774–775 carbon-14 spike

The 774–775 carbon-14 spike is an observed increase of around 1.2% in the concentration of the radioactive carbon-14 isotope in tree rings dated to 774 or 775 CE, which is about 20 times higher than the normal year-to-year variation of radiocarbon in the atmosphere.

[3][4] The event appears to have been global, with the same carbon-14 signal found in tree rings from Germany, Russia, the United States, Finland, and New Zealand.

[2][5][6] The signal exhibits a sharp increase of around 1.2% followed by a slow decline, which is consistent with an instant production of carbon-14 in the atmosphere,[2] indicating that the event was short in duration.

This year the Northumbrians banished their king, Alred, from York at Easter-tide; and chose Ethelred, the son of Mull, for their lord, who reigned four winters.

This year also appeared in the heavens a red crucifix, after sunset; the Mercians and the men of Kent fought at Otford; and wonderful serpents were seen in the land of the South-Saxons.In China, there is only one clear reference to an aurora in the mid-770s, on 12 January 776.