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Tens of thousands of ancient paintings adorn rock outcrops and shelters in the Kimberley region of Western Australia. At some sites, layers of art on rock walls record a sequence of styles and motifs that changed over thousands of years. In about the middle of that sequence, a style called Gwion depicts people in elaborate clothes and headdresses; the figures are often carrying boomerangs, spears, bags, and ornaments.
“The paintings are like a diary to me and my people,” Ian Waina, a member of the Kwini traditional owners of the region, told the Australian Broadcasting Corporation. But neither the region’s traditional owners nor archaeologists are sure exactly how old the Gwion figures are or just where they fit into the timeline relative to other types of rock art in the area. “Everyone wants to find out how old the painting is,” Waina told ABC. “They just say this is from the ‘old people.’ They know the stories, they are keeping those stories, but who is that story from? Is it from our older, older, older people?”
To figure out a more exact age, University of Melbourne archaeologist Damien Finch and his colleaguesincluding the land’s traditional owners, the Kwini and the Balanggarra Aboriginal Corporationturned to an unlikely source: the remains of mud-wasp nests.
Painted-over insect ruins
Humans aren’t alone in leaving archaeological remains behind. Around the Kimberley region, thousands of years of mud dauber wasps have left tiny ruins of their own: long-abandoned mud nests, weathered to flat stumps on the walls of sandstone rock shelters and overhangs. In a few places, the red pigments of the Gwion figures are painted right over the weathered remains of ancient wasp nests.
When mud daubers scoop up material to build their nests, it often includes bits of charcoal from nearby brushfires, which archaeologists can use to radiocarbon date the nest. And Finch and his colleagues are using that as a way to narrow the age of the rock art itself, which is otherwise a challenge to date.
Enlarge/ Ado French, from one of the families of local Traditional Owners, in front of a pair of Gwion rock art figures.
“For the older styles of Kimberley rock art, there is nothing in the remnant pigment that we can date,” Finch told Ars. “The pigment is a red ochre, mostly the iron oxide mineral hematite, or jarosite, so it does not contain any carbon that can be used for radiocarbon dating.” And uranium-series dating, which has helped archaeologists date much older rock art elsewhere, works only in limestone caves where flowing water dissolves and deposits calcium carbonatenot in the open sandstone shelters of Kimberley.
Dating a cultural phenomenon
A nest built on top of a painting is probably younger than the painting, but a nest covered over with pigment is probably older than the painting. At one site, ancient people had painted a figure over the remains of one nest, and some time later, wasps built two more mud nests atop the painting. Radiocarbon dating those nests suggested that the painting is 11,300 to 13,000 years old.
The other 20 paintings in the study only had one nest each, so Finch and his colleagues got only a minimum or a maximum age for each painting. But since the researchers wanted to know how long people had used the Gwion style, rather than figure out the precise age of any one painting, combining all those dates could still tell them something useful.
Twelve nests found on top of rock paintings suggested that the paintings were younger than 12,000 years old. Meanwhile, nests beneath five other paintings suggested that those paintings were at least 13,000 to 15,000 years old. Altogether, those dates provide a big-picture look at how long ancient rock artists used the Gwion style before moving on to other styles and motifs. After some additional calculations, which included factors like the statistical reliability of each date, Finch and his colleagues concluded that the heyday of Gwion painting was 11,520 to 12,680 years ago.
There are always more questions
That’s considerably younger than archaeologists suggested back in 1997, when a study (also using mud-wasp nests but with a different dating method) found that one Gwion painting was at least 16,400 years old. But there’s a limit to how much a single date could say about the history of rock art in the region. “As it was only one minimum age, it told us nothing about how long the Gwion style may have been in vogue,” Finch told Ars.
One of the dates in the study also stood out as much older than the rest; a nest on top of a motif dated to 16,600 years old.
“It is possible the motif really is 16,600 years old, but I am not very confident that it is,” Finch told Ars. “Radiocarbon dating is based on measurement of the radioactive isotope, carbon 14, and radioactive decay is a random process at the atomic level, so any 14C measurement depends on the statistical behavior of all the 14C atoms present. So it is possible, but very unlikely, that a sample of something that is 12,000 years old, when measured, delivers a result suggesting it is significantly different from 12,000 years old.”
Probability calculations suggest that the sample’s date was only moderately reliable, but Finch and his colleagues say it’s also possible that people in the Kimberley started painting Gwion motifs thousands of years before the style’s peak. “Later this year, after I finish my Ph.D., there is something I will try to see if we can confirm the date,” Finch told Ars. But ultimately, the questions can only be tackled properly with more dates.
The big picture
The 21 paintings Finch and his colleagues studied are the largest sample of Kimberley rock art ever dated, from an area spanning 100 kilometers (62 miles) of landscape. But they’re also just a drop in the bucket compared to the tens of thousands of paintings and galleries around the region, and the sampled paintings cover just a small part of the area where Gwion motifs are found.
Finch and his colleagues have at least another two years of fieldwork ahead of them, and they’re seeking permission from two other Aboriginal Corporations to work on their lands.
“The Kimberley rock art sequence is very complex, and there are many thousands of rock art sites, protected by their remoteness, in a region the size of Germany,” Finch told Ars. “It is such a rich record of ancient human activity, we could easily work constantly on this for another 10 years and still not have answers to all the questions we have now, much less any new ones.”
Science Advances, 2020 DOI: 10.1126/sciadv.aay3922 (About DOIs).