Young Ages for Australian Rock Art

Two years ago, archaeologists caused an international stir with their dates for a remote rock shelter called Jinmium in the Northern Territory of Australia. The dates of 116,000 to 176,000 years old made the shelter by far the earliest trace of humans in Australia and its circular carvings the oldest known rock art in the world. But in today's issue of Nature, a team reports that Jinmium's age is a completely unremarkable 10,000 years.

The very early dates for Jinmium came from a team led by archaeologist Richard Fullagar of the Australian Museum, who is also a co-author on the new paper. He used a method called thermoluminescence dating (TL), which relies on a clock driven by natural radiation in common minerals like quartz. As long as the mineral remains in the dark, radiation bumps electrons into crystal defects, or "traps," at a regular rate. Scientists can read the clock by emptying the traps in the lab, either by heating the sample (TL) or by tickling it with light (optically stimulated luminescence, or OSL). The material glows as the electrons drop back into the lattice; the more intense the glow, the more time has passed since the sediments last saw daylight.

But samples can be contaminated. In the Nature paper, a team headed by geochronologist Richard Roberts of La Trobe University in Melbourne, Australia, notes that pebbles of older rock--crumbly sandstone from the boulder wall and the bedrock below--were jumbled into the sediments being dated. When Roberts used OSL to tease dates from individual mineral grains, he was able to distinguish old grains of bedrock from the grains of sediment that would reveal the true age of the shelter. His conclusion: The base of the deposit at Jinmium is no more than 10,000 years old, and some of the quartz grains were laid down more recently.

The new dates "nail the coffin shut" on the claim that humans have been in Australia two to three times longer than previously thought, says geochronologist Jack Rink of McMaster University in Hamilton, Canada. Besides removing a puzzle in Australian prehistory, says Rink, the new dates should restore confidence in luminescence dating, which is a powerful tool when applied correctly.

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