Australia is blessed with many beautiful examples of Aboriginal cave paintings and engravings but what does science tell us about how old they are? What are the different methods used to date such artworks? And what are some of the challenges involved in dating them?
Many people will be forgiven for thinking that Australia has some of the oldest rock art in the world, but the truth there is no reliable dating to show this.
"You often hear people talk about the oldest continuous culture in the world being Aboriginal culture," says geologist Professor Brad Pillans of the Australian National University.
"Many archaeologists have thought rock art has been a part of Aboriginal culture since earliest times... but it's an inference."
Pillans and colleague Keith Fifield have argued that rocks bearing Aboriginal engravings on the Burrup Peninsula have the potential to preserve the engravings for 50,000 to 60,000 years, but they have done no direct dating of the engravings themselves.
According to archaeologist Dr Bruno David of Monash University the oldest reliably-dated rock engravings in Australia are 13,000 to 15000 years old, and are in Laura, Queensland.
"These were dated using radiocarbon dating of charcoal buried at the same depth of engravings," he says.
Beyond engravings, the oldest reliably-dated rock art in Australia is 28,000 years old. It's a fragment of a charcoal cave painting found buried in an Arnhem Land cave by David and colleagues.
The fragment was both preserved and dateable by being buried in carbon-containing soil
But dating most rock art isn't usually quite so straightforward.
Challenges of dating"Rock art is notoriously difficult to date," says David. "Most pigment art contains no dateable carbon, and therefore radiocarbon dating is usually not feasible."
What is known as the oldest rock art in the world - cave paintings in Indonesia and Spain — was dated using a more complex method that measures the age of a microscopic layer of minerals deposited after the art is created.
"It's like a laminate," says David.
Instead of measuring the decay of radioactive carbon, this method relies on measuring the decay of uranium in the microscopic layer to provide a minimum age for the art. In some cases a similar layer beneath the art gives a maximum age for the art.
In both Spain and Indonesia, the cave art appeared in deep limestone caves.
Not only did this protect the artwork from the elements, but it also provided a good environment for the production of these dateable layers.
"Australia doesn't have much art in deep limestone caves," says David.
Ochre on sandstoneIndeed, he says, the richest collections of rock art in the Kimberley, Arnhem Land, Cape York Peninsula, are on sandstone.
Take the ochre painting of a giant emu-like bird found on the Arnhem Land plateau, which some believe could be 40,000 years old — as old as the paintings in Spain and Indonesia.
There is no dateable carbon from plants, charcoal or dust caught in the ochre pigment, says David.
And it's difficult to use a similar approach to that used on the limestone art so new methods need to be developed.
"It's very likely that there is something extremely old in Australia but it's just very hard to date," says David. "And there are very few people doing research on rock art."
David says there are hand stencils in some limestone caves in North Queensland that are believed to be more than 30,000 years old, and he hopes to be involved in dating these in the future.
Circumstantial evidenceThe bottom line, says David, is that very little rock art anywhere in the world has been dated, including in Australia.
But there remain a lot of hints and circumstantial evidence around to support the idea that Australia is in fact home to the world's oldest art.
Pillans, who studies the Burrup rock engravings, describes the giant bird painting on the Arnhem Land plateau as a "hint of older rock art".
Some researchers say the creature looks like Genyornis which is believed to have gone extinct at least 40,000 years ago.
"The people who drew that animal could only have seen it more than 40,000 years ago," says Pillans.
David emphasises it is still uncertain whether the bird is actually a Genyornis and points instead to ochre crayons that date to around 50,000 years ago.
"We don't have the [dated] art itself, but we've found the tools that were used to make the art. For that reason, we rightfully assume that Australia has pigment art going back to when people first came here which is close to 50,000 years ago."