November 14, 2007

Scientists Use Tree Rings to Date 3,000-Year-Old Volcanic Eruption

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Within every tree lies a coded history to its past, and when stitched together with the information found in other trees, this record can span the past 10,000 years. Each year, a tree adds on a new growth ring to its trunk, preserving and reflecting the information about its surrounding climate and environmental conditions based in part on the width of that ring. Prof. Sturt Manning, classics, and Cornell’s Lab of Dendrochronology have been able to use tree-ring chronologies in combination with radiocarbon dating in order to place important events, such as the massive volcanic eruption at Santorini in the late 17th century B.C.
Dendrochronology, the study of these barcode-like patterns in wood samples, is able to find overlaps between the patterns in living trees and slightly older ones in order to build chronologies that go back thousands of years.
Cornell’s lab uses these chronologies to attempt to pinpoint ancient dates in areas such as the eastern Mediterranean as well as the more recent dates of various historical houses in the Northeastern United States.
Manning, the director of the Malcolm and Carolyn Wiener Laboratory for Aegean and Near Eastern Dendrochronology, presented his most recent research findings, such as the dating of the Santorini eruption, on Oct. 19 as a part of Trustee-Council Weekend.
“There have been efforts for a number of years to try and date when this eruption was. It buried the whole city on the island, so it was a lot like the eruption at Pompeii,” Manning explained.
Based on wood and seed samples found buried under volcanic ash from Santorini, Manning and colleagues found that the explosion took place 100 years before historians previously believed. The Santorini eruption was initially placed at around 1500 B.C. by archeologists who relied on inscriptions and pottery styles as their primary dating method. Based on dendrochronological findings, the event can be dated between 1660 and 1613 B.C.
“In radiocarbon terms this is very simple, the only problem is that it’s about 100 years earlier than it should be in terms of archeological history. So it must be wrong, according to many archeologists,” Manning said. “We had over 100 radiocarbon dates in that study that were based at two different laboratories. So it’s become a bit of a science versus humanities debate.”
This change in date is critical for understanding the human history of the Aegean region during the late Bronze Age.
“When you’re looking at a volcanic eruption, you’re looking at an event that affects people. And on a human lifetime, one year is quite a long time,” said Charlotte Pearson, research associate at the Cornell Tree-Ring Lab. “So it’s really important to get an absolute date.”
Combining the principles of dendrochronology with the tools of chemistry, Pearson was able to analyze the concentration of elements in each individual growth ring of the tree samples found at Santorini. Many of the elements present in the air after a volcanic eruption, such as sulfur, could be found in high levels in the growth rings added on during the years of the explosion.
The dendrochronology lab’s multidisciplinary approach to providing historical dates can be applied not only to the study of ancient history, but also to the more history of the past couple hundred years.
Dating historical buildings of the upstate N.Y. region is another area of active research in the lab. Last spring, Carol Griggs, a research associate, conducted a study at Cornell’s own McGraw Hall.
“We took samples from the hemlock beams in the attic,” Griggs said. “We didn’t have many pieces from 1830 through 1910, so this was a good addition to our collection.”
Samples from the beams dated the McGraw Hall attic to 1870, which correctly corresponded with historical data on the dates of the building’s construction.
The Tree-Ring Lab is continuously adding to its collection in order to create more fluid connections between gaps in its chronologies.
“The thing with dendrochronology is that it’s the one dating method that if you get it right, it can provide you with an absolute date, and there’s no arguing with it,” Pearson said. “So if you can get it to work, it’s probably one of the best dating methods there is.”