% of the saturation level. This difference could not be attributed to stimulation with different wavelengths, i.e. blue and green light stimulation for multi and single-grain measurements, respectively. By analysing synthetic data obtained by grouping grains according to their brightness, it was observed that brighter grains give a natural signal closer to the laboratory saturation level. This trend was confirmed for multi-grain aliquot data. Based on these findings we infer that variability in the contribution from populations of grains with different levels of brightness may represent a controlling factor in the closeness of the natural signal to laboratory saturation level for infinitely old samples.

Towards the origins of over-dispersion in beta source calibration

The ability to deliver accurate and precise calibration doses is a central part of all trapped charge dating methods. Usually, the radiation source (α, β, X-ray) used to deliver these doses is, in turn, calibrated against an absolutely known reference source (usually a γ source) and many laboratories make use of Risø calibration quartz for this purpose. We have previously described this material in detail (Hansen et al., 2015) and discussed the over-dispersion (OD) of 3.2 ± 0.3% in calibrated dose rate observed over 16 years. This dispersion highlights the danger of relying on individual calibrations, and is clearly undesirable. Here we continue our investigation into providing reliable calibration materials for trapped electron dating. A comparison of the apparent quartz β source dose rates shows that there is no significant dependence on the geological source of the quartz. However the β dose rate decreases by 25% with increasing grain size from about 100 μm to 1 mm, and backscatter leads to a dose rate increase of ∼1% per unit atomic number of the substrate. It is concluded that, for the multi-grain aliquots used in this study, the contributions to dose rate variability from grain size and substrate variations are likely to be negligible. Nevertheless there may be a practical advantage in using a high Z substrate because of the higher dose rate. Finally we test the measured to given dose (dose recovery) ratio for five heated feldspar samples and use the pIRIR290 signal for β source calibration; surprisingly this gives an apparent β dose rate 15% lower than that to quartz despite their almost identical stopping power and mass absorption characteristics. Our results are discussed in terms of their significance for reproducibility and accuracy of β dose-rate estimates.

The chronology of a sediment core from incised valley of the Yangtze River delta: Comparative OSL and AMS 14C dating

Optically stimulated luminescence (OSL) dating has gained increased use in dating deltatic deposits, however, its application can be hindered by the problem of incomplete bleaching. To address this limitation, we test the single-grain OSL method for the first time in the Yangtze River delta. A total of eight OSL and 14 AMS 14C samples were obtained from a 50.8m long sediment core from the incised valley of the delta. Quartz extracted from eight OSL samples from the core was first measured with small multi-grain aliquots using medium- (45–63μm) and coarse-grained (90–125μm or 150–180μm) fractions to test the internal consistency of their respective ages. The results showed that four of five medium-grained quartz samples appeared to be well bleached. In contrast, the coarse-grained quartz samples revealed poor bleaching, except for two samples from the delta front facies. Five coarse-grained quartz OSL samples were further analyzed using a single-grain OSL technique. Only 0.5–0.7% of the grains passed the rejection criteria. Single-grain OSL dating is appropriate for age determination of coarse-grained sediments which have been affected by incomplete bleaching. However, it consumes a lot of instrument time. Central and minimum age models (CAM and MAM) were used to calculate the burial age of the samples; MAM ages fit best in the stratigraphic sequence. Small aliquots are recommended as effective in identifying incomplete bleaching for medium- or coarse-grained quartz. A comparison of AMS 14C dates and OSL ages shows that 14C values are 1–3ka older than OSL ages. According to the OSL ages, core SD experienced rapid accumulation 10–8ka and in the last 2ka. The former is linked to rapid sea-level rise in early Holocene, while the latter is primarily due to the migration of depo-center towards the core site, rather than entirely increased sediment delivery caused by human activities.

Characterisation and OSL dating of modern fluvial sediments in the lower Vistula River: testing the zeroing assumption

Abstract In this study modern sediments of the lower Vistula River were investigated to determine the relationship between the structure and texture (grain size, rounding and frosting) of the deposits and the possibility of their zeroing. The samples of modern fluvial deposits were collected from the lower Vistula River at two sites in Toruń and Ciechocinek. Sand bars newly emerged from the river were selected for testing. The coarse quartz grains were separated for OSL measurements. The single-aliquot regenerative (SAR) technique was applied for measuring equivalent doses from multigrain aliquots. The obtained dose estimates were found to be very low, proving the reliability of the OSL zeroing assumption. The dose rates were estimated by gamma-ray spectrometry, demonstrating homogeneity of the radiation field. Analysis did not show significant relationships between the examined sediments’ capacity to zeroing and their structural and textural characteristics, or the sampling site. The obtained OSL ages of the studied sediments date back hundreds of years and are probably overestimated. The results related to fossil sediments of bars of the age of thousands of years confirm their suitability for the OSL dating method.

