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This led to the development of OSL dating which offered a number of advantages over TL methods when dating unheated sediments. With further equipment and methodological refinements, there was a burgeoning of OSL dating studies of Quaternary sediments throughout the s that saw luminescence dating emerge as a robust dating technique.
Over the last two decades, the technique has developed further [ 37 — 39 ], and today, it is the method of choice for dating detrital sediments of Late Pleistocene and Holocene age as well as previously heated archaeological artifacts. Table 1 presents some recent review studies that have examined various aspects of luminescence dating and its applications.
Many minerals such as quartz, feldspar, calcite and zircon are dielectric materials and, when subjected to ionizing radiation, they are able to store energy in their crystal lattices. In natural geological and archeological settings, the ionizing radiation emanates naturally from the immediate surroundings of the minerals.
Cosmic radiation may also contribute a small component. If the minerals used in dating are stimulated, they release the energy by luminescing and, within certain constraints, the energy released is proportional to the stored energy. In luminescence dating, the energy given out by the minerals or dosimeters following stimulation is measured using appropriate instrumentation.
This energy is referred to as the paleodose [ 50 ]. In order to determine an age, the rate at which the energy was accumulated by the dosimeter, or the dose date, is also ascertained. The quotient of the paleodose and the dose rate, as indicated in Eq. If the mineral grains were emptied of all previously accumulated energy prior to the latest energy storage episode, the age obtained will denote time that has elapsed since the start of that episode.
Hence, both in geology and archaeology, the luminescence age simply connotes time that has passed since the occurrence of a specific energy zeroing event. In geology, this might be a geomorphic event that exposed sediment to sunlight. Zeroing by sunlight is also sometimes referred to as optical bleaching [ 3 ]. In pottery, zeroing would normally occur during a firing event associated with the manufacture. Mechanisms by which minerals store energy in their crystal lattices as a result of ionizing radiation are complex [ 50 — 52 ].
However, it is thought that ionizing radiation drives mineral crystals into a metastable state where electrons are displaced from their parent nuclei. The positions from which the electrons have been evicted act as holes.
The electrons and holes then diffuse within the mineral crystals and become trapped separately at lattice defects. Examples of common defects include a negative ion missing from its lattice position, a negative ion positioned in an interstitial site or the presence of impurity atoms in the lattice through substitution [ 52 ].
Other more complex trap types exist [ 52 ]. Figure 1 depicts an energy level diagram that is used to visualize the trapping mechanisms involved in luminescence in crystalline materials. Stable traps are those that can withstand perturbations such as lattice vibrations that could dislodge the electrons from their traps. If the crystal lattice is stimulated using an appropriate mechanism, for example, by heating to an adequately high temperature or by exposure to an optical source with a suitable wavelength, trapped electrons will be evicted out of the traps.
Once evicted, the electrons diffuse within the crystal lattice until they reach a site that is attractive to electrons. Such sites are referred to as recombination centers. Some recombination centers emit energy in the form of light when they capture electrons. Where stimulation is conducted by heating, the effect would be referred to as TL. When stimulation is by optical means, OSL will be obtained. The diffusion of evicted electrons to their recombination centers occurs fairly rapidly to the extent that the time between stimulation and recombination can be viewed as instantaneous.
Effective recombination centers are usually those sites in the lattice where electrons are missing. These are the holes created when the materials are exposed to ionizing radiation Figure 1a. The intensity of the luminescence that is obtained when a material is stimulated is proportional to the number of electrons that are trapped in the material which, in turn, is commensurate with the energy absorbed from the ionizing radiation [ 50 , 51 ]. However, despite the energy storage mechanism being the same for a given mineral, the sensitivity to radiation may vary greatly between samples, an aspect that has important implications for dating procedures as will be shown later.
An energy level diagram that illustrates how ionizing radiation creates luminescence centers in crystal lattices redrawn from Ref. Both electrons and holes diffuse within the lattice. Evicted electrons that reach luminescence centers result in light being emitted. Importantly, for dating purposes, the number of electron traps within any mineral lattice is finite.
As a result, when minerals are exposed to ionizing radiation for an extended period, the traps become exhausted such that energy can no longer be stored efficiently. This effect is referred to as saturation. In dating, saturation determines the upper limit beyond which samples cannot be dated using luminescence techniques.
Materials that are subjected to very high dose rates will have the number of traps exhausted more rapidly such that the specific age representing the upper age limit will depend on both the number of traps present as well as the dose rate.
In geological and archaeological dating applications, natural sources of ionizing radiation that contribute to the trapped energy in mineral grains include isotopes of uranium U and U and thorium Th decay chains, potassium 40 K and rubidium 87 Rb. Despite the low concentrations, these radioactive isotopes collectively emit enough radiation to induce luminescence that is detectable for dating purposes. The radiation emitted includes alpha and beta particles as well as gamma radiation.
Beta particles and gamma rays have penetration ranges of about 0. An additional though smaller radiation component received by earth materials comes from cosmic radiation. Cosmic rays from outer space consist of a soft and a hard component. On earth, surface substrate absorbs the soft component such that it cannot penetrate deeper than 50 cm. The hard component, however, largely comprising muons, penetrates deeper and is lightly ionizing.
Hence, only the hard component is relevant to luminescence dating. On earth, the intensity of the hard component is also influenced by both latitude and altitude. Special formulae for evaluating cosmic ray contribution to dose rate have been developed for luminescence dating [ 53 ].
