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As part of the development of new laboratory lighting, we present a methodological approach applicable to the characterization of any light source intended for illumination in optical dating laboratories. We derive optical absorption cross-sections for quartz and feldspar from published data and compare these with the human eye response. The optimum peak wavelength giving the best clarity for non-dark adapted vision for the least trapped charge loss lies within the wavelength range 590–630 nm; we argue that it is unnecessary to consider dark-adapted vision. The predicted relative decay rates of quartz optically stimulated luminescence (OSL) and feldspar infrared stimulated luminescence (IRSL) signals by an incandescent light bulb and a compact fluorescent lamp (CFL) through an ILFORD 902 filter are first derived. These predicted decay rates are then compared with those of three relevant light-emitting diodes (LEDs); this comparison demonstrates the significant advantage of the LED sources over the filtered light sources, because essentially all of the reduction of both OSL and IRSL signals by the LEDs occurs at wavelengths to which the human eye is most sensitive. We conclude that exposure of quartz and feldspar extracts from various samples to the light from an LED with emission peak at 594 nm results in a ≤1% OSL or IRSL signal loss for a 48-h exposure at a power density of ∼0.2 μW.cm −2 . [ABSTRACT FROM AUTHOR]

We tested the suitability of the fine-grained quartz (4-11 lm) Optical Stimulated Luminescence (OSL) and thermally-transferred OSL (TT-OSL), and the fine-grained polymineral (4-11 lm) post-infrared IRSL (post-IR IRSL or pIRIR) signals for dating samples from aeolian-lacustrine deposits from the Xiaochangliang archaeological profile in the Nihewan Basin, China; these deposits include material from the Jaramillo subchron (c. 1.0 Ma). In the upper aeolian section, the OSL and pIRIR290 ages are consistent with each other, and show that the upper 8.8 m was deposited between c. 0.3 and c. 140 ka. The luminescence ages indicate a major discontinuity in deposition between the aeolian and the older lacustrine deposits. Below this hiatus at 9.4 m (i.e. in the lacustrine sediments) all three signals are found to be in field saturation (no further systematic increase in burial dose with depth) despite the TT-OSL signal (apparent mean burial dose ∼880 Gy) being well below saturation on the laboratory growth curve. This is in contrast to the pIRIR290 signal, which saturates in the field at a level consistent with laboratory saturation. This results in a practical upper limit to the measured burial dose of ∼900 Gy (2D0). Thus for the TT-OSL and pIRIR290 signals, the upper limits for dating lacustrine deposits are <260 ka and c. 240 ka, respectively. These results have major implications for the appropriate future application of these signals. The ages of our lacustrine samples cannot be regarded as necessarily accurate ones; nevertheless, these ages provide the first long series absolute chronology for study of local palaeolithic and geomorphic evolution history aside from the magnetostratigraphical results available before this research. [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]

Luminescence dating is one of the most promising technique available for studying bioturbation on pedological timescales. In this study, we use multi-grain and single-grain quartz OSL to quantify termite bioturbation processes ( Macrotermes natalensis ) in a savannah ecosystem in Ghana. Termites transport soil from depth to the surface to construct termitaria. Over time, erosion levels these mounds and returns the sediment to the soil surface. These two processes of construction and erosion together represent an upward “conveyor belt” sediment transport process. We find that the sediment is effectively bleached during the erosion process allowing us to quantify retrospectively, for the first time, the surface deposition rate, the inverse of the upwards transport rate. At this site, this is ~0.28 mm year −1 and began about 4.000 years ago. Downward mixing through subsurface galleries may replace 10–20% of the volume ka −1 below the unit formed by reburied termite deposits. [ABSTRACT FROM AUTHOR]

