Rock of ages: Photonics aims to date ancient Australian rock art

The world’s largest collection of rock art lies along a peninsula and a set of 42 islands known as Murujuga in Western Australia. There, piles of turmeric-colored stones bear millions of ancient engravings that depict everything from humans to sea turtles, to the now-extinct Tasmanian tiger.

These petroglyphs, sacred to the aboriginal groups whose ancestors made the art, were carved over many millennia. But scientists don’t explicitly know when particular engravings were done other than to say that the oldest might date back 50,000 years—when humans first arrived in Australia—and the newest had to have been created before the 1868 massacre of their creators, the Yaburara people.

Jo McDonald, director of the Centre for Rock Art Research and Management at the University of Western Australia, hopes that a new technique called optical surface exposure dating, optical-surf for short, might solve the mystery of when individual carvings, or groups of similar carvings, were made. “It’s an experimental technique,” she says, “very much a work in progress, but there’s been success with it in other places.”

The Murujuga Aboriginal Corporation (MAC) which is made up of Aboriginal people representing five different language groups in the area, lists dating the rock art as a priority in their cultural management plan for Murujuga.

“We know this…that the art goes back far before our great-grandfather’s time,” says Vince Adams, deputy chairperson of MAC and a member of the Yindjibarndi people. “It’s something that our dreaming stories tell us. It’s something we are connected to, that we see straightaway.”

This indigenous understanding, passed down through generations, he calls “the old science,” whereas he refers to Western science as “the new science.” Having new science date the art, he says, will verify for non-indigenous people that the art is as old as Aboriginal people already know it to be.

“We just need to educate the wider community,” he says. “And the only way we can educate this wider community is by sharing both what is science today and what was science a thousand (or more) years ago.”

Photo credit: Murujuga Aboriginal Corporation

Rock art dating is never without controversy. Scientists are constantly refining the various methods available as well as creating new techniques that may call into question earlier archeology studies’ conclusions about a particular artwork. Rock engravings are even harder to date than rock paintings because such carvings do not involve the use of organic substances such as charcoal or beeswax, samples of which can sometimes be scraped off and analyzed using radiocarbon dating.

“With engravings, there’s just nothing added to the rock face that you can measure,” says Luke Gliganic, a geoarchaeologist at University of Wollongong in New South Wales, Australia, who is leading the research to see if optical-surf will work at Murujuga.

The more than one-billion-year-old igneous rocks, which are dark gray-green in color, have an iron-oxide-rich patina that gives them an orange-reddish hue. When people began carving motifs on the rocks, their tools sliced through this patina exposing a whitish clay “weathering rime” underneath. Both the rime and the patina are a factor of the rock being exposed over time to sun, wind, and rain. The contrast between the rime and the patina is what makes the engravings easy to see (the carvings don’t penetrate the actual rock).

With new weathering that starts once the carvings are made, however, the contrast begins to dim. This fact has allowed the researchers to group the carvings into five general time categories. “The oldest ones are a ‘one’ because the contrast is very hard to see, and the newest ones, which have a much starker contrast, are a ‘five,’” says McDonald. “That’s really how we’ve made the leap, the interpretive leap, that the engraving started in deep time [tens of thousands of years ago] all the way up to current times.”

An OSL reader. Photo credit: Michael Meyer

McDonald’ and Gliganic’s colleague, Ken Mulvaney, took this idea even further, detailing in a 2011 study that the weathering differences also correspond with shifts in the art’s subject matter. For instance, the ones, twos, and threes have some similarities in motifs and style across a broad swath of the landscape, signaling the artists were spread out but still connected; thus, Mulvaney thought the art was made before the last ice age ended when sea levels were lower, and the region was more inland and contiguous rather than islands. That art, he believes, was created between 47,000 and 11,000 years ago.

He found that the fours show a shift in motif from land animals to sea life, so he theorized they were created as the sea was beginning to rise, around 8,000 years ago. And the fives, which depict even more sea life, as well as human figures with locally distinct headdresses, were made after the region turned into islands and people were more disconnected and separated into groups. He dated those between 6,000 years ago up to the mid-19th century massacre.

Giglanic hopes to build on Mulvaney’s chronology with his optical-surf work. The method, still in its infancy, was built off a technique called optically stimulated luminescence (OSL) dating, which archeologists have used for decades to determine the age of buried artifacts, structures, and other relics. In one recent study, for instance, researchers used the technique to determine that human footprints discovered in South Africa were some 153,000 years old.

OSL operates by the principle that sediment, essentially broken-down rocks, contain minerals such as quartz and feldspar that trap energy from cosmic rays or naturally occurring radiation in soil, air, and water. Scientists can release and measure this trapped radiation by exposing the quartz minerals to blue or green light and the feldspar minerals to infrared light in a laboratory. The minerals’ released energy is referred to as a luminescence signal.