For dating studies, the primary aim of TL and OSL measurements is to determine the amount of energy that has been stored in the mineral grains of a given material since the start of the event that is being investigated. As outlined above, two main methods are used to stimulate energy release in luminescence studies. Heating allows TL to be measured, whereas stimulation using a light source is used for OSL measurements. The basic layout of equipment used to measure luminescence in geological and archaeological dating is shown in Figure 2.
Modern luminescence dating systems commonly possess both TL and OSL measurement capabilities [ 54 , 55 ]. To conduct a measurement, samples are usually loaded on discs about 1 cm in diameter that sit on an appropriate sample holder in multiples.
These are then introduced into the device, commonly referred to as a luminescence reader [ 44 ] and selectively moved into position for measurement. The luminescence signal from the sample is captured by a photon detector system [ 1 ] for example, photomultiplier tube PMT or charge-coupled device CCD camera after passing through optical filters. When conducting TL measurements, the filters exclude infrared signals from the heating but permit blue or violet emissions to pass through. In OSL measurements, wavelengths used for stimulation are rejected by the filters, whereas violet and near ultraviolet wavelengths are usually transmitted.
Once evicted, electrons are free to be re-trapped at the same site, be trapped at a different site or get to a recombination site where luminescence occurs. The eviction temperature is depicted by a peak in emission on a plot of the luminescence signal versus temperature which is referred to as a TL glow-curve. If the heating continues, all the electron traps will be emptied. A glow-curve that is obtained after the first heating of a sample is given in Figure 3.
Heating the sample again soon after the first heating will produce a different curve. The second curve corresponds to incandescence that is usually observed when any material is adequately heated to an elevated temperature. Hence, from this second heating, there will be no luminescence from trapped electrons that had accumulated from ionizing radiation since the last zeroing event. Illustration of glow-curves obtained following thermal stimulation. In optical stimulation, electrons are expelled from their traps using a source of a chosen wavelength.
Commonly used sources in luminescence dating include blue, green or near-infrared wavelengths. The rate at which trapped electrons are evicted is influenced by the rate at which stimulating photons are emitted by the source as well as by the sensitivity of the trap types to optical stimulation. Resolving Eq. If all freed electrons reach recombination sites instantaneously, the luminescence intensity will be proportional to electrons being evicted from the traps.
Thus from Eq. The exponentially decaying emission curve obtained is referred to in OSL dating as a shine-down curve Figure 4. With continued stimulation, a point is reached where all trapped electrons that are susceptible to optical stimulation are depleted. If all the photons released during stimulation are integrated, the total luminescence energy released by the mineral can be ascertained.
A shine-down curve obtained following the optical stimulation OSL of a hypothetical mineral sample for about s modified after Ref. Factors that influence the sensitivity of a trap type to optical eviction include characteristics of the trap as well as the wavelength of the optical source.
Generally, however, eviction rates are faster when shorter wavelengths are employed. Electron eviction from some traps could require more energy than that provided by an optical source. To circumvent that limitation, thermal assistance is used to attain the energy threshold required for eviction.
This allows longer wavelengths that would not normally be employed for optical stimulation to be used in dating [ 50 ]. Generally, sources for optical stimulation are selected such that separation can be made between wavelengths of the source used for stimulation and those of signals emitted by the minerals being analyzed. That separation is usually aided by the use of optical filters. As an illustration, the main emissions for quartz and feldspar are in the near-ultraviolet nm and violet nm regions of the electromagnetic spectrum.
Thus, filters that are employed when analyzing quartz and feldspars have windows in those respective regions but exclude wavelengths used for stimulation, for example, blue for quartz and near infrared for feldspar. OSL dating has a number of inherent advantages compared with TL when analyzing sediments that have been zeroed by exposure to sunlight. Investigations have shown that solar bleaching of electron traps that are stimulated by TL proceeds more slowly than with traps that are sensitive to OSL [ 50 ].
In the study summarized in Figure 5 , after 20 h of exposure to sunlight, both quartz and feldspar were shown to have less than 0. The TL signal that remained following the same period of bleaching, on the other hand, was a few orders of magnitude higher than the OSL signal. In practical terms, the slower bleaching of TL traps by solar energy means that higher residual signals will be found in unheated sediments analyzed using TL, to the extent that it is difficult to date very young samples using TL [ 11 , 50 ].
Thus, OSL analysis is generally preferred for dating sediments that were not reset by heating [ 50 ]. For dating materials that have previously been zeroed by heating, however, such as archeological artifacts, TL remains an appropriate stimulation mechanism. The slower bleaching curves are from TL signals. Detection of TL signals was through a window with a center at nm violet.
For the OSL, both quartz and feldspar used a green laser and a detection window of nm which is violet to near-UV [ 50 ]. Many minerals will luminesce when stimulated using an appropriate source following a period of exposure to ionizing radiation. However, not all such minerals are suitable for use in luminescence dating. Today, luminescence dating primarily employs quartz and feldspar. Zircon and calcite have been tried in some studies but both minerals are associated with a number of complications.
As a result, they are not commonly used in luminescence dating at present. This section examines the luminescence properties of the four minerals. In the discussions below, natural dose refers to energy acquired from natural radiation sources by a mineral grain in its field setting. This is differentiated from an artificial dose that a sample would obtain when irradiated using an artificial source in a laboratory setting.
