Though it’s a popular vacation spot today, the Mediterranean island of Mallorca, Spain, wasn’t exactly a hot destination for prehistoric people. Humans began settling large Mediterranean islands like Crete and Sicily more than 12,000 years ago, and by 6,000 years ago, about three quarters of islands in the Mediterranean were inhabited. But even though Mallorca is the sixth-largest Mediterranean isle, our ancestors were in no rush to get there—the first archaeological evidence of human settlement dates to around 4,500 years ago.
However, an unconventional find might push that date back by more than 1,000 years. Geoscientists identified a mineral “bathtub ring” on an ancient stone bridge now submerged in Mallorca’s flooded Genovesa Cave. The ring appears to have been deposited during a period of stable sea level between 5,600 and 6,000 years ago, when the bridge—a simple walkway made of heavy limestone blocks—was only partially underwater.
“We can go back in time, and at 4,500 [years ago], the bridge would have been submerged in 70 centimeters of water,” said geologist Bogdan Onac of the University of South Florida, who led the new study. “The bridge must have been built before that. So we looked at sea level curve.”
Onac and his colleagues reconstructed ancient sea levels by dating mineral overgrowths on stalactites and stalagmites in Mallorcan caves. The results, published in Nature Communications, suggest that the bridge must have been built between 6,000 and 5,600 years ago. The research team’s interpretation is bound to be controversial, particularly because the only human-made artifacts archaeologists have unearthed in Genovesa Cave, some fragments of pottery, are 3,550–3,000 years old—several thousand years younger than the bridge appears to be. Time will tell whether archaeologists accept the finding.
“Clearly, this remains a controversial article whose findings are unlikely to find general acceptance, at least among archaeologists,” said archaeologist Thomas Leppard of the International Archaeological Research Institute via email. “More specifically, archaeological data are needed to make its claims credible.”
Phreatic Overgrowths
Onac and his colleagues didn’t set out to make an archaeological discovery. They’re mostly geologists who have been working on Mallorca for about 15 years to unravel the island’s rare record of ancient sea level.
Mallorca hosts many partially flooded sea caves full of speleothems (stalagmites and stalactites). Where these rock columns pierce the sea surface, they accumulate mineral crusts called phreatic overgrowths as water evaporates. When sea level is constant, the reliable rise and fall of the tides deposit the most mineral at the average sea level. Overgrowths formed at different heights record different periods of sea level standstill.
Phreatic overgrowths are particularly useful because their ages can be precisely determined using uranium-series radiometric dating. And on Mallorca, where there’s not much seismic activity moving the land relative to the sea, Onac’s team has shown phreatic overgrowths to be a reliable all-in-one record of sea level through time. So for 15 years, Onac and collaborators have been carefully documenting, collecting, and dating phreatic overgrowths from the island’s many sea caves to understand past sea level changes.
Underwater Bridge
In the early 2000s, divers exploring the mostly flooded Genovesa Cave discovered the stone bridge. About 8.62 meters long and half a meter high, it once connected two dry chambers of the cave. On the basis of the type of pottery shards discovered in one of the cave’s chambers, archaeologists concluded that the bridge must be no older than 3,550 years, said Onac.
“Then, kind of, everything died,” Onac said. “Archaeologists never went into this cave to look more closely, to do digging.”
Onac’s team remembered the bridge when, in 2022, they published a reconstruction of sea level based on phreatic overgrowths in Mallorcan caves going back about 4,000 years. According to their data, the top of the stone bridge would have been about 80 centimeters underwater at the time that archaeologists thought it was built.
“Who would go into a water [level] that is above them to start building something? It makes no sense.”
“Who would go into a water [level] that is above them to start building something? It makes no sense,” said Onac.
So they looked further back in time, extrapolating backward to the sea level around the earliest accepted date of human arrival on Mallorca some 4,500 years ago. The top of the bridge still would have been about 70 centimeters underwater.
A Controversial Date
Onac sent divers back to the cave to retrieve phreatic overgrowths, which revealed three periods of stable sea level. One, a period of a few hundred years that happened sometime between 5,964 and 5,359 years ago, lined up with a light-colored calcite crust visible at the top of the bridge. This “bathtub ring,” they concluded, must have formed during that period of relatively steady sea level, when the bridge was partially but not entirely submerged.
By extrapolating from their data, Onac’s team determined that sea level would have been right at the top of the bridge about 5,600 years ago. It could have been built earlier, but any later, said Onac, the bridge would have had to have been built underwater. And any earlier than 6,000 years ago, the water level in the cave would have been below 0.25 meter—too low for a bridge of this height to have been necessary.
If the bridge really was built 5,600 years ago or earlier, it would push back the arrival of humans on Mallorca by about 1,000 years.
If the bridge really was built 5,600 years ago or earlier, it would push back the arrival of humans on Mallorca by about 1,000 years. That’d be unusual for the Balearic Islands, to which Mallorca belongs. But in the broader context of the Mediterranean, it would still be quite late.
Regardless, the claim is controversial, said archaeologist John Cherry of Brown University via email. He also noted that the study team included only one archaeologist. This wasn’t for lack of trying, according to Onac. He said he contacted two, who turned him down after hearing his team’s date for the bridge.
Leppard, like Cherry, is wary of accepting the result without seeing further archaeological work to back it up.
“The Balearics have seen a lot of archaeological work; all the reliable dates, and well-dated material types, cluster after about 2,500 [CE],” said Leppard. “This find—a highly unusual artifact type in a highly unusual archaeological context—would be a remarkable outlier.”
—Elise Cutts (@elisecutts), Science Writer
Citation: Cutts, E. (2024), Underwater bridge suggests a surprising date for first migration to Mallorca, Eos, 105, https://doi.org/10.1029/2024EO240440. Published on 8 October 2024.
Text © 2024. The authors. CC BY-NC-ND 3.0
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