Mysterious radiation burst recorded in tree rings

@article{Lovett2012MysteriousRB,
  title={Mysterious radiation burst recorded in tree rings},
  author={Richard A. Lovett},
  journal={Nature},
  year={2012}
}
Spike in carbon-14 levels indicates a massive cosmic event — but supernovae and solar flares ruled out. 
2 Citations

Super solar particle event around AD775 was found

A rapid radiocarbon 14C increase of 120/00 in AD774–775 has been reported in cedar and oak tree rings. So far, the origin of the 14C increase is still uncertain and the possible origin is either

The Solar Cosmic-Ray Origin for the Rapid 14 C Increase in AD775

: The rapid 14 C increase in cedar/oak tree rings (12 × 10 − 3 ) and coral rings (10.7 × 10 − 3 ) in AD775 was found. The origin of 14 C increase world-wide is uncertain. The possible origin is

References

A signature of cosmic-ray increase in ad 774–775 from tree rings in Japan

It is argued that neither a solar flare nor a local supernova is likely to have been responsible for the increase in 14C concentrations in tree rings of Japanese cedar trees, and the data are consistent with the decadal IntCal 14C data from North American and European trees.