Quartz is the most commonly used mineral in luminescence dating because it offers a number of advantages when contrasted with alternatives. As a result, it is one of the most abundant minerals in clastic depositional environments. Additionally, it does not have an internal source of radiation as a major element of its composition. Thus, the ionizing radiation that quartz grains receive in nature is usually from an external source, which simplifies dose rate calculation procedures.
Situations exist, however, where quartz grains may contain very low levels of uranium but these are rare [ 51 ]. For TL spectra, the sharp rise in emissions beyond nm is largely from incandescence rather than from electrons evicted from traps. Quartz has been shown to luminesce when stimulated by wavelengths from any part of the visible spectrum [ 60 ].
Most current OSL studies, however, prefer using blue light for stimulation because of the higher OSL intensities it yields [ 55 ]. Investigations have demonstrated that the OSL signal of quartz can consist of at least three or more components that are referred to as fast, medium and slow, based on their decay rates [ 61 , 62 ].
Most regular dating procedures, however, employ a constant power continuous wave—CW and are unable to resolve the components. Through the use of heat treatments or stimulation for limited times to exclude the slower components , desired signals can be targeted when using CW stimulation. Main emission wavelengths for quartz and feldspars used in luminescence dating as well as wavelengths employed for stimulation. Sensitivity ranges for some detectors are also shown.
Feldspar is another widely used mineral in OSL dating. In terms of chemistry, feldspars are aluminosilicates that form solid solution series with potassium K calcium Ca and sodium Na as end members of a ternary system. Since potassium has an isotope that contributes ionizing radiation in luminescence dating, the potassium in K-feldspars has to be treated as a source of internal dose, in addition to dose contributions from sources external to the grains.
As a result, when dating feldspars, it is necessary to separate K-feldspars from Ca and Na-feldspars and analyze them separately. Compared with quartz, feldspar has a number of attractive luminescence features. First, feldspar emissions are generally brighter than those from quartz which produces stronger signals.
This means that smaller doses can be measured during analysis. Second, the internal dose from potassium is not susceptible to external influences such as variations in pore water and this allows dose rates to be ascertained more accurately.
Third, feldspar can be stimulated using infrared radiation which allows effective separation to be made between the stimulation source and emission wavelengths. The main drawback for feldspar, however, is its susceptibility to anomalous fading [ 64 ]. Anomalous fading occurs when trapped electrons reside in their traps for shorter periods than what would be predicted by physical models such that the luminescence intensity drops over time from the time of irradiation.
Ultimately, the result of anomalous fading is that most feldspar grains yield equivalent doses that are slightly lower than they would in the absence of fading. Correction methods have been developed for dealing with anomalous fading when dating feldspars [ 65 , 66 ]. In terms of emission wavelengths, K-rich feldspars have been reported [ 67 ] to show maximums in the range of — nm violet to blue. Conversely, emissions for some plagioclase feldspars have been reported to appear in the range of — nm blue-green.
Other studies, however, have intimated at a more complex emission pattern for feldspars [ 68 ]. Optical stimulation of luminescence from feldspars has been investigated using visible light. Early studies employed lasers which included the The emissions were then monitored at shorter wavelengths [ 1 , 57 ] and shown to be centered around nm [ 69 ]. The application of OSL stimulation in dating feldspars, however, has been relatively limited because near-infrared stimulation discussed below has been shown to be a more desirable approach.
This would indicate that different trap types might be involved [ 50 ]. Apart from green and red stimulation, luminescence in feldspar has been demonstrated using a range of other wavelengths in the region spanning — nm [ 71 ]. As mentioned above, wavelengths in the near infrared region peaking around nm can also be used to induce luminescence in feldspars.
Since this effect was first noticed [ 72 ], most research in optical dating of feldspars has focused on IRSL stimulation. The main advantage of using IRSL is that the rest of the visible spectrum can then be used for emission detection. Fine-grained sediments containing mixtures of both plagioclase and K-feldspars have also been demonstrated to display a major stimulation peak around nm as well as a weaker one at nm [ 73 ].
LEDs are much cheaper than lasers and are widely available, making them a desirable alternative. Sedimentary K-feldspars stimulated using IRSL show a major natural emission peak at nm Figure 6d and another minor peak in the range of — nm [ 74 ]. Some IRSL studies [ 75 ] have reported additional natural emission maxima for K-feldspars at , and nm. With plagioclase feldspar, an IRSL emission peak has been identified at nm.
Feldspars stimulated using IRSL following the administration of a laboratory dose also exhibit an emission peak at nm. That peak is not observed in feldspars that have a natural signal. When not required during dating, the peak can be removed by preheating the sample to an appropriate temperature.
Thermally stimulated calcite has an emission maximum at nm [ 60 ]. However, efforts to use the mineral in luminescence dating have been encumbered by the limited environmental occurrence of calcite. Calcite also tends to concentrate uranium in its lattice and this complicates dose rate calculations since isotopic disequilibrium of uranium has to be taken into account.
Worth noting is that uranium disequilibrium dating can yield ages from calcite that are more reliable than those obtained using luminescence techniques. As a result, the incentive to employ luminescence methods in dating calcite has been small.
It should be mentioned that some of the earliest, albeit unsuccessful, TL studies that tried to date rocks employed calcite [ 10 ]. Other attempts to use calcite in archaeological dating include a report by Ugumori and Ikeya [ 76 ]. Zircon is an attractive dosimeter because it usually has a relatively high concentration of uranium.
