The only known events that can produce a 14C spike are floods of γ-rays from supernova explosions or proton storms from giant solar flares. But neither seems likely . . . because each should have been large enough to have had other effects that would have been observed at the time.Read the rest.
A massive supernova, for example, should have been bright enough to produce a 'new' star visible even in the daytime, as was the case for two known supernovae in ad 1006 and ad 1054. Such an explosion would have needed to be brighter than either of these . . . because those events were not large enough to leave traces in the 14C record.
June 05, 2012
Mysterious Radiation in 775
It's already in Wikipedia: Something happened in or about 775 CE — a burst of radiation that affected Carbon-14 levels.