Tag Archives: Radiocarbon

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.

Radiocarbon Dating

Radiocarbon dating is used to determine the age of historic and prehistoric sites all over the world. To do this, archaeologists need uncontaminated organic samples – that is, samples that are taken straight from the ground and not touched by anyone on site before going to the lab. Most of the time, these samples are charcoal. Other organic materials that can be dated using radiocarbon are leather, shell, plant material (e.g. seeds and pollen), eggshell, fish and insect remains, paper and parchment, wood, hair, bone and ice cores.

Radiocarbon dating measures the rate of decay in the unstable, radioactive isotope ¹⁴C to determine the age of organic matter. Because the ¹⁴C isotope is unstable, calibrating precise dates is sometimes difficult, especially with older sites. There is what’s known as a ‘radiocarbon barrier’, which means that this dating method can only produce reliable dates for sites younger than about 40,000 years. If archaeologists find a site they believe to be older than 40,000 years, they must compare radiocarbon dates with different dating methods to ensure an accurate result (see luminescence dating). Comparisons are often conducted with younger sites anyway, just to ensure the accuracy of the data.

Since the first radiocarbon dates were announced in the mid-20th Century, our understanding of Australia’s antiquity has altered dramatically. As each year passes, earlier dates are being published, extending Australia’s occupation further into the past. The graph below shows the progress of radiocarbon dating in Australia since the 1950s.

Radiocarbon dates in Australia
Radiocarbon dates in Australia

Radiocarbon dating is a highly complex procedure. In order to determine the age of a site, the radiocarbon years need to be converted to solar years, and then an age range is calculated. For example, if a radiocarbon result is 5,000 years, the solar years would be about 5,750, and the age estimate for that site might be calculated at 5,700-5,800 years old. Some archaeologists prefer more accurate calibrations, but many round the totals to the nearest 50 or 100 years, meaning that sites up to 26,000 years old are often given age estimates such as 8,400 ± 100.

Sites older than 26,000 years present greater issues for radiocarbon dating. For the period before 26,000 years, it is much harder to assign accurate dates because the distance between radiocarbon and solar years becomes increasingly difficult to calculate. For older sites, age estimates are usually stated as minimums and maximums, e.g. 42,000-46,000 years old.

Today, the generally accepted baseline for people arriving in Australia sits at 50,000 years ago. But who knows – future findings may challenge this figure and suggest an even earlier arrival time.

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.