Quaternary TL Surveys

Guide to TL Date Measurement


A common feature of cave stratigraphies is the stalagmitic floor, which indicates an interval of time between depositional events coinciding with conditions favourable for calcite formation. Subsequent depositions may break up the floor or leave it intact. Where the formation remains in situ or substantially undisturbed the floor acts as a natural stratigraphic marker sealing the underlying deposits. The dating of stalagmitic calcite therefore provides a valuable method for understanding cave chronologies. Archaeological levels that are situated between two stalagmitic floors may be effectively dated within the time sequence.

The first section of this chapter forms a general introduction to the subject of stalagmite dating by TL. Considerations affecting the selection of samples for TL dating are then described. The measurement of radiation dose rates is dealt with in the following section, with particular emphasis on the problems of environmental dosimetry in caves. Palaeodose measurent procedures are then explained, and the special considerations attached to date computations for stalagmitic samples are also described. Finally, the practical age range of the TL method applied to calcite is discussed.

3.1. General Remarks

In common with many crystalline materials, calcite exhibits a TL signal whose intensity increases with exposure to ionising radiation. This TL signal is absent from newly-formed stalagmite, but grows thereafter in response to the low levels of naturally occurring radiation in its environment. The present day intensity of the TL is thus a measure of the accumulated radiation dose, or palaeodose, to which the stalagmite has been exposed since it was formed.

In order to convert the measurement of the total radiation exposure into a calculation of the age of the stalagmite, the rate at which the dose was received must be evaluated. In the vicinity of calcite formations, radiation dose rates can vary markedly in space, and the evaluation of them is subject to uncertainty. For this reason, the precision of a TL date often depends primarily on the situation in which the stalagmite was found. Under favourable conditions, overall error limts of ±10% (for 68% confidence level) can be obtained, but ±12% is more common in practice, and in difficult cases ±15% may be the best obtainable. The problems of dose rate assessment are described more fully in section 3.3.

The age range over which the technique can be applied is governed by several factors. In general terms, Holocene formations would be identifiable as such by TL, but would not be precisely datable. The useful date range extends from approximately 20,000 years to 300,000 years BP. The factors which limit the date range are set out in section 3.6.

3.2. Selection of Stalagmitic Samples for TL Dating

The suitability of a sample of stalagmite for TL dating is determined by the condition of the calcite itself and by the burial situation in which it is found. As a first consideration, the material should not be in the form of aragonite; when heated, aragonite converts to calcite, and in the process emits light which obscures any TL signal. In similarity with uranium-series disequilibrium dating, detrital contamination of the stalagmite, re-crystallisation and geo-chemical alteration are all unfavourable conditions for TL dating. The amount of contamination that is tolerable in TL dating depends on the intensity of the signal emitted by the detrital material. Visual inspection of the sample therefore provides little information for judging its suitability. From comparative studies between the TL and uranium-series dating methods, it appears that TL dates are less sensitive to sample contamination and geochemical action. However, this general statement may be subject to individual circumstances. In doubtful cases, a simple TL examination will rapidly show whether the material will be datable.

3.3. Radiation Dose Rate Measurements in Caves

While the condition of the stalagmite itself will often determine whether a TL date can be obtained, it is usually the situation of the calcite in the cave which limits the precision of the date. The basic problem arises from the inhomogeneity of the radioactivity in the cave deposits. The concentrations of the natural radioactive elements of uranium, thorium and potassium are generally much higher in cave sediments than they are in stalagmite and limestone.

Radiation dosage arrives in the form of alpha, beta and gamma rays, with an additional contribution from cosmic rays. In underground sites, the amount of cosmic radiation is low. Alpha and beta rays have ranges of approximately 0.03 mm and 3 mm, respectively, and therefore the dose which is delivered by these rays to the TL sample originates mainly in the calcite itself. Because of the relatively low radioactive content of the calcite, the dose from alphas and betas is usually a minor part of the total palaeodose. The assessment of the alpha and beta dose rates is termed the internal dosimetry.

Gamma rays have ranges of approximately 0.3 m in geological materials. Consequently, thin formations of stalagmite that are buried within cave sediments receive most of their gamma dose from the surrounding deposits. Since these deposits contain relatively high concentrations of radioactive elements, the gamma dose is usually the largest component of the palaeodose, and the precision with which it can be measured is likely to dominate the date error limits. The measurement of the gamma dose rate is performed with a portable gamma spectrometer which determines the concentrations of uranium, thorium and potassium in the burial environment. This is referred to as the environmental dosimetry.

As in all forms of TL dating, successful application depends on the preservation of representative sections of the sediments in which the TL samples were found. In the case of cave sites, this requirement may be more difficult to fulfil because of the limits set to the extent of the deposits by the cave walls. Preserved sections ideally need to stand at least 0.6 m away from cave walls so that the environmental dosimetry is not influenced by the lower radioactivity of the limestone.

Two ideal cases present themselves for the environmental dosimetry. In the first case, the stalagmitic floor is thin (less than 5 cm thick), and lies between extensive bodies of homogeneous deposits. Here, the uniform gamma dose rate within the burial sediments is not greatly affected by the presence of the less radioactive calcite, and precise estimates can be made of the spatial variations of dose rate within the floor. In the second case, the stalagmitic formation has a much larger extent (0.6 m or more across). By coring a formation of such size a TL sample can be extracted from its centre and gamma dose rate measurements taken at the same location. Because of the large size of the stalagmite, the gamma dose rate at its centre results from the radio-active content of the calcite alone, and is not altered by the removal of surrounding sediments.