Infrared stimulated luminescence dating of 19<sup>th</sup> century fluvial deposits from the upper Rhine River

Knowledge of the age of fluvial deposits is an important aspect in the understanding of river dynamics, which is pre-requisite for sustainable river management and restoration back to more natural conditions and processes. Presented here is a case study on using feldspar Infrared Stimulated Luminescence (IRSL) to date low-energy fluvial sediments that formed after correction of the Upper Rhine River in the first half of the 19<sup>th</sup> century. A rigorous testing programme is carried out to characterise the IRSL properties of the samples, including thermal transfer, dose recovery and fading. All samples reveal complex distributions of equivalent dose, implying the presence of differential bleach-ing in the samples. It is shown that multi-grain aliquots overestimate the known-age by up-to 200 years, i.e. apparent IRSL ages are twice as old as the true age of the sediment. The use of single grains results in ages that are in excellent agreement with the expected age, therefore the age overestimation in multi-grain aliquot measurements is likely explained by signal averaging effects. While the application of single grains appears mandatory for dating young low-energy fluvial deposits, the small absolute offset associated with the multi-grain approach might be acceptable when dating sediments of such type that are older than a few 1000 years.

Single-grain and multi-grain OSL dating of river terrace sediments in the Tabernas Basin, SE Spain

River terraces represent important records of landscape response to e.g. base-level change and tectonic movement. Both these driving forces are important in the southern Iberian Peninsula. In this study, Optically Stimulated Luminescence (OSL) dating was used to date two principal river terraces in the Tabernas Basin, SE Spain. A total of 23 samples was collected from the fluvial terraces for dating using quartz OSL. Sixteen of the samples could not be dated because of low saturation levels (e.g. typical 2xD0 < 50 Gy). The remaining seven samples (5 fossil and 2 modern analogues) were investigated using both multi-grain and single-grain analysis. Single grain results show that: (i) measurements from multi-grain aliquots overestimate ages by up to ∼ 4 ka for modern analogues and young samples (<5 ka), presumably because (ii) the presence of many saturated grains has biased the multi-grain results to older ages. Despite the unfavourable luminescence characteristics we are able to present the first numerical ages for two terrace aggradation stages in the Tabernas Basin, one at ∼16 ka and the other within the last 2 ka.

A new irradiated quartz for beta source calibration

For luminescence dating to be an accurate absolute dating technique it is very important that we are able to deliver absolutely known radiation doses in the laboratory. This is normally done using a radiation source (alpha, beta, X-ray) calibrated against an absolutely known reference source. Many laboratories have used the various different batches of Risø calibration quartz for the calibration of beta and X-ray sources, but these have been largely undescribed. Here we describe in detail the preparation and luminescence characteristics of a new quartz standard, based on a North Sea beach sand collected from south-western Denmark (Rømø). Two grain sizes (4–11μm and 180–250μm) have been examined in detail. These were pre-treated (annealed, dosed and annealed again) to sensitise and stabilise the luminescence signals before being given an absolutely known gamma dose from a point 137Cs source in scatter-free geometry. The luminescence characteristics are described; the very intense blue-light stimulated signal is dominated by the fast OSL component and the IR-stimulated signal is negligible. The material is shown to be suitable for measurement using SAR, and the dose recovery ratio is indistinguishable from unity with a standard deviation of <2% for multi-grain aliquots. The material is also shown to be suitable for single-grain calibration, with >80% of the grains giving a useful signal. Although there is an unexplained dispersion in our calibration data of ∼3% (which we cannot attribute to instrument variability), we nevertheless conclude that this material is very suitable for transferring absolute known doses from a standardised gamma source to in-built irradiation sources.