This yields a dose rate that is relatively constant since it is not susceptible to variations arising from external effects such as changes in water content or burial depth. An associated drawback, however, is that the uranium content of zircon varies between individual grains. Consequently, measurements for dose rate are made on single grains. Also notably, zircon crystal lattices often have natural inhomogeneities that make it difficult to make comparisons between artificial irradiation administered in the laboratory with natural doses originating from within the grain.
As outlined in Section 5, such comparisons are the standard approach for determining the paleodose when dating quartz or feldspar. To address that problem, zircon dating uses a technique called autoregeneration. With autoregeneration, after the natural signal from the zircon grains is measured, the grains are stored for a few months to allow a new dose to accrue.
Measuring the signal at the end of the storage period and comparing it to the natural signal obtained from the initial measurement allows a calibration to be made that yields an age of the natural signal. Analysis of zircon using TL includes a study by Huntley et al. OSL studies using zircon include investigations by Smith [ 78 ]. The age equation introduced in Section 1 Eq. This section examines methods that are used to determine the two variables.
The start of the accumulation of the paleodose should typically coincide with a geomorphic or archeological event that emptied or zeroed any previously accumulated energy in the sample grains. For materials that were previously zeroed by heating or firing, the start of the accumulation of the paleodose would correspond with the last time the material was heated to a temperature high enough to expel electrons from their traps.
In the case of sediments that were zeroed by exposure to sunlight, the start of the accumulation of the paleodose would correspond to the last time that a sample was subjected to the bleaching effects of the sun for a period long enough to evict all trapped electrons. As indicated earlier, the natural signal refers to the luminescence signal yielded by a sample collected from the field. In order to determine the paleodose of a sample of unknown age, the natural signal is measured first after which the sample is irradiated artificially using a well-calibrated laboratory-based source.
The signals from the artificial dose are then measured and compared with the natural dose signals in order to determine an artificial dose that gives a signal similar to that of the natural dose. This is referred to as the equivalent dose D e. Investigators use two main methods to determine D e : the additive dose and the regenerative dose or regeneration methods [ 2 , 50 ]. When determining luminescence ages using the additive dose method, a sample of unknown age is split into two sets of aliquots.
The natural signal for one set is measured first after which the second set is irradiated with incremental doses using an artificial source and also measured. Plotting the artificial signals against the dose administered produces a dose—response curve depicting the luminescence signal against the laboratory dose Figure 8a.
The curve is also known as a growth curve. The natural signal is also plotted on the growth curve against zero dose Figure 8a. Extrapolating the curve backwards until it intercepts the horizontal axis at zero signal intensity provides D e Figure 8a [ 51 ]. Growth curves are unique to each sample because luminescence sensitivity of mineral grains can vary from sample to sample. As a result, a new growth curve has to be constructed for each sample whose age is being determined.
When dating materials that were zeroed by the sun using TL, the residual TL signal that is noted following solar bleaching would have to be taken into account when extrapolating the curve backwards. With both feldspar and quartz, the relationship between the luminescence signal and the laboratory radiation portrays a linear trend for low and moderate doses. At elevated doses, however, the growth curve plateaus, indicating that luminescence traps are getting exhausted, also referred to as saturation.
Main methods employed in determining the equivalent dose. Growth curves are unique to each sample being analyzed such that new measurements have to be made and a new curve constructed for every sample being dated. The procedure used in the regeneration method is similar to that employed in the additive dose method apart from that, before the laboratory dose is applied, the sample aliquots in the regeneration method are first zeroed to remove any previously acquired dose.
Incremental doses are then applied to the zeroed aliquots and measured. The acquired signals are plotted against the administered dose to give a regenerative dose growth curve. To get the equivalent dose, the signal from the sample of unknown age is interpolated into the growth curve Figure 8b [ 51 ].
Hence, when constructing the growth curve, the laboratory irradiation doses are selected such that the signals they produce lie above and below the signal obtained from the natural dose. Examples of growth curves for quartz taken to saturation redrawn after . The parameter D 0 determines the shape of the curve.
Both the additive dose and regenerative dose methods employed multiple aliquots when they were originally developed for TL dating. Later, when OSL dating emerged, the possibility of using single aliquots only was brought up [ 1 ]. However, the concept did not take hold initially and multiple aliquots were also adopted for OSL dating.
Generally, the use of multiple aliquots assumes that all aliquots of a given sample behave similarly to the dose administered. However, in reality, inter-aliquot variations occur for a range of reasons that include changes in sensitivity [ 3 , 79 ]. Normalization is used in some cases to try and reduce the effects of the variations.
Nonetheless, the effects cannot be eliminated entirely such that uncertainties are contributed to the calculated ages. Ultimately, there was an incentive to devise an approach that only employed a single aliquot. When initially introduced for dating, single aliquot methods employed the additive dose method on feldspars [ 80 , 81 ].
Quartz had not been used because sensitivity changes that it displays during repeat measurement cycles rendered single aliquot data unworkable. Subsequently, Murray and Wintle [ 37 ] presented an enhanced version of the SAR approach introduced by Murray and Roberts [ 83 ] in which a test dose was used to monitor sensitivity changes in the quartz.
D i is the dose that yields the signal L i while D t is the test dose that produces the signal T i. Minor modifications have been made to the SAR procedure presented in Table 2 since its original inception [ 37 , 38 ]. However, over the last two decades, the protocol has been widely adopted for routine dating of both sediments and heated materials using quartz and feldspar [ 84 ].