In practice, these ideal configurations are rarely found in caves, and TL samples usually have to be taken from stalagmites which are of intermediate dimensions. In these cases, there will be significant spatial variations in the gamma dose rates. In order to understand these variations, computer modelling of the dose rates can be performed, constrained by gamma measurements at fixed locations and laboratory analyses of the geological components.

Obtaining favourable conditions for the environmental dosimetry requires a close co-operation between the excavator and TL specialist. This is true for all TL dating projects, but the complexities of stalagmite dosimetry demand special attention.

3.4. TL Measurement Procedures for Calcite

The calcite TL dating signal is observed as the sample is progressively heated. The variation of TL intensity vs temperature is called the glow curve. Figure 1 shows the distinctive shape of the calcite glow curve. The height of the TL peak at 275°C is used in the dating measurements.

[ Figure 1 ]

Figure 1. Glow curves of a sample of stalagmitic calcite, showing the characteristic peaking of TL intensity at 275°C temperature. The glow curve of natural TL results from the stalagmite's accumulated exposure to natural radiation since its formation. Also shown are glow curves obtained after the calcite has been given various radiation doses, measured in Grays (Gy), in addition to the natural dose, N.

The term natural TL refers to the TL emitted by the untreated stalagmite, i.e. that which results from the exposure of the sample to natural radiation during its history. By adding extra radiation doses in the laboratory, increased levels of TL are induced in the sample (see figures 1 and 2). The graph of this increasing TL vs radiation dose is termed the first glow growth curve.

The heating of the calcite during TL measurement releases the excess energy in the crystal which gives rise to the light emission. Therefore, a re-heating of the crystal after first glow produces no further TL emission. However, if the calcite is given an artificial radiation dose prior to its second glow, an induced TL signal is recorded. Such second glow measurements are used to observe the growth of TL vs radiation over a wide dose range. The TL growth in calcite samples is invariably found to be non-linear. An example of a second glow growth curve obtained from calcite is shown in figure 2.

[ Figure 2 ]

Figure 2. Curves describing the non-linear growth of TL intensity with increasing radiation dose. The lower curve is fitted to the second glow data points (crosses). The upper curve has the same basic form as the lower curve, but is fitted to the first glow data points (squares) which include measurements of the natural TL. The value of the palaeodose (or natural radiation dose) is obtained where this curve meets the dose axis.

The evaluation of the palaeodose must take account of the non-linear growth of TL vs dose that has occurred during the history of the stalagmite. To do this, the first glow data points are extrapolated using the form of the second glow growth curve. Mathematically, this amounts to shifting the second glow growth curve in the horizontal direction and multiplying its vertical scale by a constant factor. The horizontal shift corresponds to the off-set of the first glow data, which is simply the palaeodose, while the vertical scaling factor corresponds to the sensitivity change that the material suffers on its first glow. By adjusting these two parameters to achieve an optimum fit, the value of the palaeodose is obtained. This procedure is illustrated in figure 2.

3.5. TL Date Computations for Stalagmites

When computing the TL age of stalagmitic calcite, allowance must be given not only for the non-linear growth of TL vs radiation dose, but also for the fact that radiation dose rates in the calcite vary through time. The cause of this variation is the disequilibrium of the uranium radioactive chain, which is the basis of the uranium-series dating method. Water, which carries constituents from which the stalagmite is formed, readily transports uranium in solution but not thorium. At formation, therefore, no thorium is present in the stalagmite (except that which is contained in detrital particles that become trapped in the calcite matrix). Over time, thorium-230 appears in the calcite as the decay product of uranium-234, and, accompanying it, those elements which are below it in the decay chain. The result is a gradual increase in the rate at which the uranium-series elements inside the stalagmite contribute to the total radiation exposure of the sample. The adjustment of the TL date that allows for this effect is referred to as the thorium grow-in correction.

For thin stalagmitic floors, this correction affects only the alpha and beta contributions to the palaeodose, since most of the gammas derive from the surrounding sediments. However, where large stalagmites are concerned, higher proportions of the gamma dose rate are also subject to this effect.

3.6. Limitations on the Age Range of Datable Stalagmites

The age range over which the TL dating technique is applicable to stalagmitic calcite is limited by several factors. In young stalagmites, dating precision is limited if the intensity of the natural TL signal is comparable to that of other luminescent processes. Competing background light emissions are contributed by detrital contamination and spurious luminescences. The latter are non-radiation induced emissions which are produced chemically by reaction of the heated calcite with oxygen and water vapour. Spurious light is reduced by heating the sample in an inert atmosphere such as nitrogen, and by introducing a drying agent into the TL oven. During preparation of the sample, the stalagmite is crushed. By selecting a relatively large grain size (e.g. 100 µm) the ratio of surface area to volume is reduced, and this also suppresses spurious light emission. Natural TL intensity depends on the age of the calcite, the dose rate in the cave, and the intrinsic TL sensitivity of the material to radiation. Thus, a high dose rate and high sensitivity favour the dating of young stalagmites. With so many factors governing the relative intensities of the natural TL and the competing background, it is difficult to give a lower age limit for calcite dating. However, Holocene formations will generally be too young for useful application.

The upper age limit of the technique is not well established. TL saturation occurs when the TL intensity no longer grows in response to additional radiation doses. In calcites, saturation does not occur until very high doses are reached, and is therefore unlikely to emerge as the limiting factor in the dating of old stalagmites. The stability of the TL signal over long periods is considered to be a more significant factor in setting the upper age limit of the method. However, a reliable estimate of the maximum age has yet to be determined. In part, this is due to the difficulty in obtaining samples of stalagmite older than a few hundred millennia which have not suffered geochemical alteration, and in practice it may be this last factor which realistically sets the upper age limit.

Queries and comments on TL dating procedures and the above literature are welcome.

E-mail n.debenham@qtls.globalnet.co.uk

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