The limestone caves and rock shelters of Indonesia's southern Sulawesi island hold the oldest traces of human art and storytelling, dating back more than 40,000 years. Paintings adorn the walls of at least 300 sites in the karst hills of Maros-Pangkep, with more almost certainly waiting to be rediscovered. But archaeologists say humanity's oldest art is crumbling before their very eyes.
"We have recorded rapid loss of hand-sized spall flakes from these ancient art panels over a single season (less than five months)," said archaeologist Rustan Lebe of Makassar's culture heritage department.
The culprit is salt. As water flows through a limestone cave system, it carries minerals from the local bedrock, and the minerals eventually end up in the limestone. At the limestone's surface, those minerals oxidize into a case-hardened rocky crust. Nearly all of the oldest rock art in Maros-Pangkep—like the oldest drawing in the world that depicts an actual object—is painted in red or mulberry-purple pigment on that hard outer layer. The rock is resistant to most weathering, providing a durable canvas for humanity's oldest artwork.
But beneath the surface, trouble is brewing. Flowing water deposits minerals in the void spaces beneath the mineralized outer crust, and some of those minerals crystallize into mineral salts. As those crystals form, grow, and shrink, they push against the outer layer of mineralized limestone. Eventually, the rocky canvas where people first drew images of their world 40,000 years ago falls apart in hand-sized flakes.
To help understand the extent of the problem and confirm that salt is to blame, Griffith University archaeologist Jillian Huntley and her colleagues collected flakes from the walls and ceilings of 11 caves in the area, including Leang Timpuseng, home of the oldest hand stencil. They found mineral salts like halite and calcium sulfate on the back sides of flakes from three of the sites. And all 11 sites showed high levels of sulfur, which is a key ingredient in many of the destructive salts that worry rock-art conservators.
Exfoliation isn't a new process, but archaeologists and site custodians in Maros-Pangkep say they have watched the process speed up over the last few decades. Some of the local people who manage and protect the rock-art sites have done so for generations, and they report "more panel loss from exfoliation over recent decades than at any other time in living memory," wrote Huntley and her colleagues.
That's no coincidence, according to Huntley and her colleagues.
Here's how the process works: heavy monsoon rains drench Indonesia and the surrounding region from November to March, leaving behind water in cave systems, flooded rice fields, and brackish aquaculture ponds along the coast. The water carries a load of dissolved salts and their mineral ingredients—things like table salt or halite, along with gypsum, sodium sulfate, magnesium sulfate, and calcium chloride.
When the water begins to evaporate, the salt it carried stays behind as crystals, which expand and contract along with changes in temperature and humidity. Some geological salts, like the ones mentioned above, can expand up to three times their original size when heated, and they can put an impressive amount of pressure on the surrounding rock. The result is similar to the freeze-thaw cycles that enable water ice to crack rocks and concrete.
The whole cycle is more active and more pronounced when temperatures rise and the local weather swings from extremely wet to extremely dry every few months. And that's precisely the conditions Indonesia is experiencing as the climate gets warmer and extreme weather events become more frequent. More and more over the last few decades, severe monsoon flooding is followed by periods of intense drought.
People struggle, rocks crack, and a little more of humanity's deepest connection to itself fades away.
"We are in a race against time," said rock-art expert Adhi Agus Oktaviana of Indonesia's National Research Center for Archaeology (ARKENAS). "Our teams continue to survey the area, finding new artworks every year. Almost without exception, the paintings are exfoliating and in advanced stages of decay."
To win the race, archaeologists and conservators will have to work at a small scale, monitoring conditions in individual caves and preserving individual paintings. But the large scale, across the whole complex landscape of Indonesia, also matters. We need to understand and mitigate the impacts of climate change, mining, and intensive farming on the ancient karst landscape itself.
"The more we learn about how climate change is impacting cultural heritage, the better we will be able to tackle these issues," Huntley told Ars.
One key influence lies along the country's coast in networks of brackish ponds where aquaculture farmers raise about 15 million tons of shrimp and fish every year. Many of those ponds are dual-purpose, providing a home for farmed fish in flooded fields where rice grows. People have farmed rice on Sulawesi for at least 7,000 years, but farming has intensified over the previous few centuries—and drastically so over the last few decades.
Aquaculture and expanding rice farming may offer a bulwark against food insecurity as the world's climate grows warmer and less stable. Huntley and others say it's also important to consider—and hopefully mitigate—unintended consequences. Fish farming, especially shrimp farming, can have a devastating impact on marine environments if not carefully managed. The farming may also be indirectly threatening the world's oldest art.
"Holding surface water in these ways enhances humidity, prolonging the seasonal shrink and swell of geological salts, as well as leading to more mineral deposition," said Huntley. "All of which leads to rock art degradation." Regulations from Indonesia's government might help mitigate the problem, but conservators and policymakers need to better understand the scope and the local details of the problem before they can shape a policy that might help.
"Detailed monitoring of the rock art and microclimate on the Maros-Pangkep caves will help us quantify how rapidly the rock art is being impacted, and within the region where areas of higher impact occur," Huntley told Ars.
Conservation agency BPCP has already started a small-scale program to monitor the condition of rock art in some of the area's caves, make 3D digital scans, and measure temperature, humidity, and chemical conditions inside the caves. That kind of work is already standard in some of Europe's most famous Pleistocene painted caves, like Lascaux in France and Altamira in Spain. Huntley and her colleagues argue that Sulawesi's galleries of ancient art deserve the same protection.
With that information, conservators may be able to save some of the most threatened ancient paintings. "Conservation interventions for stone (especially built heritage) are available and well understood, so there are probably a number of options to explore," Huntley told Ars. "Of course, best conservation practice is to undertake trails and apply treatments incrementally to ensure there are no unintended effects."
But she emphasized that the battle to save humanity's earliest recorded stories won't be won in a cave-to-cave fight. "In my opinion, the scale of the salt weathering in southern Sulawesi and the Australasian region is so large that the best mitigation measures will be of an equal landscape scale."
Ultimately, of course, a lot depends on mitigating climate change on a global scale. Otherwise, we may be erasing the oldest signs that we were ever here.
This story originally appeared on Ars Technica.