SAR protocols have also been extended to determining paleodoses using individual mineral grains. The analysis of individual grains from the same sample is particularly useful for identifying differences in paleodose between grains [ 86 ]. Examples of cases where this may be expedient is when studying sediments deposited by rivers fluvial where well-bleached grains might be mixed with partially bleached fractions [ 87 ].
As outlined above, once the paleodose has been ascertained, the dose rate needs to be evaluated before an age can be determined. In Section 1, it was mentioned that ionizing radiation responsible for the energy accumulation in mineral grains in natural settings emanates from uranium and thorium decay chains as well as from potassium and rubidium isotopes. Cosmic radiation also contributes a minor component. A number of methods can be used to evaluate the total contributions of all these components.
Using the concentration approach, levels of uranium, potassium and rubidium in a given sample are quantified with the help of an analytical procedure such as atomic absorption spectroscopy AAS , neutron activation analysis NAA , flame photometric detection FPD , X-ray fluorescence XRF and inductively coupled plasma spectroscopy ICPS. Once the concentrations have been measured, dose rate is determined using special conversion tables prepared for the purpose [ 50 ].
For uranium and thorium, however, isotopic disequilibrium could render the measurements using these analytical techniques unreliable [ 50 ]. Despite being costly, these methods can provide accurate measurements, including from uranium and thorium decay chains and in cases where disequilibrium exists.
Nonetheless, lengthy measurement times may be necessary [ 50 ]. Yet another approach to determine the dose rate is to use TSAC to determine the alpha particle contribution after which a beta particle counter is employed to determine the beta contribution. The gamma dose rate is best determined in the field whenever possible.
Highly sensitive portable gamma-ray spectrometers that make such onsite measurements possible are now available [ 44 ]. These are highly sensitive materials that are left in the field for a few weeks after which they are retrieved and analyzed. The dose contribution from cosmic rays is usually minor. However, in settings where the radionuclide concentrations are low, the proportion from cosmic rays becomes significant. A methodology for calculating cosmic ray contribution to the luminescence dose rate was formulated by Prescott and Hutton [ 53 ].
Lastly, it is imperative to take the in situ moisture content of the material that is being dated into account when calculating the dose rate. This is because interstitial water absorbs part of the dose that should otherwise reach the dosimeter, with the attenuation of the dose rate intensifying as the moisture content increases. At present, luminescence dating methods can be used to date samples that are as young as a few decades [ 88 ]. Dating using the single grain approach can produce young ages that are relatively precise.
When dating such young samples, it is desirable to use mineral grains characterized by a high luminescence sensitivity and for the grains to have been completely bleached prior to the burial [ 39 ]. Minimizing thermal charge transfer during measurement also improves the accuracy of the results. Maximum ages that can be obtained using luminescence dating methods are ultimately controlled by the fact that the population of electron traps within any given dosimeter is fixed.
As a result, the number electrons stored by trapping cannot increase indefinitely [ 50 , 51 , 56 ]. This is depicted in luminescence growth curves by a flattening of the signal obtained as the dose increases and is often expressed using a saturating exponential function.
Figure 9a shows such a function expressing the fast component of a quartz signal. It shows that, once a certain dose is reached, the curve flattens. That dose is the upper limit above which the proportionality between the dose received and luminescence signal obtained breaks down.
Quartz usually saturates with a dose of around — Gy. An approach used in some studies [ 4 , 89 ] working with high doses is to model the growth curve by combining a linear function and a saturating function as shown in Figure 9b [ 3 ]. This approach has been used to report quartz ages in excess of — years. It should be noted that the limits of the ages that can be obtained are ultimately determined by the magnitude of the dose rate, with low dose rates giving higher age limits and vice versa.
With feldspar, several studies have reported ages that exceed years using IRSL [ 90 ]. In essence, with both feldspar and quartz dating, there are maximum dose limits above which reliable ages cannot be produced as a result of electron trap exhaustion.
Preceding sections explored the basics of luminescence dating. A subject that now needs to be addressed is the nature of materials on which luminescence dating methods can be applied. Before that can be looked into, however, it is pertinent to examine the topic of sample grain size, since protocols employed in luminescence analysis are contingent upon the granulometry of the material being analyzed. There are generally two broad mineral grain size ranges that are employed in luminescence dating: coarse grains and fine grains.
Grains in this size range normally receive ionizing radiation from alpha and beta particles as well as gamma and cosmic rays. When working with coarse grains, the outer rim that is affected by the alpha particles is removed by etching using hydrofluoric acid HF. For this reason, coarse grain luminescence dating is sometimes described as inclusion dating [ 58 , 91 , 92 ]. In sediment dating, the usual practice is to extract grain sizes that represent the modal size. We use a range of sampling techniques in the field.
Where possible, sediment exposures with visible stratigraphy are used or created. In addition or where exposures are not present, sampling can be carried out using an auger to drill through deep sedimentary sections. A hydraulic drive with a range of different heads can be used in conjunction with hand auguring to punch through calcrete or silcrete layers within the landform of interest. Using this technique, sampling intervals of 0.
Samples are taken in light-tight sampling pots to prevent any exposure to light. Sampling for OSL dating in i-ii exposed dune sections in the UAE and Pakistan, iii-iv archaeological sites in India and Sudan, v-vi using short cores in the Nebraska sandhills, vii-viii hand dug pits in Kalahari dunes and lake basins; ix-xi using hand and hydraulic augers in Nambia and Botswana; xii samples in light-tight packaging before shipping back from field sites and xiii-xiv using the gamma spectrometer to record dose rates within the sediment matrix.