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

The mountain building processes in Taiwan are currently among the most rapid in the world. However, the spatial and temporal dynamics of this orogen are still poorly resolved within the <0.5 Ma timescale, reflecting methodological gaps in addressing young and rapid bedrock cooling by erosion. Optically Stimulated Luminescence (OSL) from quartz is an emerging thermochronometer that could potentially provide valuable low-temperature markers on a ∼0.1 Ma timescale. Here we study four meta-sandstone samples from the Hsuehshan Range in central Taiwan. We characterize the OSL from these samples in terms of feldspar contamination, saturation dose, signal components, and thermal stability. We conclude that the OSL is dominated by the fast component, and is therefore amenable to dose measurement using the SAR protocol. Based on the luminescence characteristics and ambient dose rate, we suggest that these samples may be valuable for evaluating recent cooling rates exceeding 360 ± 100 °C/Ma, in places where the alternative thermal scenarios (instantaneous cooling or prolonged isothermal storage) may be ruled out by external geological constraints. [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]

Abstract: The classical equation of closure temperature ( ) in thermochronometry (Dodson, 1973), assumed (i) no storage limitation for the accumulating radiogenic product, (ii) a negligible product concentration at the initial temperature of cooling , and (iii) a negligible product loss at the final (present-day) temperature . A subsequent extension (Ganguly and Tirone, 1999) provided a simple correction for systems cooling from an arbitrary , at which presence of an initial concentration profile may affect final concentrations. Here, we use a combination of analytical and numerical solutions to derive a general expression for the effective closure temperature in (i) systems which cool between arbitrary initial and final temperatures, potentially still suffering from thermal product loss at (termed ‘leaky’), and (ii) systems which may contain a physical limit on the maximum amount of product that can be stored (termed ‘saturating’). While all conservative results can be easily reproduced, an extended use of our formulation provides meaningful effective closure temperatures even when the standard calculation schemes fail. For a first-order loss radiometric system governed by , where E [J mol−1] and s [s−1] are the Arrhenius parameters and R is the gas constant, we find that the effective closure temperature is given by: where and [s−1] are shorthand for and , respectively, λ [s−1] the production rate, τ [s] a time constant, and the upper incomplete gamma function. Under conventional conditions, our solution reduces to Dodsonʼs formula. Although the solution strictly applies only to systems where increases linearly with time, it is nevertheless a useful approximation for a broad range of cooling functions in systems where closure occurs close to the systemʼs initial/final thermal boundary conditions. We clarify the use and the meaning of by drawing a comparison between (i) a hypothetical application of apatite U–Pb dating ( ) on Venus (mean surface temperature of 450 °C, leaky behaviour), and (ii) the recently introduced thermochronometric application of optically stimulated luminescence (OSL) dating on Earth (both leaky and saturating behaviour). [Copyright &y& Elsevier]

Abstract: We propose a new method for in situ surface exposure dating of non-terrestrial geomorphological features using optically stimulated luminescence (OSL); our approach is based on the progressive emptying of trapped charge with exposure to light at depth into a mineral surface. A complete model of the resetting of OSL with depth and time is presented for the first time; this model includes the competing effects of both optical resetting and irradiation. We consider two extreme conditions at the time the resetting is initiated: (a) a negligibly small trapped charge population and (b) a saturated trapped charge population. The potential dating applications for (a) include dust accumulation, volcanic rocks and impact-related sediments, and for (b) fault scarps, rock-falls, landslides and ice-scoured bedrock. Using assumptions based on terrestrial observations we expect that this approach will be applicable over the last 100ka. The method is ideally suited to in situ measurement using existing technology developed for space applications, and so offers for the first time the realistic possibility of direct determination of exposure ages of young non-terrestrial surfaces. [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]

Abstract: The thermally-transferred optically stimulated luminescence (TT-OSL) responses of chemically-purified fine-grained quartz from a lava-baked aeolian sediment from Datong (China) are presented. Our main focus is to examine the suitability of the test dose TT-OSL and OSL response to monitor sensitivity changes during SAR measurements. It is found that, in contrast to the test dose OSL, the TT-OSL response to a test dose can successfully monitor sensitivity changes, and a high-temperature blue-light bleach (600s at 260°C) in the middle of each SAR cycle is necessary to minimize interference with the test dose TT-OSL signal. The revised SAR TT-OSL protocol was tested by dose recovery tests on two very young (quartz OSL D e <10Gy) sediment samples (one heated by human activity and one of aeolian origin). Dose recovery tests using different test doses indicate that it is more appropriate to use larger test doses (>20% of the given dose) in this revised SAR TT-OSL protocol. [Copyright &y& Elsevier]

