Modern-day Santorini, the site of a massive volcanic eruption in the 15th or 16th centuryMichael Virtanen / AP

The latest controversy in a bitter archaeological dispute involves—I kid you not—a literal olive branch.

The olive branch comes from the Greek island of Santorini, where a volcano erupted more than three millennia ago, spewing gas, ash, pumice, and boulders into the sky. Once depleted, the volcano collapsed in on itself. So violent was the eruption, some have speculated, that it ended the once prosperous Minoan civilization, instigated a volcanic winter as far away as China, and inspired the 12 plagues of Exodus as well as the myth of Atlantis—claims that are to varying degrees controversial. But nothing is as controversial, it turns out, as the debate over when the Santorini volcano actually erupted.

The olive branch was supposed to help resolve this. In the 2000s, the geoscientist Walter Friedrich and his graduate student Tom Pfeiffer at Aarhus University found the branch in Santorini under several feet of pumice from that ancient eruption. It looked as if it had been buried alive. They got excited. Because trees grow a new ring every year, the variation in carbon-14 from year to year can be “wiggle matched” to historical levels of carbon-14 in the atmosphere. Using this method, Friedrich arrived at an eruption date of 1627 to 1600 B.C.—a full century earlier than what archaeologists had previously decided based on pottery found near Santorini and elsewhere. The archeologists were not pleased.

It does not help that olive wood is notoriously hard to date. Olive wood doesn’t have clearly delineated rings. Its branches don’t grow in perfect concentric circles. Instead, a new paper in Scientific Reports finds, parts of a branch can stop growing for years and even decades before a tree dies. The olive branch in Santorini, depending on what part of it is sampled, may not give an accurate date for the volcano’s eruption. “It’s a beautiful tree full of significance for us,” Elisabetta Boaretto, a radiocarbon-dating researcher at the Weizmann Institute of Science who led the study, says of the olive tree. “But it’s a difficult tree. It likes to keep its secrets.”

Sturt Manning, an archaeologist and tree-ring researcher at Cornell University—who, for the record, also believes the radiocarbon dating suggests an eruption date of around 1600 B.C.—points out that accounting for this problem would only move the date a few decades. That still doesn’t get to 1500 B.C., the date many old-school archaeologists continue to defend. But he predicts the uncertainty raised in this paper will have consequences. Critics “will cite this paper for the next 50 years as one of the reasons to always be a bit worried about what scientists say,” Manning says.

In fact, archaeologists did pounce the last time scientists raised concerns about olive tree rings. In 2014, Paolo Cherubini, a tree-ring researcher at the Swiss Federal Institute for Forest, Snow and, Landscape Research, published a paper in the journal Antiquity detailing how he got 10 tree-ring experts to count the number of rings in olive trees currently growing on Santorini. Their answers varied 24.5 to 56.3 percent from the median. The review process of the paper inspired enough controversy that the journal devoted a whole section to the debate around dating the eruption at Santorini, or Thera, as it used to be called.

In response, J. Alexander MacGillivray, an archeologist with the British School at Athens, wrote, “The single piece of evidence that might have persuaded some archaeologists to support the ‘high’ 1613+/−13 BC date for the Theran eruption is hors de combat.” He titled his piece “A Disastrous Date.” Yes, he confirmed to me by phone from a shack on the island of Crete, the title was a very deliberate play on words. “When the first olive-branch data came out, I spent a weekend in the library trying to work that date into the history, and it simply wouldn’t come,” MacGillivray said. So, yeah, he was a little piqued when he wrote the comment in Antiquity.

An excavated portion of the city of Akrotiri on present-day Santorini, which was covered with ash in the volcanic eruption (Yorgos Karahalis / Reuters)
A 17th-century B.C. gold ibex found in Akrotiri (Yorgos Karahalis / Reuters)

To be fair, the carbon-14 dating of the Santorini eruption does not rely on that one olive branch alone. Research groups such as Manning’s have dated “short-lived material,” like seeds, found in storage buried under the volcanic debris of Santorini. Still others have dated insect shells in the ash to around 1600 B.C. Another group has suggested that chemical signatures in ice cores in Greenland can help date the Santorini eruption—a claim that has also been contested by other scientists.

Manning admits that scientists working in the early days of radiocarbon did make mistakes, but the technique has gotten a lot more advanced and a lot more reliable. “Now if you send off a radiocarbon sample to one of the best half-dozen labs in the world, you’d expect to get a result that comes back with an error of probably plus-minus 15, 20 years,” he says. On the other hand, he says, the process by which archaeologists came up with the 1500 B.C. date is “basically by guesswork.” “It’s just a different way of doing things. It’s a different ideology,” MacGillivray says. The Egyptologist Manfred Bietak, a staunch defender of the traditional date, framed it before as a matter of science versus archaeology. (He also called Manning’s book on the eruption “repetitive and confusing” in a 2016 review.)

“It’s an age thing,” says Manning, who says he doesn’t care for the science-versus-archaeology angle. “Most of the scholars who actively pushed for using these new techniques were probably in their 20s and 30s when they started and now they’ve reached their 50s”—including Manning—“and those defending the orthodox position … probably were in their 40s and 50s or older back then, and now in their 70s, 80s, or 90s.” For his generation, he says, it’s typical to work in integrated teams of archaeologists, historians, physicians, and chemists.

Friedrich and his group actually found a second olive branch on the island, too. The analysis on that has never been published, much to the bafflement of the rest of the field. (Friedrich did not respond to an inquiry about this.) “Very strange is the fate of the second branch,” Bietak wrote in an email to me. He went on, “One may think therefore that the dates were not in the range of expectations of the Danish team.”

Despite the mystery, there are other tantalizing hints at more conclusive evidence of the eruption’s date. Excavations at Santorini got their start in the 19th century, when crews began mining the ash to make concrete for the Suez Canal. An oak tree was supposedly found buried in the volcanic debris in the 1860s, Manning says, and later a pine tree. Both trees are much more straightforward to date than olive, but no one knows what happened to them. The mining has stopped, and Santorini is now a picturesque town with white-washed houses along the island’s cliffs. But discoveries are much fewer and further in between. If only, Manning says, an archaeologist had been there when the Suez company was doing all its mining.

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