Abstract. We have utilised the nm line from a 2 W argon ion laser in application to mineral grains of quartz, zircon and apatite, extracted. Optically stimulated luminescence in kalahari quartz: bleaching of the °c peak as the source of the luminescence. Optical dating: methodological aspects. luminescence signal of quartz: implications of dating. cal stability in order to determine the degree of optical Methodological considerations in the optical .
For young sediments, little signal will have accumulated as grains have been exposed to ionising radiation for only a short periods of time, which can be problematic with regard to the signal-to-noise ratio.
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In particular, this applies to quartz and appears to be related to a lack of dosing and exposure cycles that would cause a sensitisation of the electron traps in quartz Pietsch et al. Thus, while quartz is clearly the preferred mineral for the dating of young sediments, there are several settings where its application is not feasible because in some regions quartz does not exhibit any detectable OSL signal at all, corresponding to immature dosimetric properties of the quartz e.
In such cases, Infrared Stimulated Luminescence IRSL from feldspar might be the only option, but this signal can be unstable with time. This anomalous fading of the feldspar signal will cause underestimation of De, hence of the sedimentation age Wintle, ; Spooner However, it appears that the degree of fading depends on mineral genesis, with plutonic feldspars fading less than those of volcanic origin Visocekas and Guerin, It has been presumed that anomalous fading results from tunnelling of charge that follows strict physical laws, i.
Following these assumptions, Huntely and Lamothe developed a procedure to correct for fading. First, storage experiments are carried out to monitor the loss of signal with time.
The results of such experiments are expressed by g-values, where g is the signal loss in percent per decade, a decade being a factor of 10 in time since irradiation Huntely and Lamothe, The g-value is then used to correct for the signal loss since burial. However, several studies report that the signal loss observed in experiments does not represent the fading of the signal over geological periods e.
The implication is that the fading correction proposed by Huntely and Lamothe might not be accurate as temperature under laboratory conditions might differ from that in nature. Hence, fading correction may tend to either over- or underestimate the true age of a sample, depending on the nature and climate history of a sampling context.
Thermal transfer and recuperation, both representing a transfer of electrons from thermally shallow, light-insensitive traps to deeper, light-sensitive traps, occur when grains are preheated during the measurement procedure to remove unstable components of the OSL signal that are induced by laboratory irradiation Aitken and Smith, ; Rhodes and Bailey, As thermally transferred OSL is not present in the build-up of the signal in nature, i.
The effect of thermal transfer can be monitored by removing the light sensitive signal by light exposure and measuring the signal induced by preheat thermal transfer test.
It is monitored by including a zero dose step in the measurement procedure Murray and Wintle, Overestimation of ages will also occur when the OSL signal was incompletely reset at the time of deposition, as the signal acquired since the event to be dated will add on top of the residual signal.
However, as each grain has its own daylight exposure history, the residual level in grains will differ in the presence of incomplete resetting; grains will be differentially bleached Duller, Hence, measuring several dozen to a few hundred individual grains and investigating the spread will provide information on the degree of bleaching at the time of deposition Li, ; Murray et al.
Differential bleaching will be indicated by a positive skew and a broad spread of the data. The spread of data is expressed by the overdispersion value that describes the variability of the data beyond the one expected from measurement uncertainties cf. Galbraith and Roberst, In this context it has to be noted that usually the spread of single grain values is not just the result of incomplete bleaching, but in particular dosimetric effects Lepper et al.
As this value is not known, it can only be estimated by comparison with similar sediments that are not affected by incomplete bleaching.
However, as this approach is quite laborious, De measurements are often made on aliquots consisting of several dozen grains. Here, it has to be considered that usually only few percent of the grains in particular for quartz exhibit OSL signals e. Hence, using aliquots with not more than grains will almost mimic single grain measurements. However, in particular for feldspar separates a much larger number of grains may exhibit luminescence signals.
In such cases multi-grain approaches can cause averaging of the signal originating from individual grains and mask incomplete bleaching at least to some extent Wallinga, Several previous studies have shown that young aeolian and coastal sediments are suitable for the dating of young deposits using quartz, as partial bleaching has little effect.
There are quite a few studies that have used quartz to date young fluvial deposits e. For example, sediments filling up a harbour basin along the River Danube east of Vienna, which wooden structure has been dendrochronologically dated to the middle of the 16th century, revealed three consistent OSL ages using MAM of around years Fiebig et al.
Repeat step 2 to 5. The SAR protocol applied to coarse-grain feldspars 1.
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 Tx. The palaeodose value for this aliquot from sample LUM is These tests may vary from one laboratory to the other but basically correspond with: 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: 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.
Optical Dating: Methodological Aspects | Radiation Protection Dosimetry | Oxford Academic
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.
Determination of the equivalent dose De using statistical models 14The SAR protocol creates as many equivalent doses as aliquots, with the exception of those which had to be discarded after the tests. In the case of aeolian sediments, all of the analysed grains are assumed to be well bleached, and all the Dehave 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 Dedistribution 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.
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.
Applications and place of OSL in geomorphological research in France 15The physical principles of the optical dating method, and its reliability for quartz and for feldspars from silty to sandy sediments, have resulted in optical dating being applied to a diverse range of sedimentary environments, as described in several journal papers see for example special issue of Boreas 1, The aim of this section is to review the applied representative studies dealing with OSL in France.