There are many examples of buried rock surfaces whose age is of interest to geologists and archaeologists. Luminescence dating is a potential method which can be applied to dating such surfaces; as part of a research project which aims to develop such an approach, the degree of resetting of OSL signals in grains and slices from five different cobbles/boulders collected from a modern beach is investigated. All the rock surfaces are presumed to have been exposed to daylight for a prolonged period of time (weeks to years). Feldspar was identified as the preferred dosimeter because quartz extracts were insensitive. Dose recovery tests using solar simulator and IR diodes on both K-feldspar grains and solid slices taken from the inner parts of the rocks are discussed. Preheat plateau results using surface grains and slices show that significant thermal transfer in naturally bleached samples can be avoided by keeping preheat temperatures low. Equivalent doses from surface K-feldspar grains were highly scattered and much larger than expected (0.02 Gy to >100 Gy), while solid surface slices gave more reproducible small doses (mean = 0.17±0.02 Gy, n = 32). Neither crushing nor partial bleaching were found to be responsible for the large scattered doses from grains, nor did the inevitable contribution from Na-feldspar to the signal from solid slices explain the improved reproducibility in the slices. By modelling the increase of luminescence signal with distance into the rock surface, attenuation factors were derived for two samples. These indicate that, for instance, bleaching at a depth of 2 mm into these samples occurs at about ∼28% of the rate at the surface. We conclude that it should be possible to derive meaningful burial doses of >1 Gy from such cobbles; younger samples would probably require a correction for incomplete bleaching. [ABSTRACT FROM AUTHOR]

Abstract: This article demonstrates that violet (405nm) stimulated luminescence (VSL) signal from quartz contains contribution from deep traps that are otherwise not accessible with blue light (470nm). Additionally, it also contains the typical fast and slow components observed with the blue light stimulation. Although, the fast OSL component is measured with similar efficiency by blue and violet lights, the slower OSL components (especially S 3) are measured relatively more efficiently with the latter. New insight into the origins of quartz luminescence is presented through a comparison of violet and blue lights stimulation, and thermal stimulations. Finally, it is shown that the deep traps probed through violet light stimulation have potential for increasing the dose measurement/dating range using quartz. The post-blue VSL signal allows easy, precise measurement of dose up to at least 1kGy in our samples, while the initial BSL signal shows saturation at ∼100Gy. The violet stimulation provides a means for optically probing the traps that give rise to signals with extended growth (dose-response) characteristics such as high temperature isothermal TL (ITL), the slow component OSL, and perhaps also the thermally transferred-OSL (TT-OSL). [Copyright &y& Elsevier]

Abstract In response to the comment of Madsen and Liu (2018), we explain why our quartz optically stimulated luminescence (OSL) ages showing the early MIS 3 lake highstand are robust. We also demonstrate that the difference in the age estimates between ours (Long et al., 2018) and other studies should be attributed to contrasting equivalent dose (D e) values, rather than water contents (hence the dose rate) which was claimed by Madsen and Liu (2018). [ABSTRACT FROM AUTHOR]

We investigate on the origins of the infra-red stimulated luminescence (IRSL) signals in 3 potassium feldspars based on IR-red spectroscopy (∼700-1050 nm) using a fiber-coupled tunable Ti:Sapphire laser, in combination with different thermal and optical (pre)treatments of the samples. We also measure dose-response curves with different wavelengths and at different stimulation temperatures so as to be able to distinguish between traps based on their electron trapping cross-sections. Our data suggest that the dosimetric signals, IRSL, and the post IR-IRSL in K-feldspars arise from a single electron trapping centre. [ABSTRACT FROM AUTHOR]

Optically stimulated luminescence (OSL) ages can determine a wide range of geological events or processes, such as the timing of sediment deposition, the exposure duration of a rock surface, or the cooling rate of bedrock. The accuracy of OSL dating critically depends on our capability to describe the growth and decay of laboratory-regenerated luminescence signals. Here we review a selection of common models describing the response of infrared stimulated luminescence (IRSL) of feldspar to constant radiation and temperature as administered in the laboratory. We use this opportunity to introduce a general-order kinetic model that successfully captures the behaviour of different materials and experimental conditions with a minimum of model parameters, and thus appears suitable for future application and validation in natural environments. Finally, we evaluate all the presented models by their ability to accurately describe a recently published feldspar multi-elevated temperature post-IR IRSL (MET-pIRIR) dataset, and highlight each model's strengths and shortfalls. [ABSTRACT FROM AUTHOR]

