Optically stimulated luminescence dating of fluvial deposits

Recent advances in OSL techniques and the development of single‐grain dating capabilities have now allowed fluvial deposits, and other.
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It is reasonable to assume that the sample was sufficiently well bleached if the OSL age estimates are in agreement with the independent ages. However, such a test may not be appropriate if the sample of interest is much younger than the sample with the verified OSL age. Small age offsets are concealed by the uncertainties in the age measurements in older samples but nevertheless may be significant in younger material. Age determination in modern samples see Stokes et al, ; Colls et al, ; Jain et al, a.

Although important for dating studies on recent samples, this approach may not be very relevant to studies extending back to the Late Pleistocene, because present day bleaching conditions may not have applied in the past. There is a rapid decay of trapped charges n that give rise to luminescence L. In full daylight, the quartz OSL is reduced by about 4 orders of magnitude in s Godfrey Smith et al.

The saturation of trapped charge determines the upper limit of the dating range, c Dose estimation in a single aliquot using the Single-Aliquot Regenerative-Dose protocol SAR; Murray and Wintle, The cycle is repeated for different laboratory irradiations. The reproducibility of sensitivity corrected OSL is checked by measuring a same dose twice R2 and R6.

Any thermal transfer signal, giving rise to possible error in the dose estimation, is measured by reading OSL without giving any dose R5. A comparison of dose distributions measured from small ali- quots of a fluvial quartz and b aeolian quartz reproduced from OUey et al, OSL in the sediment. It is also important to realise that methods are all concerned with identifying incomplete bleaching and asserting its significance.

They do not offer solutions to the problem if it is identified. On the other hand, methods 4 and 5 attempt to identify those grains within a bulk sample which received the most light exposure and thus which are likely to record a dose proportional to age. Dose distribution studies, in particular, seem to form a very promising research area. Nevertheless, the examination of modern samples offers an important indicator of the degree of zeroing found in various depositional environments.

Over the last few years, it has become possible to examine the question of incomplete bleaching in more detail because it is now possible to measure doses in very small sub-samples aliquots , down to individual sand-sized grains. Here we give a brief description of the OSL dating procedure and then go on to compare the dose distributions found in modern and older samples; most of the data was obtained using recently developed dating techniques which employ small aliquots and individual sand grains.

Only samples with some independent age controls are discussed. Nevertheless, many stratigraphie studies are only able to make use of modern analogues to help assess the degree of bleaching in older material. This method employs the differences in bleaching rates of different OSL components to identify less completely zeroed materials.

Poor zeroing is thus reflected in the shape of the OSL decay curve. Unfortunately, this assumption can be complicated because the shape of the OSL signal may be governed by factors other than incomplete light exposure Jain et al, b; Bailey et al, Analysis of the dose distribution Murray et al, ; Murray and Roberts, ; Olley et al, ; Jain et al, , c; Lepper et al, ; Fuchs and Lang, ; Zhang et al, This approach assumes that, in an incompletely zeroed sample, the dose distribution will be skewed to higher doses if the sub-sample size is reduced sufficiently to make different subsamples also called aliquots heterogeneous in terms of the number of poorly bleached and well bleached grains they contain.

Generally, the lower dose end of such a distribution is relatively free of poorly-bleached grains and hence is closest to the real age. Interpretation of the dose distribution, however, may be complicated by micro-dosimetry and our ability to measure dose accurately, especially in very small subsamples Jain et al, ; furthermore the sensitivity of this method in detecting completely reset grains in older samples remains unclear.

In general, laboratory based studies suggest that residual luminescence signals should be significant, whereas most direct determinations of doses from modern environments, suggest a more complete resetting Stokes et al, Although, each of these five methods has its own drawbacks, a convergence of results from different approaches should give a reliable assessment of prede- positional zeroing of the trapped charge giving rise to. The use of OSL to estimate the time elapsed since sediment burial Huntley et al, is based on the premise that daylight exposure released electrons from light-sensitive traps lattice defects in the crystal structure prior to sedimentation fig.

Quartz and feldspars are the most commonly used minerals in OSL dating.