What is luminescence dating? Luminescence dating of arid sediments Facilities Luminescence dating is an absolute radiometric method of determining the age of a material since a key event in its history - typically burial in the case of sediments or firing in the case of ceramics or burnt stone. Luminescence sampling procedures in the field Sampling for OSL dating in i-ii exposed dune sections in the UAE and Pakistan, iii-iv archaeological sites in India and Sudan, v-vi using short cores in the Nebraska sandhills, vii-viii hand dug pits in Kalahari dunes and lake basins; ix-xi using hand and hydraulic augers in Nambia and Botswana; xii samples in light-tight packaging before shipping back from field sites and xiii-xiv using the gamma spectrometer to record dose rates within the sediment matrix.
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It was recently developed for quartz grains Murray and Wintle, and and applied also to feldspars Wallinga et al. We consequently describe in the present paper its general principles and its application to the case-study samples LUM and LUM It includes the measurement of several OSL signals for a single aliquot tab.
Basically the comparison of the natural signal with the artificial luminescence signals should make it possible to interpolate the palaeodose directly. The procedure is however more complicated, as artificial irradiation leads to the trapping of electrons in unstable traps of the mineral crystal lattice.
Since the measurement of the luminescence signal occurs immediately after irradiation, these electrons are very likely to be released during the optical stimulation, leading to an overestimate of the signal. The unstable traps must consequently be emptied before the measurement. The temperature of the preheat has to be chosen by a preliminary test that requires exposing the aliquots to different preheat temperatures or by exposing the aliquots to a given temperature during various time-spans.
The aliquots are placed on a wheel in the reader black box on the right. The black tube above the reader is the photomultiplier tube used to record the photons emitted by quartz and feldspar during a stimulation i. The heater element is located in the lower part of the reader. The results can be seen on the screen left of the picture photo: T.
The SAR protocol applied to coarse-grain quartz 1. Give various regenerative doses s, s, s and s and repeat step 1 to 6. Checking of recuperation and recycling: 8. Give a repeated regenerative dose and repeat step 1 to 6.
Checking of feldspar contamination: Give the same regenerative dose Repeat step 2 to 5. The SAR protocol applied to coarse-grain feldspars 1. Checking of recuperation and recycling: 7. Give a repeated regenerative dose and repeat step 1 to 5. Laboratory treatments may actually induce sensitivity changes in the grains, which means that the signal obtained after a subsequent stimulation may be affected by the preheat and irradiation conditions.
At this step, the SAR protocol can be summarised as follows tab. Each measurement is followed by the administration of the test dose and measurement of the test signal T x. The palaeodose value for this aliquot from sample LUM is These tests may vary from one laboratory to the other but basically correspond with: i a repeat of the very first regenerative dose which was given after the L n and T n measurement.
This shows that sensitivity changes were corrected using the test dose. A recycling ratio significantly different with 1 means that for a similar dose the two signals are not the same: the aliquot is consequently discarded for the equivalent dose determination; ii a luminescence measurement for a regenerative dose equal to zero.
If the normalised signal is theoretically equal to zero, a weak signal is often induced by the transfer of electrons during the preheat process. This detection is undertaken using infrared diodes. This test is important because feldspars are not only stimulated by infrared light, but also by the blue or green light used for quartz. Hence, the presence of feldspar contaminates the luminescence one wishes to record from quartz.
A similar test is not necessary when the analyses focus on feldspar grains, because the quartz grains which may be present in the aliquots are insensitive to infrared stimulation; iv a measurement of anomalous fading for feldspar.
This test may be performed using a SAR protocol including variable delays between irradiation and measurement of the signal to estimate the fading to be estimated. Accurate ages are then obtained by inserting this fading in a correction model Huntley and Lamothe, ; Auclair et al. In the case of aeolian sediments, all of the analysed grains are assumed to be well bleached, and all the D e have a similar value, which can be used to calculate the age of the sediment.
However, partial or incomplete bleaching is common, especially if the transport history was short or the exposure to sunlight was insufficient, as can be the case for fluvial sediments. This partial bleaching can be homogeneous all the grains being incompletely bleached in the same proportion or heterogeneous differential bleaching.
In this latter case the D e distribution shows a scattering fig. Some aliquots can present a very high palaeodose, which greatly overestimate the age of the last transport event. This explains why the mean is not appropriate in estimating the accurate equivalent dose. It is therefore necessary to use a statistical model. Several models have recently been developed. It will also overestimate the equivalent dose in the presence of a partially bleached sediment.
As for the sampling strategy the choice of the model depends upon the kind of sediments and presupposes a discussion between the field and luminescence specialists Bailey and Arnold, Comparison with independent age control may also be very useful, as shown by H. Rodnight et al. The relevance of these models increases with the number of aliquots.
The number of 50 aliquots is sometimes considered as a minimal value to ensure a reliable equivalent dose determination Rodnight, , but it is important to keep in mind that the number of aliquots to be measured depends on the sample and increases with the scattering. The aim of this section is to review the applied representative studies dealing with OSL in France. As in other countries, the first dating of sediments was based on thermoluminescence Wintle et al. The first OSL applications tab.
Loess deposits were successfully dated especially in NW France. Several loess-palaeosol sequences Engelmann et al. Most of the research focused on the last interglacial-glacial cycle Antoine et al. Coastal sands from the North Sea or Channel coastlines were also optically dated for more than one decade.
