A publication of the Archaeological Institute of America
In late 1996, Australian archaeologist Richard L.K. Fullagar and his colleagues announced startlingly early dates for a rock shelter at Jinmium in Australia's Northern Territory. Their claim that the site was first occupied between ca. 176,000 and 116,000 years ago made the shelter the world's oldest rock-art site and called into question commonly held beliefs about the arrival of modern humans in Australia and the seafaring abilities of our ancestors (see "Ancient Seafarers," March/ April 1997).
The early dates have drawn fire from scholars, some of whom believe the site is no more than 10,000 years old. The debate focuses on the dating technique, known as thermoluminescence (TL), which uses quartz or feldspar, ubiquitous at archaeological sites, as its subject material. When grains of sediment are "bleached," i.e. exposed to sunlight, their luminescence "clocks" are set to zero. Once buried, sediments accumulate energy at a measurable rate. The more energy accumulated, the longer the sediments have lain undisturbed.
TL dating depends on sediments' being fully bleached and remaining uncontaminated by grains that have never been exposed to sunlight. "It only takes one or two contaminant grains to throw off the TL dates by thousands of years," says Richard Roberts, a geochronologist at La Trobe University in Melbourne, Australia, who has dated many of the country's oldest sites. At Jinmium, scientists worry that quartz grains that were not fully zeroed mingled with the sediments in artifact-bearing layers.
To settle the issue, Roberts and his team have taken fresh samples from Jinmium, on which they are using accelerator-mass-spectrometry radiocarbon dating and an improved luminescence method known as optical dating. Meanwhile, Michael Bird of the Australian National University in Canberra has run a series of radiocarbon tests on minute traces of charcoal found in the sediments. "Preliminary results have already placed the occupation of the Jinmium rock shelter well within C-14 range, that is within the past 50,000 years," says Roberts. "We have also obtained similar ages by optical dating of the sediments, grain by grain. As this method enables us to sort old grains from new, we are quite confident of our findings."