An article published on 3 June 2012 in Nature reports on a finding that two
ancient cedar trees that were analyzed by researchers from Nagoya University in
Japan show a surge in carbon 14 dating to AD 774 ~ 775.
The source of the carbon 14 is likely to be from cosmic rays that had collided
with the stratosphere and reacted with nitrogen before being absorbed into
the biosphere.
Abstract of Nature article here -
http://www.nature.com/nature/journal...ture11123.html
The authors of the paper argue that neither a solar flare nor a supernova is likely to have
been responsible.
They suggest that it is unlikely to have been as a result of a solar flare because
it would have had to been far bigger than any known flare.
There is also no known historical record of a supernova at that time.
In an article in the Sydney Morning Herald on the finding, it is reported that
both the Cassiopeia A and Vela Junior supernova remnants appear to be too
far away to be the smoking gun either.
So the mystery deepens.
Sydney Morning Herald story here -
http://www.smh.com.au/technology/sci...604-1zr8k.html