Abstract: Feldspars are natural radiation dosimeters used in geological dating. The processes that underlie the measured natural- and laboratory induced infrared stimulated luminescence (IRSL) signals are, however, not fully understood. In this study we aim to determine the optical depth of the electron trap that is used in luminescence dating. A series of optically stimulated luminescence (OSL) experiments were performed on two feldspar minerals (single-crystal museum specimens of Na- and K-feldspar) at a low temperature (10 K). The low temperature suppresses thermal processes that may be active during excitation and facilitates the monitoring of mainly optical transitions. Additional experiments at room temperature were performed for comparison. The continuous wave OSL and the post-stimulation phosphorescence were recorded in the ultra violet (3.2–4.8 eV) for stimulation energies ranging between 1.7 and 2.8 eV. From the dependence of the OSL intensity on the excitation energy is deduced that band-tail states play a significant role during charge transport, thus complicating the precise determination of the optical trap depth. Both minerals display a small resonant transition when stimulated at ∼2.05 eV. From the OSL data, the trap depth is tentatively estimated to be ∼2.1 and ∼2.5 eV for Na- and K-feldspar, respectively. The fast phosphorescence following IRSL in both crystals is likely the result of tunnelling from band-tail states in the vicinity of the excited state of the IR-trap. The post-OSL phosphorescence decay is an order of magnitude slower, owing to the contribution from tunnelling of charge from a range of occupied band-tail states. The lack of dependence of the phosphorescence decay rate on the excitation energy (in the range of 1.7–2.8 eV), unambiguously indicates that a single mechanism causes the post-OSL phosphorescence signal. [Copyright &y& Elsevier]

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]

Abstract: The rates of post-depositional mixing by bioturbation have been investigated using Optically Stimulated Luminescence (OSL) dating in two sediment cores (BAL2 and BAL5), retrieved from a sandy tidal flat in the Danish part of the Wadden Sea. A high-resolution chronology, consisting of thirty-six OSL ages with ages ranging between 2 ± 4 and 410 ± 20 years, is presented. Slices of sediment (1–2 cm thick) have been dated at least every 5 cm, and from these data mixing depths of 20 cm and 22 cm (BAL5 and BAL2, respectively) are readily identified. Below the mixing zone there is a significant decrease in the apparent sedimentation rate at BAL5 (from 6.4 to 0.3 mm a−1) and an abrupt increase in OSL age at the other site, BAL2 (from ∼19 years–∼290 years). Apparent sedimentation rates in the upper ∼20 cm are more rapid than at greater depths, due to the age underestimation arising from the mixing processes. The significant change in sedimentation rates at BAL5 may indicate an offset in OSL age of up to 620 years. This paper uses a simple conceptual model for vertical mixing in which all the sediment excreted at the surface by lugworms is assumed to be completely reset before being re-incorporated in the sediment column. Using this approach, we conclude that the upward transport rate due to lugworms at BAL5 is 5–6 mm a−1, and that the net sedimentation rate is ∼0.3 mm a−1, whereas at BAL2 the upward transport rate is currently ∼16 mm a−1 and that the site is probably eroding. The situation below the mixing depth (22 cm) is however unclear because of a marked 300 year hiatus. This is the first time that OSL has been used to determine bioturbation rates and our data clearly show the potential of such measurements to provide retrospective field estimates of this important environmental parameter. [Copyright &y& Elsevier]

