Electron paramagnetic resonance EPR spectroscopy is a powerful and versatile technique that is frequently used for quantitative and qualitative analysis in a wide range of scientific fields such as chemistryphysics, biology, medicine, geology and archaeology. It can be especially useful for the characterisation of matter, providing information about the nature of the paramagnetic species present, as well as the structure of their local environment.
This numerical dating method is based on the study of the radioactive decay of 14 C in organisms after their death and may provide accurate ages for samples containing organic matter like fossil bones or charcoals. Radiocarbon is usually classified as a radiometric dating method, which corresponds to a group of techniques based on the measurement of "Electron spin resonance dating archaeology definition" radioactive decay or production of specific radioelements e.
But there is also another group of dating approaches that are based instead on the evaluation of the effects of natural radioactivity on some materials over time, which are quantified in terms of the radiation dose absorbed i. These are usually called palaeodosimetric or trapped charge dating methods, mainly based either on the study of radiation-induced luminescence, e. The first application of EPR for a dating purpose was carried out during the mids on a stalagmite from a Japanese cave, 1 about 30 years after the discovery of EPR by E.
Since then, numerous dating applications have been developed on many different materials such as silicates e.
A quite complete overview may be found in Reference 2. The first studies on fossil bones were published in the early s, however, these were then naturally oriented towards the teeth, since enamel was rapidly found to have more suitable characteristics for dating.
Since then, the method has progressively gained in accuracy over the following decades, especially via a better understanding of the EPR signal of fossil enamel and of its behaviour with the absorbed dose, as well as of the modelling of uranium uptake into dental tissues.
The objective of this article is to explain how EPR may be converted into a dating tool for fossil teeth. By definition, EPR spectroscopy is a technique designed for the study of paramagnetic species, i. These may be either naturally present in some materials, or created by physical or chemical reactions.
With regard to the latter, some materials can acquire paramagnetic properties under the effect of radioactivity. The interaction of ionising radiations with the matter may induce modifications of the electronic structure of some materials: These species create a signal that can be detected by EPR spectroscopy, the intensity of which is directly dependent on the amount of trapped charges in the crystalline network, reflecting thus the dose absorbed by the material.
EPR spectroscopy is now widely recognised as a reference technique for routine dosimetry by many international institutions [e. Over the last decades, many applications have been developed, including those for post-accident dose reconstruction in the environment, biophysical dosimetry using human tissues, to identify irradiated "Electron spin resonance dating archaeology definition," and some of them, such as Electron spin resonance dating archaeology definition alanine dosimetry, have reached a high-level of standardisation.
Dosimetric properties of tooth enamel have been known for more than 40 years. Hydroxyapatite, the main component of bones and teeth, is especially sensitive to ionising radiation: It is now internationally accepted as a valuable natural EPR dosimeter, and is commonly used in the field of retrospective dosimetry for persons accidentally exposed to ionising radiation. An extensive review on this aspect may be found in Reference 4.
From a mineralogical point of view, tooth enamel is mainly made of carbonated hydroxyapatite [Ca 10 PO 4 6 OH 2 ] like dentine or bones. These characteristics make tooth enamel especially stable over time, i. The EPR signal associated with fossil hydroxyapatite is an asymmetric composite signal. The main radiation-induced signal is defined by three peaks T1, B1 and B2, see Figure 1. Many contributions to this signal have been identified, mainly carbonate-derived radicals and some oxygen radicals, 4 but the major contribution comes from three kinds of CO 2 — radicals, whose precursors are very likely the carbonate groups CO 3 2— present in the hydroxyapatite.
Once an animal is dead, its teeth become progressively buried by sediment during the geological periods, during which the enamel is naturally exposed to ionising radiations from cosmic rays and from the radioactivity of the sediment and the tooth itself.
This natural radioactivity is due to the radioelements, mainly U-series, Th-series and 40 K elementsthat are not only naturally present in the sediment, but are also progressively incorporated into the dental tissues. Ionising radiations emitted by these radioelements are alpha and beta particles as well as gamma rays Figure Electron spin resonance dating archaeology definition. Together with cosmic rays, they contribute to build up a dose in the enamel over time, the magnitude of which will mainly depend on two main parameters: This relationship may be converted into an EPR age equation as follows:.
This work is carried out in two different ways: To obtain an accurate evaluation of the total dose rate, it is important to divide it into several components. The specificity of teeth dating relies on the complex system that has to be considered, because a tooth is usually Electron spin resonance dating archaeology definition by several tissues enamel, dentine and, sometimes, cement; Figure 2having various thicknesses and composition.
