More than 1,200 years ago, some mysterious event was recorded in tree rings in a Japanese cedar forest.
While one study suggested a solar flare was to blame, a new group of researchers are pointing toward a gamma-ray burst, a powerful space explosion.
The ancient cedar trees record a rare event around 774 or 775 A.D. This shows up in a sharp rise in the amount of radioactive carbon-14 and beryllium-10 recorded in the trees' rings, which can be created by incoming particles from space.
But what caused an influx of radiation?
Tree ring mystery
According to astronomers Valeri Hambaryan and Ralph Neuhauser of the Astrophysics Institute of the University of Jena in Germany, the most likely culprit was a gamma-ray burst. [Amazing Solar Flare of Oct. 22, 2012 (Photos)]
These bursts can be caused when two compact objects, such black holes or neutron stars, slam into each other and release a flood of high-energy gamma-ray radiation.
Such an interpretation, the scientists argue, fits the tree ring mystery, because a gamma-ray burst would be powerful enough to cause the uptick in carbon-14 and beryllium-10. It also fits with the fact that no rare celestial event was observed that year on Earth, at least according to records available now.
The researchers calculated that a gamma-ray burst at a distance of 3,000 and 12,000 light-years from Earth best fits the data.
"If the gamma-ray burst had been much closer to the Earth it would have caused significant harm to the biosphere," Neuhauser said in a statement. "But even thousands of light-years away, a similar event today could cause havoc with the sensitive electronic systems that advanced societies have come to depend on."
The rare forms of carbon and beryllium, which are heavier than the normal varieties of those elements, are created when radiation from space collides with nitrogen atoms in Earth's atmosphere, which then decay into carbon-14 and beryllium-10.
Both chemicals are unstable and decay on predictable time scales, allowing scientists to trace these particular tree rings back to such a specific time in the past. The fact that the jump in carbon-14 and beryllium-10 was only seen in one year's rings means whatever sparked their creation was short-lived.
"The challenge now is to establish how rare such carbon-14 spikes are, i.e., how often such radiation bursts hit the Earth," Neuhauser said. "In the last 3,000 years, the maximum age of trees alive today, only one such event appears to have taken place."
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