Abstract Reconstructing dust Mass Accumulation Rate (MAR) from loess deposits is critical to understanding past atmospheric mineral dust activity and requires accurate independent age models from loess deposits across Europe and Asia. Previous correlations of loess in Europe and China have tended to focus on multi-millennial timescales, with no detailed examination of dust MAR at the two ends of the Eurasian loess belt on shorter, sub-orbital scales. Here we present a detailed quartz optically stimulated luminescence (OSL) chronology from the Serbian Titel Loess Plateau (Veliki Surduk loess core) and the Chinese Loess Plateau (Lingtai section). The luminescence ages pass internal tests and show consistent increases in age with depth, with no obvious major hiatuses. However, as reported before, it seems the quartz OSL technique is only capable of accurate age determination up to accrued doses of ca. 150 Gy (ca. 30–40 ka) due to approaching field saturation of the quartz OSL signal. Two age-depth models were used to reconstruct dust MARs, where one utilises OSL data solely and the other additionally makes assumptions about sedimentation rates. Although short-term fluctuations in MAR are model dependent, general MAR patterns between the two sites are very similar, with peak MAR occurring rather late in the last glacial (ca. 13–25 ka). This suggests that at least broad scale trends in dust activity within the last glacial period may be similar at a continental scale. [ABSTRACT FROM AUTHOR]

Glaciolacustrine sediments in the Clark Fork River valley at Garden Gulch, near Drummond, Montana, USA record highstand positions of the ice-dammed glacial Lake Missoula and repeated subaerial exposure. During these highstands the lake was at greater than 65% of its recognized maximum capacity. The initial lake transgression deposited a basal sand unit. Subsequent cycles of lake-level fluctuations are recorded by sequences of laminated and cross laminated silt, sand, and clay deformed by periglacial processes during intervening periods of lower lake levels. Optically stimulated luminescence (OSL) dating of quartz sand grains, using single-aliquot regenerative-dose procedures, was carried out on 17 samples. Comparison of infrared stimulated luminescence (IRSL) from K-rich feldspar to OSL from quartz for all the samples suggests that they were well bleached prior to deposition and burial. Ages for the basal sand and overlying glaciolacustrine exposure surfaces are indistinguishable within one standard deviation, and give a weighted mean age of 20.9 ± 1.3 ka (n = 11). Based on sedimentological and stratigraphic analysis we infer that the initial transgression, and at least six cycles of lake-level fluctuation, occurred over time scales of decades to ∼2 ka. Bioturbated sandy slopewash dated at 10.6 ± 0.9 ka and 11.9 ± 1.2 ka unconformably overlies the upper glaciolacustrine deposits. The uppermost sediments, above the glaciolacustrine section, are younger than the Glacier Peak tephra (13.7–13.4 cal ka B.P.), which was deposited across parts of the drained lake basin, but has not been found at Garden Gulch. Our study indicates that glacial Lake Missoula reached >65 percent of maximum capacity by about 20.9 ± 1.3 ka and either partially or completely drained twelve times from this position. Rapid lowering from the lake's highstand position due to ice-dam failure likely led to scour in the downstream portions of the glacial Lake Missoula basin and megafloods in the Channeled Scabland. [ABSTRACT FROM AUTHOR]

Interest in the optically stimulated luminescence (OSL) dating of rock surfaces has increased significantly over the last few years, as the potential of the method has been explored. It has been realized that luminescence-depth profiles show qualitative evidence for multiple daylight exposure and burial events. To quantify both burial and exposure events a new mathematical model is developed by expanding the existing models of evolution of luminescence–depth profiles, to include repeated sequential events of burial and exposure to daylight. This new model is applied to an infrared stimulated luminescence-depth profile from a feldspar-rich granite cobble from an archaeological site near Aarhus, Denmark. This profile shows qualitative evidence for multiple daylight exposure and burial events; these are quantified using the model developed here. By determining the burial ages from the surface layer of the cobble and by fitting the new model to the luminescence profile, it is concluded that the cobble was well bleached before burial. This indicates that the OSL burial age is likely to be reliable. In addition, a recent known exposure event provides an approximate calibration for older daylight exposure events. This study confirms the suggestion that rock surfaces contain a record of exposure and burial history, and that these events can be quantified. The burial age of rock surfaces can thus be dated with confidence, based on a knowledge of their pre-burial light exposure; it may also be possible to determine the length of a fossil exposure, using a known natural light exposure as calibration. [ABSTRACT FROM AUTHOR]