A.G. Wintle

Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol

Single aliquot protocols are now widely used as a means of measuring the equivalent dose (De) in quartz and feldspar optical stimulated luminescence (OSL) dating of both heated and sedimentary materials. The most recent of these is the single-aliquot regenerative-dose (SAR) protocol, first suggested by Murray and Roberts (Radiation Measurements 29, 503–515, 1998). In this approach, each natural or regenerated dose OSL measurement is corrected for changes in sensitivity using the OSL response to a subsequent test dose (10–20% of De). If the sensitivity correction is adequate, then the corrected OSL response should be independent of prior dose and thermal/optical treatment, i.e. there should be no change in the sensitivity-corrected dose–response curve on remeasurement. Here we examine the interpretation of the sensitivity corrected growth curve as a function of dose, and the effect of changing measurement conditions (e.g. preheat temperature, size of test dose, stimulation temperature) on the estimation of De. The dependence of the dose response on prior treatment is tested explicitly, and the significance of thermal transfer discussed. It is concluded that a robust SAR protocol is now available for quartz, and that it is applicable to a wide range of heated and unheated materials.

The single aliquot regenerative dose protocol: potential for improvements in reliability

Abstract This paper examines the effects of optically stimulated luminescence (OSL) components, other than that usually termed the fast component, on dose determination by the single-aliquot regenerative-dose (SAR) protocol. Results are presented for “dose recovery tests”, in which a known laboratory dose, delivered after optical bleaching at room temperature, is measured using the SAR protocol. Data obtained using either the initial OSL signal or the fast component, derived by curve fitting, are compared. Dose recovery tests are also carried out when an additional step is added to the SAR protocol, aimed at reducing recuperation, i.e. the residual signal observed in a SAR cycle when no regenerative dose is applied. The results for quartz from various sources indicate much improved dose recovery when only a well-separated fast component is analysed.

Luminescence dating : Laboratory procedures and protocols

Abstract In the last 30 years, from 1967 to 1997, the use of luminescence signals from naturally occurring minerals has gone though a major metamorphosis, from thermoluminescence (TL) dating of pottery to optically stimulated luminescence (OSL) dating of sediments. Laboratory procedures for dating sediments have been adapted from those for pottery and new procedures have been developed as the need arises. The majority of sediment dating applications are carried out on quartz and potassium-rich feldspars and the general characteristics of the TL and OSL signals from these minerals are reviewed. For sediments some new problems were encountered, with some grains perhaps not being completely bleached at deposition. For OSL signals there is no simple procedure for the selection of a thermally stable signal, as there had been in the case of pottery. Many different laboratory protocols have been developed as our understanding of the fundamental behaviour of luminescence signals from quartz and feldspar has improved. These protocols are explained and discussed, giving the advantages and disadvantages of each procedure as applied to different types of sediment. This review is presented as a guide to the selection of the most appropriate procedure for a particular dating application.

Thermoluminescence dating of sediments

Abstract The potential for using the thermoluminescence behaviour of sediments for dating them was first recognized by Soviet scientists G.V. Morozov and V.N. Shelkoplyas, and for over a decade their TL dates, obtained from a variety of sediments, have appeared in the Soviet literature. Since 1977 TL sediment dates have been published by six additional groups using a variety of methods. In this review we describe the principles of TL dating, the various methods used, and contrast TL dating of sediments with the now well-accepted TL dating of pottery. We conclude that while TL dating has the potential to solve many sedimentary problems, more fundamental research needs to be carried out before such dates should be accepted. A set of criteria for acceptable dates is proposed.

The single-aliquot regenerative-dose (SAR) protocol applied to coarse-grain feldspar

The application of the single-aliquot regenerative-dose (SAR) protocol to equivalent dose determination in coarse-grain feldspar is investigated. An outline of the protocol is presented, and basic tests of its performance are discussed. It is shown that the method is robust and yields results similar to those obtained with a single-aliquot additive-dose (SAAD) procedure. The use of the SAR procedure for fading tests is also discussed. We conclude that the SAR protocol for coarse-grain feldspar yields several advantages over previously used multiple-aliquot and single-aliquot methods. The method is fast and precise, the equivalent dose is obtained by interpolation, and the method corrects for sensitivity changes. The luminescence age obtained by SAR and additive-dose dating of the coarse-grain feldspar separates used in this study do, however, underestimate the independently estimated geological age of the samples.

Last Glacial loess in the conterminous USA

The conterminous United States contains an extensive and generally well-studied record of Last Glacial loess. The loess occurs in diverse physiographic provinces, and under a wide range of climatic and ecological conditions. Both glacial and non-glacial loess sources are present, and many properties of the loess vary systematically with distance from loess sources. United States’ midcontinent Last Glacial loess is probably the thickest in the world, and our calculated mass accumulation rates (MARs) are as high as 17,500 g/m 2 /yr at the Bignell Hill locality in Nebraska, and many near-source localities have MARs greater than 1500 g/m 2 /yr. These MARs are high relative to rates calculated in other loess provinces around the world. Recent models of Last Glacial dust sources fail to predict the extent and magnitude of dust flux from the mid-continent of the United States. A better understanding of linkages between climate, ice sheet behaviour, routing of glacial meltwater, land surface processes beyond the ice margin, and vegetation is needed to improve the predictive capabilities of models simulating dust flux from this region. r 2003 Elsevier Ltd. All rights reserved.