The dating of raised beaches Balescu et al , ; Regnault et al , ; Coutard et al. At the same time the dating of young Holocene dunes Clarke et al. The first OSL dating of fluvial sediments from French rivers also started at the end of the s, despite the problem of differential bleaching being for a long time considered as a major hindrance in applying luminescence dating to fluvial sediments Wallinga, Improvements in the detection of the partial bleaching e.
Fluvial deposits of the Seine River have been locally dated both in its lower reach middle Pleistocene estuarine silts; Balescu and Lamothe, ; Balescu et al. A more extensive dataset was provided for the Loire basin Straffin et al. The same sediments were subsequently used to improve the dating method using the far-red IRSL of feldspars Arnold et al. Study area. Dating technique.
Main topics. Loess and palaeosoils. Engelmann et al. Normandy and Brittany. Antoine et al. Regnauld et al. Balescu and Tuffreau in Bahain et al. Sun et al. Cliquet et al. Guette-Marsac et al. Coastal sediments. Balescu et al. Clarke et al. Geoarchaeology, palaeoenvironmental reconstructions. Coutard et al. Fluvial sediments. Balescu and Lamothe ; Balescu et al.
Folz et al. Straffin et al. Loire and Arroux valleys. Moselle and Meurthe valleys. Vernet et al. Tufa and travertines. Bahain et al. About 20 samples from the Meurthe and Moselle fluvial terraces in France and in surrounding countries Germany and Luxembourg were dated Cordier et al. These samples originate from a section exposing the sediments from the lower terrace M1.
The fluvial sedimentary sequence exposed here is typical for the M1 alluvial terrace in this area, exhibiting a succession of three units: a coarse lower unit unit A up to 5 m thick, a sandy unit unit B 1 to 3 m thick, and an upper unit unit C less than 2 m thick, which corresponds to an alternation of sandy and silty thin laminae.
The samples LUM and were taken from a sandy horizon on top of unit A and in unit B, respectively. We applied the typical SAR protocol tab. The equivalent dose distributions are presented in fig. They reflect incomplete bleaching of some aliquots, which is typical for fluvial sediments. In our case study, however, quartz surprisingly shows a greater scattering than feldspars.
This may be explained by the presence of two distinct populations of quartz grains: one being fully bleached since it originates from the Vosges Massif ca. In contrast, all the feldspars are assumed to come from the Vosges Massif. The consequence of the scattering is that D e values fig. Each D e value is represented by a point. The precision is indicated by the x-axis and increases to the right. The D e value is plotted on the y-axis as the number of standard deviations away from a chosen central value.
In contrast, the radial plots for quartz from both samples show the presence of high palaeodoses corresponding with incompletely bleached grains. While the results are more or less similar for feldspars, the values are highly variable for quartz. As for current luminescence dating of Rhine sediments where such an age control is available Lauer et al. For feldspars the age was corrected using the model of D. Lamothe to take into consideration the anomalous fading tab. This example demonstrates that the palaeodose calculation should take into consideration the geomorphological context of the sample in order to understand the mechanisms for the scattering.
Based on the Minimum Age Model, the age estimates show that the sediments from below the lower terrace M1 were deposited during the Weichselian upper Pleniglacial. This chronological framework is consistent with the age estimate for the present river floodplain M0 Lateglacial to Holocene ages; Carcaud, , and with the previous OSL dating of higher fluvial terraces of the French Moselle River and its mains tributaries: the Meurthe River fig.
These results still have to be improved especially by obtaining an independent age control. However, they emphasise the climate control on terrace formation for the Moselle River and its tributaries, each terrace formation being allocated to a glacial-interglacial cycle. They also confirm the sedimentological results, which show that the younger terraces of the rivers Meurthe-Moselle have age similarities between the Paris basin and the Rhenish Massif, suggesting tectonic stability along the river valleys during the middle and upper Pleistocene.
The generalised results see also Cordier et al. The fading was estimated using the model of D. Lamothe to calculate the fading rate. This reflects the percent decrease of intensity per decade, with a decade being a factor of 10 in time since irradiation. Corrected feldspars ages are in good agreement with the quartz results for both samples. Huntley et M. At a shorter time scale e. Finally, historical phases of sediment aggradation were dated, with the accuracy of the age results being confirmed by historical archives.
The method was integrated into several research topics tab. In parallel it allowed methodological improvements e. However, it is worth noting that most of the data are used for local reconstructions, and that quantitative large-scale studies e. This might be explained by the methodological limits of the OSL method, which in particular relate to the anomalous fading of feldspars, the partial bleaching, and the age uncertainty.
This latter derives from the addition of uncertainties which concern both the dose rate e. As a consequence it is important especially for old sediments not to exclude it from the interpretation. Despite these limits the weak number of publications including OSL dating in France should however not be interpreted in terms of the accuracy of the method, as it may be successfully compared with other common dating methods: i the thermoluminescence methods are commonly used for archaeological or volcanic materials, especially because of the mineral composition of the studied material.
For other sediments OSL dating is typically used as it focuses on the light-sensitive part of the signal which is faster and more completely bleached while TL also measures the non-bleachable signal Duller, ; hence OSL dating makes it possible to date younger sediments, and to reduce the occurrence of partial bleaching; ii in opposition to radiocarbon, OSL dating is applied to sediment sequences that contain no organic material. It can also be used for longer time-scales up to several hundreds of ka , while radiocarbon cannot be used for materials older than ka.
