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Tree ring dating confirms historical accounts of 'blood aurora'

A woman cuts a tree with a chainsaw

Amy Hessl collects tree ring samples in Mongolia in 2012. (Photo credit: Neil Pederson)

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In a paper published today in Nature Communications, a worldwide team of researchers has used tree ring dating to confirm that two significant "cosmic events" occurred in 774 and 993 CE. Cross-cultural eyewitness accounts of red or "blood" aurora correspond with these years. The study measured carbon-14 content in 44 wood samples taken from five continents, including two samples from Mongolia provided by West Virginia University geographer Amy Hessl, a co-author on the paper. 

The findings are important to many fields of study, including astronomy, ecology and history. The research validates the accuracy of dendrochronology, or tree ring dating, to identify the year a given ring formed. The paper also establishes global carbon “bookmarks” that will show up in almost any tree with rings that lived through 774 or 993, which means archaeologists and preservationists can find accurate ages of more first-millennium samples and artifacts than ever before. Finally, this study may also help assess the threat of space weather on our society.

Amy Hessl
Professor, Dept. of Geology and Geography
WVU Eberly College of Arts and Sciences

“Dendrochronology, the science of tree ring dating, has been used for more than a century to reconstruct climates, date archeological sites and provide histories for a host of other environmental and chemical phenomena preserved in the rings of trees. In this method, we compare the variability in ring width among living and dead trees to find times where both were alive. This is the first time that an independent method—high-resolution radiocarbon values—has been used to confirm the annual precision of cross-dating, so it’s a tremendous validation for our discipline.” 

Amy Hessl audio file

Maura McLaughlin
Eberly Distinguished Professor of Physics and Astronomy, Dept. of Physics and Astronomy
WVU Eberly College of Arts and Sciences

“Some of the most important and enigmatic solar observations were recorded not by sophisticated instruments, but through geologic history. The increased abundance of highly energetic particles in the tree rings Amy Hessl and her colleagues studied are almost certainly due to extremely bright ‘superflares’ from the sun. These superflares are brighter than any we have observed with our telescopes and hence offer unique insights into solar energetics and solar activity levels that have just not been possible with modern technology. If we experienced a superflare today, it could knock out GPS satellites and even our electric grid, temporarily sending us back in time.”

Jennifer Thornton
Assistant Professor of Public History, Dept. of History
WVU Eberly College of Arts and Sciences

“Trees are a type of living archive, recording historical information as they add new growth rings each year. Applying this information to our understanding of the past, however, requires creative and interdisciplinary collaboration. Amy Hessl and her colleagues have been able to identify global markers for two key cosmic events, which were described by eyewitnesses in 8th and 10th century chronicles. These exciting new developments mean that historians and archaeologists will be able to write more richly nuanced histories, even for places and time periods lacking extensive written records.”


“Tree rings reveal globally coherent signature of cosmogenic radiocarbon events in 774 and 993 CE” by Ulf Buentgen et al. Nature Communications, 6 Sept. 2018. DOI: 10.1038/s41467-018-06036-0. 

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