A tree-ring expert from the University of Arizona is closer than ever to determining the date of the infamous eruption of the Thera volcano, a goal she has been pursuing for decades.
Charlotte Pearson, associate professor in the Tree Ring Research Laboratory, is the lead author of a new paper in Nexus PNAS which combines a patchwork of techniques to confirm the source of a volcanic eruption in 1628 BC.
The discovery helps researchers determine when Thera’s actual eruption took place.
The massive Thera eruption, known to have occurred some time before 1500 BC, buried the Minoan city of Akrotiri under more than 130 feet of debris. But the exact date of the eruption and its impact on the climate have been debated for decades.
If a volcanic eruption is large enough, it can eject sulfur and debris called tephra in the stratosphere, where both can circulate to very distant places. the sulfur dioxide of the eruption penetrating the upper atmosphere reflects the sun’s heat and drops temperatures around the world. This climate change is reflected in the trees, which show reduced growth or frost rings that effectively mark the year in which the eruption occurred.
Sulfur and tephra can also rain down on the Earth’s poles, where they are kept in layers of ice. When ice cores are analyzed, the amount of sulfate they contain can also be used to estimate the likely impact of an eruption on the climate. High sulfate eruptions have a greater potential to cause short-term changes in climate. At the same time, ice core tephra, which has a unique geochemical fingerprint, can be used to bind sulfur in ice to an exact volcanic source.
Pearson and his collaborators, including Michael Sigl of the University of Bern and an international team of geochemists, ice core tephra experts and chronologists—aligned data from tree rings and from ice cores in Antarctica and Greenland to create a comprehensive record of volcanic eruptions during the period Thera must have occurred – 1680 to 1500 BC They used sulphate and tephra evidence to rule out several events like Thera potential dates resolution techniques to confirm geochemically through ice cores that the eruption recorded in 1628 BC was Aniakchak II.
The exact date of Thera’s eruption remains unconfirmed, but the team narrowed it down to a handful of possibilities: 1611 BC, 1562-1555 BC, and 1538 BC.
“One of them 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 has always been this gap between multiple lines of dating evidence. Now that we know what the possible dates, this evidence can be reassessed, but we still need a geochemical fingerprint to confirm it.”
A blast from the past
As an undergrad in 1997, Pearson read two papers that not only sparked her interest in the science of dark circles, but also marked the starting point for the larger debate over the date of Thera.
The first paper, by UArizona tree-ring researchers Valmore LaMarche and Katherine Hirschboeck, identified frost damage in the tree rings of California bristlecone pines that corresponded to the year 1627 BC. a period of very narrow growth rings in the oaks of Ireland which began in the year 1628 BC.
Both groups of authors linked tree ring anomalies to Thera because, at the time of the studies, Thera was the only known eruption during that approximate time period. But Pearson’s latest paper confirms that these tree-ring anomalies are actually evidence of a different, unusually sulfate-rich eruption, Alaska’s Aniakchak II volcano.
“We looked at this same event that manifested in tree rings 7,000 kilometers away, and we now know once and for all that this massive eruption is not Thera,” Pearson said. “It’s really nice to see that origin connection resolved. It also makes perfect sense that Aniakchak II turns out to be one of the biggest sulphate ejections of the past 4,000 years – the trees have always told us that. .”
Thera Eruption Hunt Continues
Archaeological evidence suggests the date of Thera’s eruption is closer to 1500 BC, while some radiocarbon dating suggests it is closer to 1600 BC.
“I favor the middle ground. But we’re really close to having a definitive solution to this problem. It’s important to stay open to all possibilities and keep asking questions,” Pearson said.
“The construction of evidence in this research is better compared to criminal cases, where suspects must be shown to be linked to both the scene and the time of the crime,” Sigl said. “Only in this case, the traces are already more than 3,500 years old.”
The study also confirms that any climate impact from Thera would have been relatively small, based on comparisons of sulphate peaks during the period with those from more recent documented eruptions.
The next step is to target the possible Thera eruption years and extract other chemical information from the sulfur and tephra in the ice cores. Somewhere in one of these sulphates, there could be a piece of tephra that would have a chemical profile matching Thera.
“It’s the dream. Then I’ll have to find something else to obsess over,” Pearson said. “Right now it’s just nice to be closer than we’ve ever been before.”
The study is part of a project funded by the European Research Council and led by Sigl at the Oeschger Center for Climate Change Research at the University of Bern in Switzerland. The project is called THERA, short for Timing of Holocene volcano Eruptions and their Radiative Aerosol forcing. In addition to Arizona, the study was carried out by an international network of experts from the University of Bern, the University of St. Andrews, the University of Swansea, the University of Maine, the University of South Dakota State and the University of Florence.
Charlotte Pearson et al, Geochemical Ice Core Constraints on the Timing and Climate Impact of the Aniakchak II (1628 BCE) and Thera (Minoan) Volcanic Eruptions, Nexus PNAS (2022). DOI: 10.1093/pnasnexus/pgac048
University of Arizona
Quote: Researchers return to Thera volcano eruption date (May 2, 2022) Retrieved May 3, 2022 from https://phys.org/news/2022-05-home-thera-volcano-eruption-date.html
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