Tag Archives: OSL dating

New dates for earliest human occupation of Australia

When modern humans first reached Australia has an importance beyond the starting date for the island continent’s archaeology and confirmation that their ancestors are the oldest known migrants from Africa. The first native Australians carried within their genome important information about the minimum date at which some non-Africans interbred with more archaic Neanderthal and Denisovan humans, traces of whose DNA are are present in that of living Australian aborigines. Most dating of when modern humans first reached different parts of the non-African world has relied on the radiocarbon method, which is suspect from beyond 40 to 45 ka as 14C produced earlier has decayed to levels that are now below the practical limit of detection and measurement. It is therefore no accident that the bulk of ‘first-arrival’ dates for Eurasia and Australasia are around 45 ka. In fact, any accurate age, however old, for the earliest skeletal remains only indicates the minimum date of arrival until other remains are discovered.

Reliable dating of earlier events in the Pleistocene relies on other methods, the most important for settings other than speleothem from caves being optically stimulated luminescence (OSL) applied to soil minerals that estimates their time of burial. Briefly, molecules of soil grains made of a mineral such as quartz are ‘charged-up’ with energy by radiation emitted by unstable isotopes in the soil. Exposure to light releases that stored energy in the form of luminescence. Measuring the amount of luminescence emitted by optically stimulated grains therefore gives a measure of the time since they were buried and ceased to be exposed to sunlight.

Madjedbebe rock shelter

The Madjedbebe rock shelter in Arnhem Land, Northern Territories, Australia. (Credit: Chris Clarkson, University of Queensland)

A re-evaluation of the Madjedbebe site in the Northern Territory, widely accepted as having yielded Australia’s oldest artefacts in 1989, takes back human occupation more than 20 thousand years before previous estimates (Clarkson and 27 others 2017. Human occupation of northern Australia by 65, 000 years ago. Nature, v.  547, p. 306-310; doi:10.1038/nature22968). The soil profile in the Madjedbebe rock shelter turns out to be littered with artefacts – including hearths, tools and blocks of ochre and reflective mica pigments, plus remnants of plant foods – to a depth of ~2.5 m, with three particularly dense accumulations. Carbon-rich remains are also present throughout the profile which provided a means of accurate calibration and confirmation of OSL dates back as far as the radiocarbon method allows, giving confidence in the older OSL dates that extend to 65.0±5.7 ka in the earliest zone of dense artefact finds. Because the modern DNA of Australia’s first native people shows no sign of mixture with other modern humans, this places the timing of modern human interbreeding with archaic people before this time. The age also predates the range when the continent’s megafauna began to decline to eventual extinction, which supports the view that it was anthropogenic.

See also: Marean, C.W. 2017. Early signs of human presence in Australia. Nature, v.  547, p. 285-287; doi:10.1038/547285a.