Mathieu Duval raises the question “Dating fossil teeth by electron paramagnetic resonance: how is that possible?”. Whilst we are all familiar. Here, the electron spin resonance (ESR) dating method is often the only one method permitting to . tooth enamel and bleached sedimentary quartz are par-. International Conference on Luminescence and Electron Spin Resonance Dating Partial bleaching and the decay form characteristics of quartz OSL . Beta doses in tooth enamel by “one-group” theory and the ROSY ESR dating software.ELECTRON SPIN RESONANCE SPECTROSCOPY INTRODUCTION AND INSTRUMENTATION
Therefore, sands transported by clear water reveal being the most appropriate for ESR analyses. It means that it is better sampling pure sands than silty sands. However, even if bleaching is not complete at the deposition time for fluvial, marine or aeolian sands, the low values of the residual dose observed does not prevent dating these sediments by ESR see further details in Voinchet et al.
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Grain size and bleaching levels may also be closely correlated. Finest and coarser grain size fractions can also be used for dating by ESR but taking into account that the residual dose is higher than for the intermediate particle size.
Loess is a special case in which an overestimation of ages is systematically observed due to insufficient bleaching turbulent transport clouds preventing sunlight contact.
In the field, sediment should be examined for evidence of disturbance such as bioturbation from roots or animalspedogenic processes clay illuviation or segregation or post-depositional reworking because it can mix grains of different ages in a sedimentary profile or alter dose rate conditions over time, respectively Bateman et al.
A metallic cap may be used so that the tube can be easily hammered horizontally into the outcrop. Following the sample collection, both ends of the tube should be sealed with tape to prevent light exposure and loss of sediment.
Clearly label the sample using preferably a permanent dark-colored pen by indicating for example the acronym obtained from the name of the site, year of collection and sample number e. Finally, the tube sealed and labelled should be introduced into an opaque ziplock bag labelled with the same code. If the sediment is too hard or compact to insert a tube, then other sampling techniques can be used.
For example, a block of sediment can be carved and securely wrapped with aluminium foil and tape. Then it must be placed in opaque plastic bags or bigger containers for transport to the laboratory where it will be prepared under controlled light conditions. In the case of coarse-grained alluvial deposits where sand lenses are too thin to sample with a tube or a block, the sandy matrix within the gravel or loose sediment can also be collected in a light-proof container under an opaque plastic cover.
In this context, bulk sediment samples for D analyses and moisture content should be collected from the ESR sampling site tube hole or from a 30 cm radius sphere around the sample fig. Samples should be bagged and clearly labelled using the same acronym to that ESR sample. In addition, the evaluation of the in situ gamma dose rate can be done using either a field portable gamma spectrometer fig.
Consequently, the site should be accessible and secured to avoid any perturbation or loss during that period. Another advantage is that the gamma dose rate registered over such a long period is also by definition indirectly taking into consideration any fluctuations of the water content of the sediment with time. In contrast, the field portable gamma spectrometer is more frequently used because the measurement can be completed almost immediately around half an hour usually.
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.
- Dating fossil teeth by electron paramagnetic resonance: how is that possible?
- Electron Spin Resonance Dating of Toxodon Tooth from Upper Ribeira Valley, São Paulo, Brazil.
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.
Electron Spin Resonance Dating of Toxodon Tooth from Upper Ribeira Valley, São Paulo, Brazil.
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. 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. This is a key period in European Prehistory, marked by the arrival of the first hominids in the continent and who very likely spread from Georgia, about 1. EPR has definitely an important role to play for the improvement of the chronological framework of the oldest hominids settlements in Europe.
Electron spin resonance dating
This is very likely due to the long and complex analytical process that requires a large diversity of equipment e. U-series analyses facilities, EPR spectrometer, gamma irradiation source, high resolution gamma spectrometer, portable gamma spectrometerwhich make it especially complicated to setup a complete and autonomous laboratory.
However, recent developments in the field have demonstrated the potential of this method for Quaternary geochronology.
Among them, the development of almost non-destructive direct dating of hominid fossil teeth is perhaps the most promising. If EPR spectroscopy is not a destructive method per se, the standard procedure consists in working with enamel powder, mainly for practical reasons, to avoid the complexity induced by EPR signal anisotropy.