The geometry of the enamel and its surrounding thus has to be considered in the dose rate reconstruction. In the case of a tooth with an enamel layer surrounded by cement and dentine, the dose rate equation may be expressed as follows:. Consequently, with this specific configuration, the internal dose rate within the enamel comes from alpha and beta particles, while the surrounding Electron spin resonance dating archaeology definition only provide an external beta contribution.
The gamma rays contribution only comes from the sediment, since the absorption by the enamel of the gamma rays coming from the enamel itself and the other tissues can be neglected.
In the case of a tooth with an enamel layer in direct contact with the sediment on the outer side i. Dental tissues are usually assumed to be free of Th and 40 K, since their incorporation into the crystalline network is very complicated, owing to their mobility and atomic radius, respectively.
Consequently, the dose rate components associated to dental tissues are directly, and only, dependent on the uranium concentration. However, dental tissues behave as open systems for U, i.
It is therefore crucial not only to measure the actual U-content but also to know its evolution in the past. Indeed, one may intuitively understand that the total dose absorbed by "Electron spin resonance dating archaeology definition" enamel will be somewhat different if the uranium was accumulated in the dental tissues shortly after the death of the animal or if it happened only very recently.
The US model defined by these authors is based on the following equation:. Examples of dating applications may be found in Reference 9. An EPR age estimate is the result of a long analytical process, made by five main steps associating fieldwork and laboratory procedures: Fossil teeth are usually collected either on site or chosen from collections.
Large mammal teeth, and especially from herbivores, are usually preferred, since they offer a thicker enamel layer. Then, in situ measurements of the natural radioactivity at the exact place where the sample was collected during excavations, or at least the closest possible, is carried out to evaluate the gamma dose rate.
Classically, various techniques may be employed: In the laboratory, the fossil tooth is prepared by separating mechanically each dental tissue.
The enamel layer is then cleaned, usually with a dentist drill, and gently powdered, in order to avoid significant angular dependence of the EPR signal within the resonator and to improve sample homogeneity. This is why EPR must be considered as a destructive dating method. Each aliquot is then measured at room temperature by EPR spectrometry in order to study the behaviour of the EPR signal with the increasing dose values see Figure 1.
Routine quantitative measurements are usually performed by X-band EPR spectrometry, since it offers a good compromise between sensitivity and measurement repeatability in comparison with higher frequency bands. The experimental setup for quantitative EPR measurements is specifically designed to ensure the stability of the system, including air conditioning and chiller to control the temperature of the water circulating in the magnet. Measurements are thus performed under controlled experimental conditions and following a standardised analytical protocol, in order to minimise any sources of uncertainty that could affect the repeatability of the measurements see further details in Duval et al.
EPR intensities are Electron spin resonance dating archaeology definition extracted from each spectrum, usually by peak-to-peak measurements between T1 and B2 Figure 1 and plotted vs the irradiation doses in order to obtain a growth curve or dose response curve.
A given function, usually a single saturating exponential or a double saturating exponential function, is fitted through the EPR experimental data points. By definition, this function is supposed to describe the behaviour of the radiation-induced EPR signal of tooth enamel since the death of the animal i. If the gamma dose rate is assessed in situthe beta dose rate from the sediment if it applies should preferably be assessed in the laboratory from the sediment sample that was collected around the tooth.
Various laboratory analytical techniques may be used to calculate the radioelement contents of the sediment, e. Other techniques, like beta counting for example, may directly provide a total beta or gamma dose rate value. To do so, mass spectrometry techniques are now usually employed [e. This is done via tables, and the value depends on the depth of Electron spin resonance dating archaeology definition sample, as well as the density of the sedimentary matrix, latitude and altitude of the site.
EPR age calculation of fossil teeth is not so straightforward since it may involve up to
Electron spin resonance dating archaeology definition parameters.
In addition to this, the dose rate in dental tissues is not constant over time but has to be modelled from the U-series data collected. An EPR age may be obtained by iteratively solving the integral Equation 1since there is only one solution for which the total dose rate built up over time will match the D E value. Not every chronometric dating method can be used on a given Prehistoric site, since by definition it depends on the presence or absence of suitable materials for this purpose, which is closely related to the geological context and the presumed age of the site.
Figure 3 shows the time range applicability for some of the most used dating techniques in Quaternary studies.