Electron spin resonance (ESR) dating is a trapped charge dating method. to bones and teeth and is therefore used for the direct dating of human fossils. ( we use the radioactive isotope K for dating as well, but this is another story). Unlike radiometric methods based on the measurement of radioactive growth or Indeed, for Electron Spin Resonance (ESR) dating of tooth enamel, the origin. Dating fossil teeth by electron paramagnetic resonance: how is that Radiocarbon is usually classified as a radiometric dating method, which.
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 made 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 tissues 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 40K, since their incorporation into the crystalline network is very complicated, owing to their mobility and atomic radius, respectively.
Dating fossil teeth by electron paramagnetic resonance: how is that possible?
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 the 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. Standard analytical procedure 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 then 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 situ, the 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 the 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 25 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 DE 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.
- Electron spin resonance dating
EPR is one of the very few dating methods that may be applied to fossil remains. By definition, the dating of a tooth remains provides a direct dating of hominid or animal occupations, whereas other numerical methods can only date the sedimentary matrix that is enclosing the archaeo—palaeontological materials.
Consequently, this application may be also potentially used in any sedimentary context, while other methods like Argon—Argon or cosmogenic nuclides dating can only be used on volcanic minerals and quartz grains, respectively.
In addition, the EPR technique is one of the very few possibilities to date fossil remains beyond the C and U-series dating time range. C, U—Th, Ar—Ar; an overview may be found in Reference 12this is nevertheless one of the few methods that can be used for the Early Pleistocene period 2.
However, external beta and gamma dose rates are very dependent on the sedimentary context of the sample, and have to be determined precisely. If the enamel layer is in direct contact with the sediment on one side, then the beta dose rate has to be calculated from a sediment sample, from which radioactive element contents U, Th and K are measured.
Consequently, the true cosmic dose rate received by the sample decreases according to the thickness of these deposits.
Electron spin resonance dating - Wikipedia
Therefore, it is recommended to report the GPS coordinates of the sampling point. Sampling form for ESR dating of fossil tooth. As explained above, it is essential to collect all the basic information relating the tooth to its surrounding environment to ensure the most accurate dose rate estimation.
This has been summarised in a form addressed to non-specialists in ESR dating e. This includes the location of the tooth within the excavation area and its position within the stratigraphic section.
For the evaluation of the cosmic dose rate, it is necessary to get an estimation of the thickness of the overlying deposits.
Sometimes, the actual situation is not representative of the past history, especially if erosion took place and removed most of the sediment cover. In this case, the maximum sediment thickness above the sample should be estimated as well. Nevertheless, as far as possible, the gamma dose rate should always be derived from in situ measurements.
Sometimes, fossil teeth are taken from collections and their exact original location spot may no longer exist. Consequently, in situ measurements have to be performed as closely as possible to the tooth original position. Indeed, a tooth cannot be dated without a good knowledge of its sedimentary context, since the dose rate needs to be accurately reconstructed.