Luminescence dating laboratory procedures and protocols

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This paper aims to provide an overview concerning the optically stimulated luminescence OSL dating method and its applications for geomorphological research in France. An outline of the general physical principles of luminescence dating is given. A case study of fluvial sands from the lower terrace of the Moselle valley is then presented to describe the range of field and laboratory procedures required for successful luminescence dating.

The paper also reviews the place of OSL dating in geomorphological research in France and assesses its potential for further research, by focusing on the diversity of sedimentary environments and topics to which it can be usefully applied. Hence it underlines the increasing importance of the method to geomorphological research, especially by contributing to the development of quantitative geomorphology. They are now largely used to date not only palaeontological or organic remains, but also minerals that characterise detrital clastic sedimentary material.

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The most common methods applied to minerals are cosmogenic radionuclides, electron spin resonance ESR and luminescence techniques. The latter were first applied to burned minerals from archaeological artefacts [thermoluminescence TL method]. Improvements of this technique led to the development, for more than twenty years, of the optical dating method [commonly referred to as Optically Stimuled Luminescence OSL ] which is now applied to sediments from various origins Wintle, The aim of this paper is to provide people involved in geomorphological research a global overview about the principles and procedures of optical dating, from the field sampling to the age interpretation.

Most of the publications actually focus on one part of either the method e. The general principles of the method are described first. The paper then explains how OSL dating is applied to obtain a depositional age, through the field and laboratory procedures employed. These procedures are described as clearly as possible in order to provide useful information for geomorphologists interested in the method, and illustrated by a case study that has involved luminescence dating of fluvial sands samples LUM and LUM from the lower alluvial terrace of the Moselle River M1 terrace as defined by S.

Cordier et al. However, it is essential to keep in mind that the protocols preparation of the sediments, measurements may vary from one laboratory to the other: the presentation does not aim to be exhaustive, Luminescence dating laboratory procedures and protocols to reference the main procedures at each step of the dating.

Finally the paper reviews the recent applications of OSL dating in France, and assesses the potential of applying the luminescence dating technique to a range of geomorphic research. The main minerals Luminescence dating laboratory procedures and protocols are quartz and K-rich feldspar, which can be found in almost all sedimentary environments. They lead to the emission of electrons which are subsequently trapped in crystalline lattice defects.

On the contrary, defects situated deeper inside the lattice have a higher thermal lifetime. The total amount of trapped electrons within a crystal is proportional to the total energy absorbed and retained by the crystal or dosehence the time it was exposed to radiation. As soon as the mineral is exposed to sunlight, for example during its transport, trapped electrons absorb the photon energy from the Sunand are released from the traps.

The recombination generates the emission of light the luminescence al which can be measured in laboratory through heating for TL or through light stimulation for OSL Huntley et al. The intensity of the al is proportional to the amount of released electrons. The wavelength of the al is allocated to the nature of the mineral: the OSL from quartz is typically measured in ultra-violet nm wavelengthwhile quartz also emits in blue nm wavelength and in orange-red nm wavelength; Huntley et al.

These latter wavelengths are however less used to avoid interactions with the stimulating light. Feldspars mainly emit in the nm wavelength Huntley et al. It is exposed again to radiation and accumulates trapped electrons. Much later the grain is sampled in the field and stimulated in the laboratory using either visible light for quartz OSL stricto sensutypically performed with blue or green LEDsor infrared light for feldspars Infrared Stimulated Luminescence IRSL.

The observed OSL al from quartz is however made up of several exponentials related to different levels of traps Bailey et al. This reflects the existence of a fast component associated with the emptying of the most light-sensitive trapsa medium component, and several slow components corresponding with less bleachable traps; Bailey, The analysis of the OSL al makes it possible to estimate the time elapsed since burial, i.

