Carbon 14 dating artifacts
Until recently, most scientists used the "multi-grain" OSL technique – analysing thousands of grains at once to obtain an average date for that bundle.
But over the past two decades, a laser-based apparatus has enabled analysis of single grains.
Madjedbebe rock shelter in Australia's Northern Territory, for instance, has recorded single-grain OSL dates of between 50,000 and 60,000 years, seemingly making it Australia's oldest site of human occupation.
But debate still rages about whether the stone tools recovered from this ancient sediment really are as old as the sand grains that surround them, or whether they slid down into older sediment over time.
Carbon-14 is continually formed in nature by the interaction of neutrons with nitrogen-14 in the Earth’s atmosphere; the neutrons required for this reaction are produced by cosmic rays interacting with the atmosphere.
Radiocarbon present in molecules of atmospheric carbon dioxide enters the biological carbon cycle: it is absorbed from the air by green plants and then passed on to animals through the food chain.
Single-grain OSL needs specialised equipment and skilled personnel to analyse results, making it twice as expensive and more time-consuming than multi-grain analysis.
It finds the age of the sediment surrounding artefacts – sediment which may have once been outside sand trampled into caves tens of thousands of years ago – by measuring when it was last exposed to the sun.
This luminescence of the burst provides a measure of how long ago the sample was buried.
“Eventually a crystal becomes saturated with trapped charge – all the defects are filled – but this technique is usually capable of going back more than 100,000 years,” Spooner says.
Radiocarbon is an isotope with two extra neutrons, created by cosmic rays interacting with nitrogen in Earth's atmosphere.
When a plant or animal is alive, it constantly replenishes trace amounts of radiocarbon in its tissues.