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Immediately after burial cut off from light , energy begins to be absorbed from the natural radiation flux in the sediment fig. The radiation comprises of alpha a , beta P and gamma y rays originating from the radioactive decay of Uranium, Thorium, their daughter products, and the radioactive isotope of Potassium '"'K , and cosmic rays. Interaction of this radiation with the crystal results in creation of free charge ionisation. Some of this free charge is trapped at lattice defects traps where it can remain for some period of time depending on the trap depth.

The trapped charge density at any point is thus a function of time. Generally the process of trap filling eventually saturates because of the limited number of defects in the crystal fig. In the laboratory, the sample is stimulated with light green, blue or infrared. Eviction of charges from traps and their subsequent recombination at luminescence centers results in emission of light, usually at a higher frequency than the stimulation light. Calculation of age then involves calibrating this light output in terms of an estimate of total energy per unit mass dose absorbed by the crystal, and an estimate of the rate at which this energy is absorbed during the burial history.

The OSL from the crystal is proportional to the number of trapped.

References

The dose rate, or the total dose absorbed in one year, can be calculated by measuring the radionuclide U, Th, K concentration in the sample and estimating the cosmic ray radiation at the particular latitude, altitude and sample depth. The absorbed dose rate is then corrected for average water content during the history of the sediment. Each of these estimates effects the precision of the obtained ages see Aitken, for details.

It is usually assumed that the dose rate remained constant during the lifetime of the sample, one of the positive indications of this is secular equilibrium in the activities of various daughter nuclides of U and Th. Calibration of OSL in terms of dose has undergone significant refinement as our understanding of OSL processes has improved. In principle, it requires measurement of a luminescence - dose response curve in the laboratory so that the luminescence in the natural sample can be converted to dose using the so obtained sample- specific dose response curve fig. In earlier methods, generally termed multiple-aliquot techniques, measurements of luminescence both natural and laboratory induced used several separate sub-samples aliquots from the same sample.

Landauer OSL Technology Movie

This was because samples usually undergo changes in the relative importance of charge recombination pathways sensitivity changes after the measurement of the natural OSL and so, it is not possible to make repeated measurements on the same sub-sample see Aitken, and Jain et al, d for status reviews on multiple aliquot dating. The biggest drawback of all multiple aliquot techniques is in the case of poorly bleached samples; any multiple-aliquot age on a poorly bleached sample is inevitably an average of several thousands of grains, many of which will overestimate the true age.

Research over the last decade Duller, ; Mejdahl and Better- Jensen, ; Wintle and Murray, ; Murray and Wintle ; Murray and Roberts, have helped overcome this problem of sensitivity change, thereby allowing the possibility of repeated measurement on the same aliquot and even the same grain. The most widely used method is often referred to as the single- aliquot regenerative-dose SAR protocol Murray and Wintle, ; see fig. Because it makes all measurements needed to estimate the dose on a single subsample, it makes possible the rejection of grains that had significant residual doses in them prior to deposition see Jain et al, c; Zhang et al, ; Lepper et al, ; Fuchs and Lang, for various dose selection criteria from single aliquot and single grain dose results.

One also needs to be aware that when dealing with samples only a few hundred years old, some dose offsets can be caused by laboratory thermal treatment resulting. The OSL technique can date samples from few tens of years to about 1 Ma.

Pré-publication

The lower end of this range is governed by photon counting statistics which in turn depends on the sensitivity i. The upper end of the range is controlled by saturation of traps, and by stability of charge in the traps. It is therefore a function of the dose rate and the total elapsed time; generally the upper range is about ka for quartz and up to 1 Ma for feldspars. Unfortunately, feldspar dating is also complicated by an unexpected loss of charge from the traps, a process called anomalous fading Wintle, , which often leads to significant age underestimates.

Since the advent of these procedures involving single- grains and single-aliquots, a few studies have been made on modern and older fluvial sediments with independent age control. These methods have made it possible, at least in principle, to detect partial bleaching either by using the relationship between aliquot luminescence intensity and equivalent dose De or the actual distribution of De estimates e. Some of these studies, using quartz, are reported in recent technical reviews by Murray and Olley and Wallinga The present paper focuses on the detection of incomplete bleaching in modern and ancient fluvial samples using quartz.

