Tree Rings Could Pin Down Thera Volcano Eruption Date

A new study that combines a mosaic of techniques to confirm the source of a volcanic eruption in 1628 B.C.E. has shown that it wasn’t Thera volcano, but instead Aniakchak II volcano in Alaska.

The modern islands of Santorini in the southern Aegean Sea are the remains of an enormous volcanic explosion that destroyed the earliest settlements on what was formerly a single island called Thera. The Thera eruption occurred sometime before 1500 B.C.E. and was one of the most violent volcanic eruptions in European history.

But the exact date of the eruption and its impact on climate have been debated for decades.

If a volcanic eruption is large enough, it can eject sulfur and debris called tephra into the stratosphere, where both can be circulated to places very far away. The sulfur dioxide from the eruption that makes it into the upper atmosphere reflects heat from the sun and causes temperatures around the world to drop. This climatic shift is reflected in trees, which show reduced growth or frost rings that effectively mark the year in which the eruption occurred.

Frost damage in bristlecone pine tree-rings from California and narrow tree-rings in oak trees from Ireland suggested that a large volcanic eruption cooled the climate about the year 1627-28 B.C.E. Scientists linked the tree ring-anomalies to Thera because at the time of the studies, Thera was the only known large eruption in that approximate time period.

But a new study published by Charlotte Pearson, an associate professor in the Laboratory of Tree-Ring Research of Arizona University, shows that the geochemical fingerprint of the 1628 B.C.E. eruption doesn’t match Thera, but instead Aniakchak II, a large volcanic caldera located on the Alaska Peninsula.

The sulfur and tephra of a large eruption can also rain down on Earth’s poles, where they are preserved in layers of ice. As every layer of ice represents one year, layers showing traces of volcanic tephra can be precisely dated. The ice cores’ tephra, which has a unique geochemical fingerprint, can be used to link the volcanic eruption to an exact volcanic source.

“We’ve looked at this same event that showed up in tree rings 7,000 kilometers apart, and we now know once and for all that this massive eruption is not Thera,” Pearson said. “It’s really nice to see that original connection resolved. It also makes perfect sense that Aniakchak II turns out to be one of the largest sulfate ejections in the last 4,000 years—the trees have been telling us this all along.”

Pearson and her collaborators – which included Michael Sigl of the University of Bern and an international team of geochemists, ice core experts and tephra chronologists – then applied the same method to try to pinpoint the Thera eruption.

They aligned data from tree rings and from ice cores in Antarctica and Greenland to create a comprehensive record of volcanic eruptions across the period – 1680 to 1500 B.C.E. – when Thera must have occurred.

The exact Thera eruption date remains unconfirmed, but the team has narrowed it down to just a handful of possibilities: 1611 B.C.E., 1562-1555 B.C.E. and 1538 B.C.E.

“One of these is Thera,” Pearson said. “We just can’t confirm which one yet, but at least we now know exactly where to look. The challenge with Thera is that there’s always been this discrepancy between multiple lines of dating evidence. Now that we know what the possible dates are, this evidence can be re-evaluated, but we still need a geochemical fingerprint to clinch it.”

Archaeological evidence has suggested the date of the Thera eruption is closer to 1500 B.C.E., while some radiocarbon dating has suggested it’s closer to 1600 B.C.E.

“I favor the middle ground. But we are really close to having a final solution to this problem. It’s important to stay open to all possibilities and keep asking questions,” Pearson said.

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