radiocarbon dating is inapplicable or of dubious value. This review examines the role of electron spin resonance (ESR) dating of tooth enamel, which, together with obtained so far for fossil hominid sites relevant to the origin of modern . magnetic moment of a paramagnetic centre relative to the external magnetic field. features used in radiometric age estimation are marked by arrows, with g = . ( EPR, also known as electron spin resonance or ESR) dating of tooth enamel (see Cetin, et al. . (The sign of the amplitude relative to the baseline is irrelevant.). Electron spin resonance (ESR) dating of sea shells has been made (b) The relation between the magnitude of the signal and the radiation.
Dating fossil teeth by electron paramagnetic resonance: how is that possible?
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 [Ca10 PO4 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 CO2— radicals, whose precursors are very likely the carbonate groups CO32— present in the hydroxyapatite.
This natural radioactivity is due to the radioelements, mainly U-series, Th-series and 40K 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 1.
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 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. 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.
The original DA in the sample is obtained from the negative intercept of the regression line with the dose axis; is estimated as being the total radiation provided by all radioactive elements present in a sample and soil and also cosmic radiation Ikeya ; Jonas ; Walker ; Grun Materials and Methods The dentine was initially removed from teeth using a dental drill and water cooling.Radioactive Dating
The molar enamel powder was divided into nine aliquots. The spectrometer parameters used were: Soil samples were also collected from the Aguazuque site and sent for U, Th and K content analysis by gamma spectrometry.
Radiometric analysis is widely used for determining natural radioisotopes in geological samples by means of spectroscopic methods and, especially, for quantifying the uranium, thorium and potassium present in samples. A linear model was used for fitting ESR signal intensity to dose, as has been done by other authors Baffa et al.
ROSY software Brennan et al. Soil water content was The software's default values were used for the remaining parameters. Finding the dose rate to convert DA into an age is a crucial step after DA has been found.
Electron spin resonance dating of animal and human bones.
ROSY software allows age to be calculated from an enamel sample by taking DA due to radioisotopes from adjacent layers into account. The software computes age based on three uranium absorption models linear, early uptake and a combination of both. Table 1 shows the soil analysis results. The present work did not determine U, Th and K concentration in enamel and dentine as their percentages have been negligible for calculating age in previous work, especially in dates for the Holocene period Kinoshita et al.
Electron spin resonance dating
Typical soil density, enamel and dentine values were used for calculating age 2. Using these values and the radioisotope concentrations listed in Table 1ROSY software was used for obtaining the alpha, beta and gamma radiation dose rate contribution for each radioisotope in sediment the results are listed in Table 2.
These results were the same for any uranium absorption model. It should be noted that ROSY software calculates these ages using three uranium accumulation models: The way that uranium accumulates in a tooth is established by these models; accumulation increases at the same rate as time in the linear model and accumulation occurs in a short period of time in the early model, relative to a tooth's age and then remains constant after this short period of time.
The tooth's age was the same when calculated by these three methods; this result was consistent if taken into account that the accumulation of radioactive elements inside the tooth was negligible. Conclusions Age determined at many countries' archaeological sites is the result of radiocarbon 14C analysis of particular items found around the burials and not direct dating of individuals found there.
The results regarding either physical or geological phenomena may not be chronologically equal, hence the importance of having techniques like ESR which can directly and absolutely determine the age of individuals of interest. This result agreed with stratigraphic analysis at sites providing soil ages ranging from 2, to 3,y BP for 50 and 80 cm depths Correal, ESR dating accuracy was obtained relatively straightforwardly, using just a few grams of tooth enamel, thereby highlighting some of the advantages and characteristics of the ESR dating method.
The results so obtained have contributed towards joining links in the long chain of Colombian and American history where there is still much to do, due to the rich pre-Hispanic and colonial past.
Quaternary dating by electron spin resonance (ESR) applied to human tooth enamel
This paper thus represents a pioneering effort aimed at promoting the dating of our ancestors. Rodriguez, for providing facilities and allowing the use of his samples, as well as AM Groot archaeologist for her support during this research. ESR dating of a toxodon tooth from a Brazilian karstic cave.
Direct Dating of Human Fossils. Free Radicals in Biology and Medicine. Advances in ESR Applications. Use of electron paramagnetic resonance dosimetry with tooth enamel for retrospective dose assessment. Report of a coordinated research project. New applications of electron spin resonance: