06/04/2012 02:44 pm ET

Cosmic Ray Burst In 775 A.D. Seen In Japanese Tree Rings

In 775 C.E., while Charlemagne was ruling his Frankish kingdom, something mysterious struck Earth.

An analysis of the rings of two Japanese cedar trees (typical tree rings shown above), reported online today in Nature, reveals that from 774 to 775 C.E., the atmospheric level of radioactive carbon-14 jumped by 1.2%. This indicates that cosmic rays—high-speed, charged particles from space—bombarded our planet and converted some atmospheric nitrogen-14 into carbon-14.

Earlier work had found a rise in radioactive carbon during the same decade; the new discovery means that scientists have been able to narrow down the date of impact to within just 1 year. The scientists argue against two logical suspects: solar flares are too weak to do the job, and no supernova explosion was seen at the time, nor do any nearby supernova remnants date back to Charlemagne's time.

There's also nothing unusual in the history books. So the cause remains a mystery, but whatever it was, something similar could presumably strike again.

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