Unprecedented last-glacial mass accumulation rates determined by luminescence dating of loess from western Nebraska

Abstract A high-resolution chronology for Peoria (last glacial period) Loess from three sites in Nebraska, midcontinental North America, is determined by applying optically stimulated luminescence (OSL) dating to 35–50 μm quartz. At Bignell Hill, Nebraska, an OSL age of 25,000 yr near the contact of Peoria Loess with the underlying Gilman Canyon Formation shows that dust accumulation occurred early during the last glacial maximum (LGM), whereas at Devil’s Den and Eustis, Nebraska, basal OSL ages are significantly younger (18,000 and 21,000 yr, respectively). At all three localities, dust accumulation ended at some time after 14,000 yr ago. Mass accumulation rates (MARs) for western Nebraska, calculated using the OSL ages, are extremely high from 18,000 to 14,000 yr—much higher than those calculated for any other pre-Holocene location worldwide. These unprecedented MARs coincide with the timing of a mismatch between paleoenvironmental evidence from central North America, and the paleoclimate simulations from atmospheric global circulation models (AGCMs). We infer that the high atmospheric dust loading implied by these MARs may have played an important role, through radiative forcing, in maintaining a colder-than-present climate over central North America for several thousand years after summer insolation exceeded present-day values.

River response to high-frequency climate oscillations in southern Europe over the past 200 k.y.

A 200 k.y. chronology of river response to climate-related environmental change has been established for northeast Spain using newly developed luminescence dating techniques. This constitutes the best-documented record of late Quaternary river behavior currently available for the North Atlantic region and enables fluvial stratigraphies to be compared with high-resolution ice core and marine oxygen isotope climate series. Pleistocene and Holocene river aggradational episodes coincide with stadial or neoglacial events, while phases of river incision occur during interstadial or interglacial periods. Alluviation and erosion cycles would appear to track variations in sediment supply controlled by vegetation cover and winter storm frequency.

The relationship between quartz thermoluminescence, photo-transferred thermoluminescence, and optically stimulated luminescence

The effects of green light stimulation on the thermoluminescence (TL) of a sedimentary quartz from the Kimberley region of Western Australia are reported. The loss of TL at 310°C and the concomitant growth and decay of the photo-transferred TL (PTTL) at both 110 and 160°C are presented for optical stimulation of up to 200 s in a Riso TL/OSL reader. The optical decay lifetime for the 110°C peak was about 130 s, and we deduce that this decay probably arises from optical stimulation of electrons rather than loss of recombination centres. A linear correlation was found between the loss of TL from the rapidly bleaching peak at 310°C (following a natural (N) + 43 Gy dose) and the integrated optically stimulated luminescence (OSL), for stimulation at 125°C and for stimulation times between 0.1 and 100 s. For another N + 43 Gy sample, the OSL signal decreased by about 20% with increasing preheat temperature from 220 to 280°C, after correction for thermal activation of luminescence centres (monitored by an additional experiment). The TL signal associated with this OSL (i.e. the TL lost as a result of optical stimulation) decreased by about 30% with preheat temperature. For repeated 0.1 s stimulation at 25°C, there was a linear relationship between the OSL, and the PTTL at 110°C obtained after each stimulation. The effects of different doses and stimulation times on this relationship are reported, and the implications for dating protocols using the OSL signal are discussed.

Equivalent dose measurement using a single aliquot of quartz

Abstract The principles behind an additive-dose single-aliquot protocol and the need for such a protocol are outlined. It is shown for two Australian sedimentary quartz samples that the decay of OSL during a repeated measurement cycle, consisting of a 10 s preheat at a given temperature followed by a 0.1 s exposure to green light at 110°C, can be well represented by exponential decay. The decay constant of about 8% per cycle is insensitive to preheat temperatures below about 250°C, but increases to about 35% per cycle at 280°C. This increase is attributed to increasing thermal erosion of the main OSL trap. The decay constant is also shown to be insensitive to dose. An additive-dose protocol is presented which requires only one aliquot for the estimation of the equivalent dose ( D e ). This protocol has been applied to quartz from 11 Australian sites. To illustrate the value of the single-aliquot approach, the apparent values of D e for 14 samples, containing doses of between 0.01 and 100 Gy, have been measured in triplicate at preheat temperatures of between 150 and 300°C, using a single aliquot for each D e measurement. It is shown that low temperature preheats (≈200°C for 10 s) are appropriate for the younger samples containing the smaller doses, but a much higher temperature preheat (≥280C for 10 s) is required for the older samples containing the larger doses. Excellent agreement is found between these single-aliquot estimates of D e and those from additive-dose multiple-aliquot protocols, over the entire dose range.