This age is derived using the following equation:. It is estimated in the laboratory by the determination of the equivalent dose D ei. D r is the dose rate the rate at which the sediment is exposed to natural radiations. However, the measured al does not always reflect the time elapsed since the burial of the sediment fig. The mineral becomes saturated, and its equivalent dose will consequently underestimate the event of interest. Quartz generally saturates at lower doses than feldspars. Therefore it is possible to date feldspars that are several hundreds of thousands of years old Juschus et al.

On the contrary, it Luminescence dating laboratory procedures and protocols difficult to date quartz beyond ka except when the dose rate is low, i. Recent research focusing on the recuperated al of quartz Re-OSL however opens the way to overcome satisfactorily the problem of quartz saturation Wang et al. This phenomenon, which might be explained by a quantum mechanic tunnelling process, is associated with a spontaneous eviction of electrons in deep traps, at room temperature, and without exposure to light.

Anomalous fading generates an underestimate of the equivalent dose, and consequently of the burial age estimate. It is highly recommended to apply measurement procedures in the laboratory to detect the fading. A successful mathematical model was proposed by D. Huntley and M. Lamothe to compensate for anomalous fading and get more reliable age estimates see also M.

Auclair et al. This decay is not a single exponential curve as several levels of traps are involved, some of them being more rapidly bleached fast component than the others medium and slow Luminescence dating laboratory procedures and protocols. The choice of these sizes is justified by the procedures used for the dose rate determination see next section. The best sampling strategies are those that involve communication between a geomorphologist and luminescence dating expert prior to and during the field sampling process.

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The sampling is best undertaken from a pit or a natural sectionor using cores. In a pit, samples should be taken from an homogeneous layer more than cm thick, away from coarse deposits, which may complicate the dose rate calculation. Sampling should also be avoided for sedimentary layers affected by bioturbation or pedogenic activity, as it may generate in situ bleaching and variations in the dose rate through time fig.

The sampled sediment should be cleaned first by removing the outermost 10 cm. It is possible to operate by night especially for consistent and hardened sediments which can be dug with a shovel but also by day since the outer parts of the sample will be removed before the measurements.

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Once the sample has been removed the remaining hole can be used for an in situ determination of the dose rate see below. The dose rate may also be determined in the laboratory. When the profile presents several sedimentary units, further samples should also be taken so that the luminescence ages can be compared. This comparison is however not sufficient to assess the accuracy of the age, which Luminescence dating laboratory procedures and protocols be only obtained using an independent technique see below. A second possibility is to collect luminescence samples from sedimentary cores.

It is essential to protect from light the sediment inside the core, until it is opened in laboratory under subdued light conditions. The sedimentary layering within the core should be checked to ensure that sedimentary horizons have not been disturbed during the coring process. As for other analyses it is especially important not to sample the area between different cores junctions. A part of the sample should also be used for dose rate estimates. A: General map of the study area. C: Log of the studied profiles. D: Sampling of LUM Evaluation of the time-averaged water content present in the sediment being sampled throughout its burial is also an important, yet difficult parameter to assess.

Field observations for water content by a geomorphologist may therefore be useful, e. Good estimates of modern moisture content can also be achieved by comparing the weight of the dried sample with that of the natural sample. However, the value is typically not indicative of the entire period of burial, especially if the sediment is thousands of years old.

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It is also important to mention that the effective dose rate varies with the grain size. The dose rate may be measured in Luminescence dating laboratory procedures and protocols using a radiation dosimeter or a field gamma spectrometer Aitken, ; Hossain, It may also be evaluated in the laboratory from the representative sediment sampled close to the luminescence one. The fundamental assumption is that the dose rate remained constant during the whole burial period, i. This assumption can be considered as valid if the decay series show a radioactive equilibrium, i.

Hossain, The contents of radionuclides within the sediment as well as information about their decay behaviour can be obtained by high resolution gamma spectroscopy. In that case the sediment has to be dried, homogenised by grinding, cast inside an air-tight container to prevent from the loss of Rn by gaseous diffusion and stored for one month.