There are several advantages in using recent samples to assess the importance of location- and environment-specific bleaching conditions:. Due to the obvious advantages of this approach, there has been a strong tradition in TL and OSL studies to use modern reference samples as proxy indicators of the bleaching conditions in the past, especially where no other chrono-stratigraphic control exists.

This section discusses results obtained from fluvial deposits younger than years tab. The dose estimates reported here have been calculated using the following protocols listed in the order of frequency of usage and the extent of averaging involved in the measurement of dose:. Sensitivity changes occurring due to lab heat pre-treatment are monitored after each OSL measurement and corrected for fig.

Single aliquots are of varying sizes, typically between and grains, the limiting case being a single grain. The proportion of light producing grains in different quartz samples generally varies between 0. There have been various earlier versions or simplifications of SAR, when dealing with modern and very young samples.

Luminescence Dating of Fluvial Deposits from the Weser Valley, Germany : Geochronometria

In some of these, sensitivity correction is not used, or only one dose point is used for construction of the dose response curve Murray et al, This is justified as the dose response curve is linear at low doses, and sensitivity changes are likely to be small at the low preheat temperatures used for modern samples this is intended to avoid any thermal transfer. In this procedure a minimum of two aliquots usually many more are used for construction of the growth curve, hence some averaging effect is inevitable.


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Due to the time involved in laboratory irradiations and measurement usually only one dose estimate is made; this is then considered as representative of average dose in the sample. These techniques have been largely superceded by SAR, especially in the measurement of sand-sized quartz grains. In the following sub-sections we discuss case studies on a samples taken from the modern depositional context considered as zero age samples ; in most cases these represent-flood or in-channel deposits that formed in last few years, and b samples with known ages in last years.

Samples taken from modern depositional contexts: Ages based on the average OSL from multiple grains from modern fluvial samples range over four orders of magnitude from 4 a to 12 ka, with the majority of results lying below 1. They attributed this low residual age to good daylight conditions, and a coexistence of active fluvial and aeo- lian processes in the semi-arid tropics. Unpublished dose estimates Jain et al. Both the samples show a dose distribution centered on zero. Calculation of age from these samples has assumed dose rates from unpublished data M. Jain and from Rittenour et al However, such well-zeroed dose populations are unfortunately not very common.

The average ages obtained on all the results showed a range between 95 to years, while the minimum age estimates were between 85 and years. Partial bleaching in these samples was attributed to flashy, seasonal flow associated with high - amplitude, low- frequency storm events. In the Loire, France, Colls et al. In another study on Loire sediments, Stokes et al.

The dose distributions derived using a simplified SAR method Murray et al, , were all found to be positively skewed. The source sediment had an average residual age of about years; after about m of transportation, this reduced to 1 , years, and after about km of transport, the mean age reduced to about years fig. The minimum residual ages varied between years at the source to 77 years at km.

In general, the mean, median or minimum dose estimates reduced rapidly over the first km and then varied relatively slowly in the remaining km. Stokes et al found a slightly greater degree of scatter in residual ages from the samples in the urbanised catchment reaches of the Loire, and speculated that lack of complete zeroing is perhaps due to input from bank erosion and exposed bedrock or building materials from the urbanised areas.

Better bleaching in Loire as compared to that in Colorado was attributed to more stable flows and rare, extreme flood events Stokes et al, Excluding the source proximal river sediment from Loire all the above studies give residual ages consistent with a range from zero up to years.


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Good bleaching in these systems is perhaps because they are sand dominated, low-latitude river systems; these conditions help to ensure effective bleaching during transport because of low turbidity, due both to low turbulence and the dominance of sand grade load, and good daylight conditions. Pioneering studies examining partial bleaching using single aliquots and single grains have come from various Australian river systems Murray, ; Murray et al, ; Olley et al, , Murray et al examined the scale dependence of the residual doses in 4 modern samples from fluvial systems dominated by turbid flood events.

Their small aliquot dose distributions were positively skewed with some results that were consistent with zero. Average residual ages varied between 60 to years; the extreme cases were an overbank and a small gully deposit, respectively. Channel deposits from the much larger scale Murrumbidgee and Barwon rivers had intermediate bleaching characteristics. The data of Murray et al. While the former hypothesis needs validation, clear support for better bleaching with transport distance comes from Stokes et al Average and minimum residual OSL ages measured in R.