Science-based Dating in Archaeology by Aitken M. J

Author:Aitken, M. J.
Language: eng
Format: epub
ISBN: 978-1-317-87148-4
Publisher: Taylor & Francis (CAM)

Coral and mollusc shells

Although coral is not often of archaeological relevance, mention of it is appropriate because it is the material par excellence for uranium-series dating, it being a reliable closed system as long as material in which recrystallization from aragonite to calcite has occurred is avoided: living coral incorporates about 3 ppm of uranium and there is no change with age; a negligible amount of thorium-230 is incorporated; the initial (uranium-234/uranium-238) ratio is known for sea-water and hence the method based on that ratio is applicable also (see section 5.2.6). Uranium-series dating of coral reefs has been an important tool in the study of sea-level changes associated (in part) with variations in global ice volume such as discussed in section 2.3; the high-precision results obtainable with mass spectrometry are particularly important here (e.g. Edwards et al. 1987).

On the other hand, mollusc shells, like bone, are an open system and despite attempts at correction for uranium movement ages obtained are of questionable reliability, more so for aragonitic shells than for the calcite variety – which are less subject to uranium uptake; the uranium content of fossil aragonitic shells ranges from 0.1 to 50 ppm, whereas the content in living examples is an order of magnitude lower.

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