Exposure to sunlight can also release the signal. When atmospheric processes such as wind and water lift sediment and carry it to new places, it gets more sunlight exposure than when it’s on the ground, sometimes buried by intact rocks or shaded by trees. Sunlight exposure that causes the luminescence signal to leach out of the sediment is what scientists call “bleaching.” And when that bleached sediment gets redeposited on the ground and then buried by accumulating sediment so that it is no longer exposed to natural light, it begins to once again reaccumulate a luminescence signal. In the mid-1980s, scientists recognized that measuring this post-bleaching/rebuilding luminescence signal in buried sediment would tell them when the sediment was buried and, by proxy, when nearby artifacts were buried.

Some “very, very high-profile studies” have made use of luminescence dating for answering human evolution questions, says Reza Sohbati, a physicist who pioneered the application of luminescence dating to rock surfaces in 2011 at Aarhus University and the Technical University of Denmark. Researchers at the latter institution created the Risø TL/OSL reader, which has an array of light-emitting diodes for infrared and blue light optical stimulation to excite the luminescence signals from quartz and feldspar. The machine also has optical filters to separate those intense LEDs from the photomultipliers that detect the signal for measurement.

The rock surface method works a bit like this: Say that after years of heavy rainfall, a chunk of a cliff face falls to the ground. The new cliff face is made of rock that has never been exposed to the Sun and thus has a fully intact luminescence signal. But as the Sun shines on this new rock face, it begins to bleach that luminescence signal, and, over time, the bleaching penetrates deeper and deeper into the rock. If scientists take cores from the rock, they can measure the remaining luminescence signal as a function of depth into an exposed rock surface and infer how long the rock’s surface has been exposed to sunlight.

Sohbati and colleagues first demonstrated the potential of this technique in 2011 using rocks collected from a beach in Denmark. Later, while dating ancient rock art in Canyonlands National Park in Utah, he and colleagues showed that the technique worked even better if they could determine a rock type’s bleaching rate. They took a core sample from the same rock type as the one that the art had been created on, and that had been cut into to build a nearby road. They knew the road had been built 80 years prior, so they knew when the rock’s surface had first been exposed to light. This information allowed them to calculate the bleaching rate and add it to their model to better calculate the date of the art.

“We were able to really constrain the window when this rock art must have been created,” he says. “And that was somewhere around 800 to 1,500 years ago.”

In 2019, Gliganic moved the technique forward even more by using it to date what are called archaeological surface assemblages—related artifacts that are found scattered across a particular area. Such relics are hard to date, in part because water, wind, and other geological processes can move them from their original positions. It can also be hard to find appropriate nearby organic substances for radiocarbon dating.

Gliganic and colleagues focused on a tool quarry site north of the Mount Everest-Cho Oyu massif in Tibet that is also near an ancient trade and pilgrimage route. Using optical surface exposure dating, they were able to date not only when some stone tools were made—some 5,000 years ago—but also when people may have later discarded the tools.

They knew the original rock from which the tools were made might have been shielded from sunlight and thus have a deep luminescence signal until the quarry workers cut into the boulder or cliff face and extracted part of it. Later, being made into a tool and being used would have exposed all sides of the rock to sunlight, bleaching the luminescence signal. But when the tool was discarded, they hypothesized, the side that fell against the ground would have started reaccumulating a luminescence signal.

This would mean that “there’s a signal in that rock for how long the rock was used after it was made,” Gliganic say, “Theoretically, if someone were to then come and pick it up  again, say 3,000 years later, expose it to sunlight and then throw it away again, it would now retain a signature of that usage as well.”

Core drilling on a non-cultural rock. Photo credit: Luke Gliganic

After the Tibet study, Gliganic moved to Australia and began doing some rock surface dating there. At a 2018 conference, he explained the process to McDonald and suggested it might be helpful for dating Murujuga rock art. For the past several years, he’s been gathering evidence to ascertain whether optical surface luminescence dating will work on the Murujuga art. In some ways, the project is simpler than the one in Tibet. The engravings on the rocks would have started the bleaching process, so if he takes core samples, he should be able to measure the remaining luminescence signal and thus figure out when the bleaching started.

However, because the art is connected and sacred to local aboriginal groups—in many other places the connection between rock art and indigenous groups has been lost—Gliganic knows he must be very careful in his research and very sure that it will work before approaching MAC to ask if they want him to take actual samples from the engravings.

Gliganic has spent the last two years doing things like conducting laboratory and field bleaching experiments, where he made his own simple carvings into the different rock types in Murujuga that do not contain any heritage engravings. He then returns at later dates to take core samples and measure the bleaching rates, similar to what Sohbati did in Utah, to calibrate the model to the rocks’ bleaching rate.

“By combining multiple calibration sites across the archipelago, we’re hopefully going to be able to build a calibration model,” he says. “Then we’ll be able to collect core sample from engravings in different areas and say, ‘Oh, the bleaching is five millimeters deep, that means it’s 1,000 years old or whatever.’”

Gliganic hopes to have his field experiments wrapped up in a few months and then present his results. Whether MAC will allow him to take core samples from the art, he can’t say for sure. However, the organization has applied to make Murujuga a UNESCO World Heritage Site, so having scientific data on the age of the rock art could bolster their case. “Murujuga is an incredible place,” Gliganic says, “so dating the art won’t make or break the case for World Heritage recognition. But it would certainly contribute to it.”

Nancy Averett is a Cincinnati-based science and environmental writer.