Extracts from a talk given at the symposium "Neanderthals in Europe" held in the Gallo-Roman Museum in Tongeren, Belgium, 17th - 19th September 2004.
The techniques for dating archaeological materials can be divided into two classes; those which provide absolute dates and those which give relative ages. Absolute dating methods yield results directly in terms of calendar years. The principal absolute dating techniques used for Neanderthal sites include uranium series disequilibrium dating, thermoluminescence (or TL) and electron spin resonance (or ESR) dating.
In contrast to these, relative dating methods provide floating chronologies, which require a process of calibration before their findings can be presented as calendar dates. Methods that require calibration include radiocarbon dating and amino acid racemisation. While relative dating methods are valuable in many cases, it is clear that calibrated results cannot be more precise or reliable than the absolute dates against which they are calibrated.
The uranium series method is most commonly used to date speleothems. In many archaeological cave sites, in situ stalagmitic floors provide easily identifiable marker horizons. These floors indicate an interval of time between the emplacement of the underlying sedimentary unit and the deposition of the overlying unit. It is often difficult to measure the ages of cave deposits directly, but the stalagmitic floors can be dated with relative ease, and will usually provide a reliable chronology of the emplacements.
The TL technique is also applicable to dating stalagmites, but is additionally used to date the heating of flint and stone by fire, and the deposition of sediments. When dating flint and stone it is sometimes difficult to establish whether the heating was caused by human agency or by natural fire. However, it is certain that, in order for the material to reach the necessary temperature, it must have been lying on the surface. Therefore, at sites where flint débitage has been rapidly buried, the information provided by TL dating can be directly related to the industry.
In the case of sedimentary material the dated event is the exposure to daylight that often accompanies its deposition. The necessary light exposure can only occur if the sediment is transported in a dispersed state over some distance. Thus, the TL method is well suited to dating loess depositions, but can also be applied to fluvial and colluvial sediments. In addition, we find that it can be used to date the formation of palaeosols, since the constant cycling of material to the surface by bioturbation eventually results in the whole body of the soil being exposed to light.
The ESR method is closely related to TL, and has been applied mainly to dating teeth.
Let us remind ourselves what the date measurement means. The information that the archaeologist receives from the dating laboratory is summarised by two numbers. The first is the "age" of the dated event. This should be understood as the central value of the age range which is indicated by the date measurement. The second number gives the half-width of the measured age range, often referred to as the "error". Adding the two numbers gives the upper limit of the age range, and subtracting them gives the lower limit. The meaning conveyed by the two numbers is that there is a 68% probability that the actual date of the event lies between the upper and lower limits of the age range. (It should be noted, however, that radiocarbon laboratories normally quote date limits which correspond to a probability of 95%.)
Using this information, it is theoretically possible to estimate the likelihood that, for instance, one site predates another. Alternatively, the archaeologist may want to compare the date information for the human occupation with other measurements attached to environmental and climatic data. For these purposes, it is important that the age range indicated by the date measurement should be as realistic as possible. The question therefore arises: Do the error limits express all the uncertainties which affect the date measurement?
The second category includes uncertainties which are less easily quantified. In all dating procedures, there are complicating factors which adversely affect the measurement of the true age. In uranium series dating, attempts are sometimes made to estimate the effect of the Th-230 which was already present in the speleothem at its formation, and to correct the measured dates accordingly. In TL dating, we need to consider how radiation levels may have varied in the past and to estimate the effect of such variations on the calculated date. All of these corrections carry an additional uncertainty which should be included in the error limits of the final date.
There is a third category of uncertainties which dating laboratories are unable to quantify, and which are therefore never expressed in the error limits of the age. These uncertainties result from failures of the basic assumptions on which the dating method rests. Let us examine these more closely.
It is worth remembering that dating laboratories do not measure time directly. Instead, they measure such things as isotopic ratios and luminescence intensities. Time is an inference which is drawn from these measurements, based on assumptions about the initial state of the measured system, and the manner in which the system has evolved from its initial state. It follows that technical advances in our ability to measure isotopic ratios or signal intensities do not automatically lead to improvements in dating accuracy.
What are the basic assumptions? In uranium series dating, and also in radiocarbon dating, it is assumed that the sample has behaved as a perfect time capsule throughout its history; in other words, that there has been no movement of the relevant isotopes into or out of the sample.
In TL dating, various assumptions are taken depending on the material under examination. When dating heated flint, we assume that the sample was not reheated at a time significantly later than its use. In the case of stalagmite, there is the possibility of the material undergoing recrystallisation after its initial formation. If this has happened, the TL date may refer to the recrystallisation, rather than to the original formation of the sample. The dating of sediment is based on the assumption that the material was exposed to light at the time of its deposition. However, some mechanisms of deposition do not result in an adequate exposure, and the TL date will then refer to an earlier transportation of the material.
In all these cases, it is not possible to detect failures of the basic assumptions directly, or to quantify the uncertainties that they transfer to the date measurement. However, this does not mean that we are unable to make progress in reaching a more realistic understanding of the uncertainties surrounding date information. Fortunately, a way forward is suggested by the fact that, while all date measurement techniques are subject to unquantifiable uncertainties, they are not affected in the same way. The methods can, in varying degrees, be regarded as independent of each other. To the extent that the errors are uncorrelated, comparisons between the results of different dating techniques can reveal the hidden uncertainties that affect each of them.
In conclusion, I would emphasise that the production of date measurements is essentially an impossible scientific task, because much of the information required to produce the result is inaccessible to measurement. Therefore, we should understand that no dating technique has a monopoly of reliability. However, use of the widest possible range of dating methods provides a means of detecting the hidden uncertainties that affect all of them. There are significant benefits when dating Middle Pleistocene sites of employing a number of different methods. When we are able to compare dates from several sources, the archaeologist gains a more truthful account of the age of the site, and the dating laboratories gain a greater understanding of the limitations of their techniques.
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