Tag Archives: Luminescence

The Jinmium Controversy

JinmiumIn 1996, a team of archaeologists excavated Jinmium rock shelter, which is located in a remote area of the Kimberleys. They found a rich array of stone artefacts, as well as some stone engravings called ‘cupules’ that are thought to be examples of some of the oldest form of rock art in the world.

In their preliminary findings, the archaeologists published the following:

We report stone artefacts found in sediments dated by thermoluminescence (TL) to greater than 116,000 ± 12,000 years. If that correctly dates the human presence in Australia, it almost doubles previous luminescence age estimates and is beyond any possible overlap with radiocarbon determinations (Fullagar et al, 1996).

The archaeology team concluded that this finding signalled a need for further investigation into the occupation dates at Jinmium.

As soon as these preliminary findings were published, the media pounced, reporting that Australia had been occupied for up to 176,000 years. Here are some extracts from journal publications and news reports:

Jinmium response

Jinmium’s sediments were recalculated for verification, and it turned out that the thermoluminescence dates were correct – but the artefacts found in the lower layers were much younger than their surrounding stratigraphy.

It turned out that the sediments at Jinmium had shifted, meaning that lower layers were contaminated with material from the upper layers. The TL dates were correct, but archaeologists concluded that sand grains weren’t the only things moving up and down in the ground. The artefacts had moved as well.

The artefacts excavated from the deepest, most ancient layers at Jinmium had in fact been pushed down from the upper levels, possibly as a result of the soft, crumbly bedrock, which meant that artefacts could easily have fallen through the gaps in the stone. It could also be because many people over thousands of years had been trampling on the dirt and this pressure had pushed the artefacts deeper into the ground.

After extensive analyses of sediments and repeated dating methods (about six dating methods were used at Jinmium), it was determined that Jinmium’s artefacts were only around 10,000-20,000 years old. The media’s misrepresentation of the science had turned a tentative hypothesis into an explosive news headline.

eResources for Jinmium:

ABC’s Quantum ‘Sands of Dreamtime’

ABC: ‘Sands of Dreamtime’ transcript

ABC Shop (‘Sands of Dreamtime’ DVD)

New York Times: Art, but for Whose Sake?

New York Times: In Australia, Signs of Artists Who Predate Homo Sapiens

Science Daily: Tests Reveal True Age Of Controversial Jinmium Aboriginal Rock Shelter

Science News: Australian site jumps forward in time

World Archaeological Congress

Further reading:

Bahn, P 1996, ‘Further Back Down Under’, Nature, vol. 383, p. 557-8.

Bowler, JM & Magee, JW 2000, ‘Redating Australia’s oldest human remains: a sceptic’s view’, Journal of Human Evolution, vol. 38, p. 719-726.

Burke, H & Smith, C 2004, The Archaeologist’s Field Handbook, Allen & Unwin, Crows Nest.

Fullagar, R, Price, D & Head, L 1996, ‘Early human occupation of northern Australia: archaeology and thermoluminescence dating of Jinmium rock-shelter, Northern Territory’, Antiquity, vol. 70, p. 751-773.

Hiscock, P 2008, Archaeology of Ancient Australia, Routledge, New York.

Holden, C 1996, ‘Art Stirs Uproar Down Under’, Science, vol. 274, p. 33-34.

Mulvaney, J & Kamminga, J 1999, Prehistory of Australia, Allen & Unwin, Crows Nest, NSW.

O’Connell, JF & Allen, J 2004, ‘Dating the colonization of Sahul (Pleistocene Australia-New Guinea) a review of recent research’, Journal of Archaeological Science, vol. 31, p. 835-853.

Roberts, R et al. 1998, ‘Optical and radiocarbon dating at Jinmium rockshelter in northern Australia’, Nature, vol. 393, p. 358-362.

Spooner, NA 1998, ‘Human occupation at Jinmium, northern Australia: 116,000 years ago or much less?’, Antiquity, vol. 72, no. 275, p. 173-178.

Watchman et al. 2000, ‘Minimum ages for pecked rock markings from Jinmium, north western Australia’, Archaeology in Oceania, vol. 35, p. 1-10.

Wilford, JN (1996, September 21), ‘In Australia, Signs of Artists Who Predate Homo Sapiens’, New York Times, available here.

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Luminescence Dating

There are two types of luminescence dating: thermoluminescence (TL) and optically stimulated luminescence (OSL). Both measure the number of trapped electrons that have accumulated in quartz (sand) grains since the last time they were exposed to sunlight.

In order to release these trapped electrons, the quartz grains must be subject to intense heat (TL) or intense light (OSL). Luminescence dating is often used to determine age-estimates of sediment in stratigraphy where artefacts have been located.

The graph below shows the progress of luminescence dating in Australia since the 1950s. (Note the similarity with radiocarbon dates.)

Luminescence dates in Australia
Luminescence dates in Australia

Where radiocarbon dating has a ‘barrier’ of about 40,000 years, luminescence can be used to date material millions of years old. Many older sites in Australia are being dated using a combination of luminescence and radiocarbon, to ensure the accuracy of dates.

While luminescence is an accurate method of dating the sand grains in sediment, there are a number of pitfalls associated with using this dating method to determine the age of sites. For a start, 3000 sand grains are required, and the sample will likely contain a mixture of earlier and later grains, especially if it is taken from less than 20cm below the ground surface, or within 20cm of bedrock. Contamination from earlier or later layers can sometimes misrepresent the correct age of the sediment, hence the age-estimates for the associated artefacts may be inaccurate.

Some sites that were originally dated using luminescence have since been re-examined and found to be older or younger than initially thought. A good example of this type of error occurred in 1996 when archaeologists excavated the Jinmium rockshelter and preliminary findings suggested artefacts as old as 116,000 years. Later analysis debunked this finding to reveal a much younger site of between 10,000 and 20,000 years. For more on this, see the Jinmium entry.

References:

Burke, H. & Smith, C. (2004) The Archaeologist’s Field Handbook. Crows Nest: Allen & Unwin.

Hiscock, P. (2008) The Archaeology of Ancient Australia. New York: Routlege.

Mulvaney, J. & Kamminga, J. (1999) Prehistory of Australia. Crows Nest: Allen & Unwin.