Electron Spin Resonance (ESR) dating method
We need teethFor ESR dating, we need teeth. They preserve the ESR signal in the hard minerals of the enamel. When we dig out a tooth at an archaeological site, it has an ESR signal. That signal was generated over the time the tooth was buried, from the radioactivity in the sediment around the tooth (external component) and the radioactivity within the tooth itself (internal component).So how can we get to the age of the tooth?
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A Question of Time: How We Date Human Evolution
- Was the bathtub empty when the process began? (Were there electrons already trapped in the tooth at the death of the animal?)
- Did the flow of water into the bathtub change at any time? (Did the dose rate into the tooth vary over the millennia?)
- Are there any leaks in the bathtub? (Was the tooth subjected to processes that changed the level of electrons trapped within the lattice – such as heating?)
- Did anyone add water to the bathtub later? (Has the tooth sample been irradiated after sampling?)
Using the analogy of a bathtub – potential issues appear on the column to the right
Complicated calculationsThe calculation of the dose rate is actually quite complicated. The vast majority of radioactivity in nature derives from three elements: uranium, thorium and potassium (we use the radioactive isotope K-40 for dating as well, but this is another story). The challenge here is that the concentrations of radioactive elements within the sample are usually very different from its surroundings – which means that we need to assess the internal dose rate and the external dose rate separately.The external dose rate needs to be calculated from any sediment attached to the fossil, along with samples from the site where the fossil was located, which is not always possible. Even the fact that the teeth may be located within their original jaw adds complications, as the jawbone will both shield the tooth from environmental radiation – and add its own radiation to the tooth. In many cases we are forced to reconstruct this external dose rate from museum samples, which adds the possibility of very large errors.The internal dose rate is calculated using the same principles as U-series dating – the equilibrium between the uranium and the thorium. And with that we inherit the same challenges that require us to model the uranium uptake of the fossil. We do this by combining ESR and U-series dating results on the tooth. Both dating methods depend on U-uptake, but to a different extent. As we know from school, if we want to solve for two unknowns, we need two independent equations. Here the two unknowns are the age of the sample and the way the uranium migrates into the sample (that is described by a one-parameter diffusion equation). By putting everything together we can solve for the age and the diffusion parameter. Does that sound complicated? It should – the equations run over several pages. Don’t worry, we have computer programs that do the hard math.We asked Mathieu to give us a quick summary of the process of ESR dating.
This is an additional video, hosted on YouTube.
Your taskWell, you made it through a lot of science here. As you probably suspect, there is a lot more underpinning the process and the modelling.As we said when we looked at the limitations of dating methods, when it comes to the general reliability of dating methods other than radiocarbon, we still have a long way to go. But that’s the theory behind ESR dating, and where we are at with its application.What do you think of the ESR dating method?Have any questions for Rainer or Mathieu?Select the comments link below and share your thoughts.
ReferencesAitken, M.J. (1990). Science-based dating in Archaeology. Longman Inc., New York.Duval, M. (2014). “Dating fossil teeth by electron paramagnetic resonance: how is that possible?” Spectroscopy Europe 26(1): 6-13.Grün, R. (1989). “Electron Spin Resonance (ESR) Dating”, Quaternary International, 1: 65-109.Grün, R. (2006). “Direct Dating of Human Fossils”. Yearbook of Physical Anthropology, 49: 2-48.
A Question of Time: How We Date Human Evolution
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