Dating of archaeological sites
“I think there is a strong compelling argument to re-date these key sites using single-grain OSL,” he says.
Previous multi-grain OSL dating at a number of ancient sites have suggested humans arrived in Australia well over 50,000 years ago, but Spooner is sceptical of many of these dates.The mean ages were found to be 5500 ± 150 years, 2950 ± 50 years, 1200 ± 50 years, 1300 ± 150 years, 3300 ± 100 years and 3400 ± 200 years for the examined pottery from the sites Al-Shermanieh, Tell Serah, Der Al-Hajar, Tell Al-Shekdakah, Tell Al-Souwirieh and Khurbet Al-Kulieh, respectively.The results were in good agreement with the ages estimated by archaeologists except for one sample which belongs to Der Al-Hjar site.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.Scratching around in a cave in the middle of nowhere, you find a bone.
How do you find out if it’s the remains of an ancient animal that stomped the land tens of thousands of years ago or a discarded scrap from a cooking fire only a few hundred years back?
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.
While a crystalline grain such as quartz – found in desert sand – is buried and tucked away from sunlight, natural radiation from surrounding soil and rocks knocks electrons in the crystal out of position.
A few of these electrons become snagged in defects in the crystalline structure and build up over time – and i To date a buried grain, scientists heat the crystal or stimulate it with light, releasing energy from the accumulated trapped charges.
Then, only exceptionally well-preserved, pristine samples can provide reliable dates.