Researchers home in on Thera volcano eruption date

Date:
May 2, 2022
Source:
University of Arizona
Summary:
Tree-ring, ice core and volcano experts teamed up to identify one of
the most climatically impactful volcanic eruptions in 4,000 years -
- Aniakchak II. In the process, they narrowed down potential dates
for the Thera volcano eruption.



FULL STORY ==========================================================================
A University of Arizona tree-ring expert is closer than ever to pinning
down the date of the infamous Thera volcano eruption -- a goal she has
pursued for decades.


========================================================================== Charlotte Pearson, an associate professor in the Laboratory of Tree-Ring Research, is lead author of a new paper in PNAS Nexus that combines a
mosaic of techniques to confirm the source of a volcanic eruption in
1628 B.C. While the eruption was previously thought to be Thera on the
Greek island of Santorini, Pearson and her colleagues found instead that
it was Alaskan volcano Aniakchak II.

The finding helps researchers narrow down when the actual Thera eruption
took place.

Thera's massive eruption, known to have occurred sometime before 1500
B.C., buried the Minoan town of Akrotiri in more than 130 feet of
debris. But the exact date of the eruption, along with 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.

The sulfur and tephra can also rain down on Earth's poles, where they
are preserved in layers of ice. When ice cores are analyzed, the amount
of sulfate in them can also be used to estimate the likely impact of
an eruption on climate. High-sulfate eruptions have greater potential
to cause short-term shifts in climate. At the same time, the ice cores'
tephra, which has a unique geochemical fingerprint, can be used to link
the sulfur in the ice to an exact volcanic source.



========================================================================== 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 -- aligned data from tree rings and
from ice cores in Antarctica and Greenland to create a comprehensive
record of volcanic eruptions across the period when Thera must have
occurred -- 1680 to 1500 B.C. They used sulfate and tephra evidence
to rule out several of the events as potential Thera dates and used high-resolution techniques to geochemically confirm through the ice
cores that the eruption recorded in1628 B.C. was Aniakchak II.

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

"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." A blast from the past As an undergraduate
student in 1997, Pearson read two papers that not only sparked her
interest in tree-ring science but also marked the starting point of the
larger Thera date debate.



==========================================================================
The first paper, written by UArizona tree-ring researchers Valmore
LaMarche and Katherine Hirschboeck, identified frost damage in bristlecone
pine tree-rings from California that corresponded to the year 1627
B.C. The other paper, by Queen's University's Mike Baillie and UArizona's Martin Munro, identified a period of very narrow tree-rings in oak trees
from Ireland that started in the year 1628 B.C. Both tree-ring anomalies indicated the sort of abrupt, severe climatic shift that occurs when
volcanoes spew sulfate into the stratosphere.

Both sets of authors linked the tree ring-anomalies to Thera because,
at the time of the studies, Thera was the only known eruption in that approximate time period. But Pearson's latest paper confirms those
tree-ring anomalies are actually evidence of a different, unusually high-sulfate eruption -- Alaska's Aniakchak II volcano.

"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." The Thera eruption hunt continues Archaeological evidence has suggested the date
of the Thera eruption is closer to 1500 B.C., while some radiocarbon
dating has suggested it's closer to 1600 B.C.

"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.

"Building evidence in this research is best compared to criminal cases,
where suspects must be shown to be linked to both the scene and 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 climatic
impact from Thera would have been relatively small, based on comparisons
of sulfate spikes within the period with those of more recent documented eruptions.

The next step is to home in on the possible Thera eruption years and
extract further chemical information from the sulfur and tephra in the
ice cores.

Somewhere in one of those sulfates there might be one piece of tephra
that would have a chemical profile matching Thera.

"That's the dream. Then I'll have to find something else to obsess over," Pearson said. "For now, it's just nice to be closer than we have ever
been before." The study is part of a European Research Council-funded
project led by Sigl at the Oeschger Centre for Climate Change Research
at the University of Bern in Switzerland. The project is named THERA,
short for Timing of Holocene volcanic Eruptions and their Radiative
Aerosol forcing. In addition to UArizona, the study was carried out
by an international network of experts from the University of Bern,
University of St. Andrews, Swansea University, University of Maine,
South Dakota State University and University of Florence. Funding at
UArizona was provided by the Malcolm H. Wiener Foundation.


========================================================================== Story Source: Materials provided by University_of_Arizona. Original
written by Mikayla Mace Kelley. Note: Content may be edited for style
and length.


========================================================================== Related Multimedia:
* Ice_cores,_tree_rings_and_maps ========================================================================== Journal Reference:
1. Charlotte Pearson, Michael Sigl, Andrea Burke, Siwan Davies, Andrei
Kurbatov, Mirko Severi, Jihong Cole-Dai, Helen Innes, Paul G Albert,
Meredith Helmick. Geochemical ice-core constraints on the timing
and climatic impact of Aniakchak II (1628 BCE) and Thera (Minoan)
volcanic eruptions. PNAS Nexus, 2022; DOI: 10.1093/pnasnexus/pgac048 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/05/220502142621.htm

--- up 9 weeks, 10 hours, 50 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)