From a geomorphological point of view optical luminescence also presents the advantage that it dates directly the deposited material; iii cosmogenic dating methods are used principally for age estimates of surfaces e. Furthermore, it is more difficult to obtain several ages on a given vertical profile, while the OSL dating methods makes it possible to get a high-resolution chronological reconstruction; iv the Electron Spin Resonance method is applicable to material that is up to several hundreds of thousands years old, but is typically applied to palaeontological remains bones or teeth.
Even if accurate ages have been obtained for aeolian or fluvial sediments Antoine et al. It is however important to try and get as often as possible an independent age-control to improve the reliability of the results. It may derive from archaeological or historical data, from the recognition of palaeoenvironmental evidence pollens, periglacial features; Wintle, , or from comparison with other absolute dating techniques e.
It is however important to note that the comparisons between optical dating of quartz and independent age controls often show good correlations e. The above-mentioned methodological limits should not be considered as fixed in regard to the constant improvements which have characterised the OSL method for more than twenty years SAR protocol, development of the small aliquots and single grain analyses, improvements in the anomalous fading estimate and in the statistical treatment, applications to new kinds of sediments.
Hence the reason justifying the rarity of studies using OSL should very likely be attributed to the lack of laboratories in France in comparison with other countries 2 laboratories in France but ca. This fact is all the more regrettable in that research worldwide shows that the optical dating method represents one of the most useful geochronological tools in geomorphological studies. However, it is important to keep in mind that not all sediments from all sections can be successfully dated: geomorphological field expertise before sampling remains essential in order to target the sediments showing a higher probability for complete bleaching, since only these sediments will allow an accurate age to be obtained.
Methodological improvements have enlarged the field of applications, but have also supported the development of quantitative geomorphology. In the same way it is an essential tool for research focusing on the existence of morphological thresholds, or dealing with the influence of forcing e.
For example L. Clemmensen et al. Based on these statements, the perspectives for applying the OSL method to geomorphological research in France are various. This can subsequently be used for sediment budget estimation during the Pleistocene climatic cycles, completing the results presented by M.
Frechen et al. The reconstruction of coastal dune formation may actually help us to understand their dynamics, which in turn can be used to assess their response to an increase of storms and their management in a context of sea-level rise; iii in fluvial environments OSL may be applied both to sediments which lack age control as it is the case for many terrace staircases and to sediments which have already been dated using other methods such as ESR e.
From an applied point of view, the acquisition of an extensive dataset is essential for a better understanding of fluvial response to environmental changes and for sediment budget reconstructions e. This is especially the case of glaciogenic deposits especially fluvio-glacial to glacio-lacustrine which are preserved in all the French mountains and often include sand-sized deposits potentially suitable for OSL dating; v the dating of slope-deposits lato sensu may be used to achieve a better understanding of the influence of human societies on slope dynamics, as well as for hazard management.
Evidence for this is shown by the worldwide increase in the number of laboratories and publications. A similar observation has been made in the general review of S. Stokes , but associated with the persistence of methodological problems. Ten years later, most of these problems seem to have been overcome: even if the operating procedures may vary from one laboratory to another e.
The method should therefore contribute to the development of quantitative geomorphology Singhvi and Porat, , as it makes it possible to recognise sedimentary discontinuities identification and dating of the thresholds separating erosive and aggradational periods , or to estimate aggradation rates. He also wishes to gratefully acknowledge the four anonymous reviewers for their very constructive comments and advice on the first version of the manuscript.
Adamiec G. Ancient TL 16, Aitken M. Academic Press, London, p. Oxford University press, Oxford, p. Antoine P. Boreas 28, Quaternaire , 14, Quaternaire , 17, Quaternary International Arnold L. Quaternary Science Reviews 22, Auclair M. Radiation Measurements 37, Bahain J. Quaternary Geochronology Bailey R. Radiation Measurements 27, Radiation Measurements 32, Quaternay Science Reviews 25, Balescu S.
Quaternary Science Reviews 7, Quaternary Research 35, Quaternary Geochronology 13, Boreas 26, Archeologicheskii almanach, Donetsk, 16, Banerjee D. Radiation Measurements 33, Radiation Measurements 23, Brocard G.
Earth and Planetary Science Letters , Carcaud N. PhD thesis, University of Nancy 2, p. Clarke M. Supplement Band , The Holocene 12, Clemmensen L. Sedimentary Geology , Cliquet D. Quaternaire , 20, Colls A. Quaternary Science Reviews 20, Cordier S. Quaternaire , 16, Quaternary Science Reviews 25, Coutard S. DeLong S. Quaternary Geochronology 2, Duller G. Radiation measurements 37, Journal of Quaternary Science 19, Ancient TL 25, This is obtained from the formula:.
This accumulated signal results in luminescence i. Stimulation can be achieved by heating thermoluminescence or TL or exposure to light optically-stimulated luminescence or OSL. Luminescence dating has been applied depending on conditions from sediments ranging from 10 - 10 6 , although more commonly the upper limit is ka. It has been applied to aeolian, fluvial, lacustrine, glaciogenic, coastal and marine applications, in addition to a wide range of research in archaeology and art antiquity.
We use a range of sampling techniques in the field. Where possible, sediment exposures with visible stratigraphy are used or created. In addition or where exposures are not present, sampling can be carried out using an auger to drill through deep sedimentary sections. A hydraulic drive with a range of different heads can be used in conjunction with hand auguring to punch through calcrete or silcrete layers within the landform of interest. Using this technique, sampling intervals of 0.