Insights into TT-OSL signal stability from single-grain analyses of known-age deposits at Atapuerca, Spain

Single-grain thermally transferred optically stimulated luminescence (TT-OSL) dating has recently been applied to a number of the Atapuerca palaeoanthropological sites. As yet, however, there have not been any direct assessments of TT-OSL signal stabilities for specific grains used for dating. In this study, we undertake a series of TT-OSL suitability assessments on known-age samples from Gran Dolina and Sima del Elefante. Our results suggest that the Atapuerca samples contain populations or sub-populations of grains with suitably stable TT-OSL signals for dating over late-Early and Middle Pleistocene timescales. Equivalent dose (De) distribution analysis in combination with pulse-annealing assessments provides a useful means of identifying inter-grain and inter-sample differences in TT-OSL signal stabilities. We also show that obtaining De values using different preheat conditions may help to identify potentially problematic TT-OSL behaviours. Analyses of multi-grain aliquot TL curves for these samples reveal that 'bulk' TL signal loss experiments may provide limited insights into TT-OSL source trap lifetimes due to averaging effects, the dominance of grain populations that do not produce TT-OSL, and interference from slowly bleaching OSL components. Our results improve the robustness and precision of existing TT-OSL chronologies for units TD6-3 and TE16-TE17 at Gran Dolina and Sima del Elefante, and support the broader suitability of the single-grain TT-OSL approach at the Atapuerca sites.

Re-evaluating luminescence burial doses and bleaching of fluvial deposits using Bayesian computational statistics

Abstract. The optically stimulated luminescence (OSL) signal from fluvial sediment often contains a remnant from the previous deposition cycle, leading to a partially bleached equivalent-dose distribution. Although identification of the burial dose is of primary concern, the degree of bleaching could potentially provide insights into sediment transport processes. However, comparison of bleaching between samples is complicated by sample-to-sample variation in aliquot size and luminescence sensitivity. Here we begin development of an age model to account for these effects. With measurement data from multi-grain aliquots, we use Bayesian computational statistics to estimate the burial dose and bleaching parameters of the single-grain dose distribution. We apply the model to 46 samples taken from fluvial sediment of Rhine branches in the Netherlands, and compare the results with environmental predictor variables (depositional environment, texture, sample depth, depth relative to mean water level, dose rate). Although obvious correlations with predictor variables are absent, there is some suggestion that the best-bleached samples are found close to the modern mean water level, and that the extent of bleaching has changed over the recent past. We hypothesise that sediment deposited near the transition of channel to overbank deposits receives the most sunlight exposure, due to local reworking after deposition. However, nearly all samples are inferred to have at least some well-bleached grains, suggesting that bleaching also occurs during fluvial transport.

OSL dating of Brazilian fluvial carbonates (tufas) using detrital quartz grains

Robust chronologies for Quaternary fluvial carbonate sediments (tufas) from Brazil are still challenging. We obtain Optically Stimulated Luminescence (OSL) ages of Brazilian tufa deposits using the Single Aliquot Regeneration (SAR) dose protocol applied to single grains and multigrain aliquots of quartz extracted from tufa matrix. Additionally, radiocarbon (14C) ages were obtained for the carbonate matrix and organic matter from tufas. The methodology was applied in samples from tufa deposits in southwestern and southeastern regions with Quaternary carbonate deposition. Equivalent doses distributions have overdispersion between 19% and 81%, indicating that the studied samples comprise well bleached as well as poorly bleached sediments. The quartz grains extracted from the studied tufas show an OSL signal dominated by the fast component and reliable dose response curves, with saturation doses (2D0) ranging from 35 to 210 Gy. Most of the samples has low radiation dose rates (0.40 ± 0.02 to 0.78 ± 0.05 Gy/ka), with variation in the water content due to compaction and matrix recrystallization, being the major factors that could induce dose rate changes through time. Hard tufa samples with intense carbonate cementation show quartz OSL ages from 51.9 ± 4.8 to 150.3 ± 35.9 years. These ages are much older than the radiocarbon ages of the carbonate matrix and organic matter. The OSL ages of soft tufa samples related with the active fluvial system ranged from 0.8 ± 0.1 to 5.5 ± 0.4 years and are in the same interval of radiocarbon ages presented in the previous works. We interpret that carbonate recrystallization or uptake of organic matter after tufa formation are responsible for the younger radiocarbon ages compared to quartz OSL ages in the older tufa samples. For these samples, age differences vary up to tens of thousands years between deposition of quartz grains and the last episode of carbonate recrystallization or organic matter uptake. Consequently we believe that the obtained OSL ages are the best estimates for the starting deposition time of the studied tufa samples.