History Stories

A new study says that tree rings bearing evidence of intense solar storms could allow archaeologists to pinpoint the exact dates of historical events.

Shortly after sunset one evening in 775 A.D., a strange “red crucifix” appeared in the western skies over Great Britain, according to records in the Anglo-Saxon Chronicle. Scientists theorize the unusual celestial display could have resulted from a violent solar storm that erupted on the Sun and unleashed a major burst of high-energy radiation that blasted the Earth’s upper atmosphere. A similar event occurred again centuries later in 994 A.D.

While those intense radiation bursts unleashed from the Sun caused an unusual light show in the skies centuries ago, they might also revolutionize the study of ancient civilizations. According to a new study published in the Proceedings of the Royal Society A, trees across the world that were living during massive solar storms experienced a rare spike in levels of the radioactive isotope carbon-14—up to 20 times normal levels—that were absorbed into their rings. The Oxford University researchers who authored the study say that those rings could act as “secret clocks” to allow archaeologists to pinpoint exact dates of major events that occurred in ancient history, such as the building of the Great Pyramid of Giza, the collapse of Mayan civilization and the arrival of the Vikings in North America.

The dramatic jumps in carbon-14 produced by strong solar storms would be present not just in tree rings but in the surviving tissue of any plant growing at the time. That means radiocarbon spikes could be found not just in timbers used to construct ancient buildings but in reeds used to make papyrus and baskets and flax woven into linen. Scientists who spotted a radiation signature of a solar storm in an ancient object made from an organic material would be able to precisely date the artifact as well as historical events connected with it.

“Variations in atmospheric radiocarbon concentration are largely the result of carbon dioxide emissions from activity from volcanoes and the ocean, but they are also influenced by changes in solar activity,” explains Oxford University archaeologist Michael Dee, lead author of the study. “The spikes in 775 and 994 A.D. were almost vertical and of comparable magnitude all around the Earth. Such markers can be easily identified in known-age tree-rings and are fixed in time.”

According to the researchers, archaeologists have been forced to rely on relatively sparse evidence to date the history of Western civilization before 763 B.C. and Chinese history before 841 B.C. Scientists have been forced to use ancient records of rare astronomical phenomena, such as solar eclipses, to date historical events. And even with high-tech tools such as radiocarbon dating, the best that scholars can often do is date objects and events to within a range of 50 to 100 years or more. In addition to being imprecise, dating archaeological finds by comparing the ratio between two isotopes of the element carbon—slowly decaying carbon-14 and stable carbon-12—in organic materials is expensive.

Dee and co-author Benjamin Pope believe that by mining tree-ring data, scientists will be able to detect similar spikes to those of 775 A.D. and 994 A.D. going back thousands of years. Even a handful of these time-markers would improve the dating framework for ancient civilizations, they say.

One challenge facing scientists is that tree-ring data are only available in 10-year blocks not year-by-year, so the study’s authors have proposed the adoption of a new science called “astrochronology” to mine solar storm data through the use of advance mathematics to re-examine existing tree-ring data.

Another potential obstacle is that it’s uncertain just how often intense bursts of solar radiation have struck the Earth. Still, Dee is optimistic about the applicability of the research team’s findings. “In the past, we have had floating estimates of when things may have happened,” he says, “but these secret clocks could reset chronologies concerning important world civilizations with the potential to date events that happened many thousands of years ago to the exact year.”

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