Radiocarbon dating

Radiocarbon dating is one of the most common methods of dating used in archaeology. It relies on measuring the relative amounts of different types of carbon in the object being dated, as these change in a predicable manner through time.

Carbon atoms exist in three sizes (or isotopes), weighing 12, 13 and 14 atomic mass units each. These occur in air (as carbon dioxide) in the proportions

• ¹²Carbon - 99%
• ¹³Carbon - 1%
• ¹⁴Carbon - one part per billion

¹⁴Carbon is weakly radioactive, and so it decays at a predictable rate (the half life). ¹⁴Carbon is also formed constantly in the upper atmosphere. It forms carbon dioxide, travels throughout the atmosphere and becomes part of every living being. This means that the proportion of ¹⁴Carbon to ¹²Carbon is stable in every living thing (including plants).

When an organism dies, it ceases to breathe, and ceases to take in ¹⁴Carbon, so the proportion of ¹⁴Carbon to ¹²Carbon starts to decrease, at a predictable rate. If you know the rate of decay, and measure the proportion of ¹⁴Carbon to ¹²Carbon, you can work out how old the object you are dating is.

The rate of ¹⁴Carbon formation is not constant, so radiocarbon dates must be calibrated. Calibration curves are derived from looking at carbon isotope ratios in ice cores and tree rings. These are used to create a calibration curve.

For more information on radiocarbon dating and calibration, have a look at the Oxford Radiocarbon Accelerator Unit's webpage, via the link on the right side of this page.

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