Human Fossils: Uranium-Series Dating Method
Remember the cave popcornU-series dating was principally used for dating the formation of stalagmitic calcite – like our “cave popcorn”. And it is probably simplest to first explain the dating principles of this method from this perspective. Stalagmites grow because of the formation of calcite crystals from ground water. As the water flows through – say a crack in a cave roof – it leaves behind deposits of the calcite crystals, which build up over time to form different shapes, such as stalactites and stalagmites. These mineral deposits commonly found in cave environments are called speleothems.The water that carries these calcite crystals also contains traces of the naturally occurring uranium, because uranium is soluble – it is able to be dissolved in water. However, thorium (the daughter isotope) is not soluble, so it is not present in the water. This means that while the water that is creating the speleothem is also depositing traces of uranium in the calcite, it is not depositing thorium. Which in turn means that any thorium in the speleothem has been formed by the gradual decay of uranium to thorium (U-234 to Th-230). The thorium is growing inside the speleothem.
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A Question of Time: How We Date Human Evolution
Some assumptionsSo what’s the challenge? Well, here are our assumptions. We assume that at crystal formation the thorium content is zero. We also assume that over the thousands of years, uranium and thorium have not been moved into or out of the material we are now testing. This is called a closed system assumption.The closed system assumption is particularly relevant to applying U-series dating to human fossils, as bones and teeth do exchange uranium with the environment. This is unlike speleothem, that usually remain closed to any subsequent migration after they have been formed. Fossils can contain hundreds of times more uranium than modern bones, due to exposure to ground water. When a bone is buried in sediment, it acts a bit like a sponge for uranium. Uranium can migrate into the bone (a phenomenon known as ‘incorporation’ or ‘uptake’). Uranium can also move out of the bone (leaching).This has an effect on our process. If uranium (the parent isotope) has been leached from a bone – we may face a situation where there is more thorium (daughter isotope) than uranium. When this happens – well, we can’t actually calculate a U-series age. One of the main challenges of U-series dating of fossil bones is identifying samples that haven’t experienced uranium leaching.
So… what are we actually dating?U-series analysis of fossils dates the moment when uranium migrates into the bones, not the moment of the death of an organism. It is possible that the uranium entered the bone a long time after the death of the organism (called a ‘delayed uptake’). This means that any U-series age that is calculated will always provide a minimum age possible for the bone. The age could be similar to the age of the death of the fossil – if the uptake occurred right after the death of the organism. In the case of a delayed uptake, the fossil will be older than the calculated U-series date.We do attempt to reconstruct the uptake of uranium into a fossil sample. We use a model to do this (called a diffusion-adsorption, or DA model), which predicts the distribution of uranium across a bone or tooth enamel section. This adds a margin of error that is difficult to calculate.We asked Mathieu to give us a quick summary of U-series dating.
This is an additional video, hosted on YouTube.
Some questionsHow convincing do you find U-series dating to be?We’ve already briefly mentioned U-series dating with the work of Maxime on the rock art of Sulawesi. What do you think of its application to human fossil specimens?
ReferencesAitken, M.J. (1990). Science-based dating in Archaeology. Longman Inc., New York.Duval, M., Aubert, M., Hellstrom, J. and Grün, R. (2011). “High resolution LA-ICP-MS mapping of U and Th isotopes in an early Pleistocene equid tooth from Fuente Nueva-3 (Orce, Andalusia, Spain).” Quaternary Geochronology 6(5): 458-467.Grün, R. (2006). “Direct Dating of Human Fossils”. Yearbook of Physical Anthropology, 49: 2-48.Grün, R., Aubert, M., Hellstrom, H. and Duval, M. (2010) “The challenge of direct dating old human fossils”. Quaternary International, vols 223-224, 87-93.Grün, R., Aubert, M., Joannes-Boyau, R. C. & Moncel, M. (2008). High resolution analysis of uranium and thorium concentration as well as U-series isotope distributions in a Neanderthal tooth from Payre (Ardeche, France) using laser ablation ICP-MS. Geochimica et Cosmochimica Acta, 72 (21), 5278-5290.
A Question of Time: How We Date Human Evolution
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