Tree rings reveal mysterious burst of cosmic radiation

By Chelsea Whyte on June 4, 2012 2:57 PM EDT

Tree Rings
Tree rings reveal the secrets of the stars (Photo: Albert Bridge)

A tree's rings hold glimpses of history, if you know how to read them. Two ancient cedar trees have given Japanese researchers insight into a mysterious radiation burst that buzzed our planet over 1200 years ago.

Looking at the amount of the radioactive isotope carbon-14 in tree rings that formed between 774 and 775 A.D., Fusa Miyake and his team at Nagoya University, Japan found a 1.2% increase in just one year, 20 times more than the normal rate of increase, as reported in a new study published in Nature.

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Traces of cosmic rays end up in tree rings when subatomic particles are blasted across space and collide in Earth's atmosphere with oxygen and nitrogen, producing new particles that include carbon-14. Trees then take in the carbon-14 through photosynthesis, and their consumption shows in their annual growth rings.

So what kind of cosmic event could lead to the supersized burst of rays?

The only known events that could produce such a glut of carbon-14 are supernova explosions or storms of protons raining down in the wake of giant solar flares. Miyake tells Nature that neither seems likely, because each should have been large enough to have been observed at the time.

A supernova, which is a dying star that explodes with a burst of gamma radiation at the end of its life, would have left a bright visible star, which isn't on record. And if it were a solar flare, it would have to have been larger than other seen to date.

"I cannot imagine a single flare which would be so bright," Igor Moskalenko tells New Scientist. Moskalenko, an astrophysicist at Stanford University, who was not involved in the work says, it could have been a series of weaker flares over a period of a few years.

"With our present knowledge, we cannot specify the cause of this event," says Miyake, reports AFP. "However, we can say that an extremely energetic event occurred around our space environment in AD 775 ... (but) neither a solar flare nor a local supernova is likely to have been responsible."

"It would be fascinating if there were some record in China or in the Middle East that reported powerful aurora or some other such event" that coincides with the observed increased in carbon-14, said Daniel Baker, an astrophysicist at the University of Colorado's Laboratory for Atmospheric and Space Physics in Boulder, Colorado, according to Nature.

Miyake and his team plan to look further into historical record to see if a wider search reveals any mention of a bright flare in the sky around the 8th century. 

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