Vast cosmic event leaves record in ancient trees

The wooden hearts of two cedar trees hold a 1200-year-old cosmic mystery ? evidence of an unexplained event that rocked our planet in the 8th century.

Cosmic rays are subatomic particles that tear through space. When they reach Earth they react with the oxygen and nitrogen in the atmosphere, producing new particles. One of these ? carbon-14 ? is taken up by trees during photosynthesis and is "fixed" in the tree's annual growth ring.

Fusa Miyake at Nagoya University, Japan, and his colleagues examined the carbon-14 content of two Japanese cedar trees and were surprised to find that there was a 1.2 per cent increase in the amount of the isotope between AD 774 and 775. The typical annual variation is just 0.05 per cent.

Miyake also found an increase in the carbon-14 record of North American and European trees around that time, as well as an increase in the isotope beryllium-10 in Antarctic ice cores ? another isotope produced by cosmic rays.

What cosmic event led to the ray boost? A supernova would do it, but Miyake points out that such an event would have left a visible trace in today's sky. It could have been a solar flare ? but only if the flare was more energetic than any discovered so far.

"I cannot imagine a single flare which would be so bright," says Igor Moskalenko, an astrophysicist at Stanford University, California, who was not involved in the work. "Rather, it may be a series of weaker flares over the period of one to three years."

This is not the first time that tree records have suggested a cosmic event occurred in the mid 770s. Researchers from Queen's University Belfast, UK, also recently found an increase in carbon-14 in tree rings at that time but their work has yet to be published.

Mike Baillie, a tree ring researcher at Queen's, has found evidence in the historical record that suggests something unusual did indeed happen at that time. The 13th-century English chronicler Roger of Wendover is quoted as saying: "In the Year of our Lord 776, fiery and fearful signs were seen in the heavens after sunset; and serpents appeared in Sussex, as if they were sprung out of the ground, to the astonishment of all."

Journal reference: Nature, DOI: 10.1038/nature11123

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