Tests Reveal True Age Of Controversial Jinmium Aboriginal Rock Shelter

Jinmium became world headline news in 1996 when researchers investigating the site estimated it may have been inhabited by humans as long as 120,000 years ago or even earlier.

Their claim challenged existing archaeological evidence on when the Australian continent was first settled, and generated wide debate. Prior to the Jinmium excavation, the oldest site known to have been inhabited was at Malakunanja, near Kakadu, and is dated at between 50,000 and 60,000 years old.

The Jinmium site consists of a large, tilted block of sandstone, the northwest side of which is adorned with an early artform, a pattern of indentations known as pecked cupules. Some archaeologists consider these predate all other forms of rock art. Deposits below the shelter contained artefacts down to a depth of 1.6 metres, including an engraved sandstone fragment.

Initial tests used a technique known as thermoluminescence and indicated a possible age for the lowest layers containing artefacts of between 116-176,000 years.

However these layers have now been intensively re-dated using the latest and most sophisticated techniques by a dating team from La Trobe University, the Australian National University, CSIRO, the Australian Nuclear Science and Technology Organisation (ANSTO), the Australian Museum and University College, London.

"These findings mean that all the 'hype' about modern humans being in Australia before they had apparently even left Africa has been put to rest, and we no longer need to debate the issue of human origins on the Jinmium evidence," says team leader Dr Richard Roberts of La Trobe University.

"Jinmium is clearly a much younger site. But that still leaves us with the fascinating question of when humans entered Australia - and whether it has any connection with changes in the pollen and charcoal records we see taking place about 120,000 years ago. That remains a mystery waiting to be solved."

Samples from Jinmium were tested using both radiocarbon (C14) and optically-stimulated luminescence - or OSL, a new, powerful dating method developed largely in Australia, capable of dating the time of burial of individual grains of sand.

OSL works on the principle that sunlight releases the natural energy stored as trapped electrons in the crystal lattice of tiny grains of quartz sand. The moment the grains are buried away from sunlight, they begin to accumulate energy from radioactive decay in the soil around them.

This minute amount of energy can be artificially released by stimulating the sand grains with a green light, explains CSIRO scientist Dr Jon Olley. The resulting sparkle of light can be measured, and used to estimate the time elapsed since the grain was concealed from the sun.

Where grains have been eroded out of bedrock underground and never seen the light of day, conventional thermoluminescence will give dates that are falsely old. Also grains not exposed to sufficient sunlight prior to burial can yield deceptive dates.

The team considers this is how the age of the Jinmium deposit originally came to be overestimated. The latest technique of single-grain analysis by OSL is considered far more accurate and reliable. Standard OSL methods gave a maximum date of 22,000 years for the bottom layer of the Jinmium deposit, and single grain analysis showed that the true age was younger still, no more than 10,000 years for the whole deposit.

As a cross-check, fragments of charcoal from the site were dated using a second method, C14 or radiocarbon dating. These indicated the top half of the deposit was inhabited from 100 to 4,000 years ago - the same age as was found by OSL dating of single grains. The older OSL dates are probably due to some grains not being exposed to sufficient sunlight before burial.

Based on these results, the researchers conclude that human habitation of the Jinmium site is certainly no older than 10,000 years (ie end of the last ice-age), and probably quite a bit younger, with the oldest inhabited level being perhaps 6000 years old - about the time farming began in the Middle East.

More information:

Dr Richard Roberts, La Trobe University, 03 9479 2649

Dr Jon Olley, CSIRO Land & Water, 02 6246 5826

Dr Michael Bird, Australian National University, 02 6249 5171

Dr Ewan Lawson, ANSTO, 02 9717 3111

Margaret Bryant, CSIRO Land & Water, 08 9333 6215

Rhonda Dredge, La Trobe University, 03 9479 1111/1574

EMBARGO: 1AM, Thursday, May 28, 1998