Such a storage is required to establish a radioactive equilibrium between Rn and Ra. Then the activity of several key radionuclides for example Th, Bi, Pb and Pb for the U chain is measured during at least 24 h. It is shown by the spectrum of energy in keV of gamma emissions and must be equal for all the measured radionuclides to assume a radioactive equilibrium.

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This activity is proportional to the dose rate which can then be calculated using conversion factors Adamiec and Aitken, Other main methods for dose rate calculation include the thick source alpha counting TSACwhich consists of counting the alpha particles emitted by the U Luminescence dating laboratory procedures and protocols Th decay series; the GM-beta counting to measure the beta dose rate from decay of U, Th and K; and the instrumental neutron activation analysis INAA which consists of irradiating the sample with neutrons prior to measuring the rate at which gamma-rays are emitted from an element, this rate being proportional to its concentration Hossain, It is important to note that most of these determinations are indirect, i.

In the same way the cosmic dose is calculated using mathematical equations Prescott and Hutton, After the removal of the top and the bottom of the sampling tube since both parts were exposed to light or sampling in the core, the sediment is sieved in order to select a representative grain size to be studied fig.

As the energy absorbed by a grain throughout burial highly depends on its size, it is important that the grain size is as homogeneous as possible. We subsequently exposed this grain size fraction to HCl, sodium-oxalate, and H Luminescence dating laboratory procedures and protocols O 2 to dissolve or remove carbonates, aggregates and organic materials, respectively.

Following these treatments, heavy liquid solutions were used to separate plagioclase, heavy minerals, quartz, and K-rich feldspars, the latter two minerals being commonly employed in OSL dating. Finally the purified quartz was sieved again to remove grains whose diameter was ificantly reduced by the etching.

Alternative chemical treatments exist Roberts,for example using fluorosilisic acid H 2 SiF 6. In case of partial bleaching, even fewer grains can be used, corresponding either with small aliquots several tens of sand-sized grains or with single-grain aliquots. Single grain aliquots can also be prepared with a purposed-built single-grain disc which is made up of a regular array of holes, with one grain per hole; Duller, The size of the aliquot is actually an important parameter Duller, : medium or large aliquots may lead to an averaging of the palaeodose estimate in case of partial bleaching, since most of the aliquots will be likely to include both well bleached and poorly bleached grains.

They are however successfully employed for fully bleached sediments.

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On the contrary, measuring an insufficient of grains may lead to the obtaining of too weak a luminescence al, influenced also by the background noise of the measuring system. Despite ificant improvements Rittenour,the use of single grain aliquots is still subject to ongoing debates: on the one hand, single grain dating provides a more precise equivalent dose and Luminescence dating laboratory procedures and protocols for partial bleaching to be detected.

On the other hand, since only a very few quartz grains emit luminescence, up to several hundreds of grains have sometimes to be measured to record a sufficient OSL al. The use of single grain aliquots is difficult when the exposure to sunlight is unequal from grain to grain e. DeLong and L.

Arnold found ificant differences in age estimates between small aliquots and single grains for only very young samples less than 1 kathe being very similar for older sediments. It is finally important to note that the choice of aliquot size depends both upon the depositional environment to be dated e. Two types of procedures can be distinguished: additive and regenerative. However, their precision is restricted due to methodological limits the use of several aliquots to obtain only one equivalent dose, obtained by extrapolation of the natural al on a curve derived from various laboratory doses.

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On the contrary, the regenerative procedures provide more precise ages as they make it possible to obtain by interpolation a similar of equivalent dose measurements to aliquot measurements. It was recently developed for quartz grains Murray Luminescence dating laboratory procedures and protocols 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 als for a single aliquot tab. Basically the comparison of the natural al with the artificial luminescence als should make it possible to interpolate the palaeodose directly. The procedure is however more complicated, as artificial irradiation le to the trapping of electrons in unstable traps of the mineral crystal lattice. Since the measurement of the luminescence al occurs immediately after irradiation, these electrons are very likely to be released during the optical stimulation, leading to an overestimate of the al.

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.

Luminescence dating laboratory procedures and protocols

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