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. It was discovered during a study of Japanese cedar tree-rings, with the year of occurrence determined through dendrochronology. A surge in beryllium isotope 10Beberyllium-10, detected in Antarctic ice cores, has also been associated with the 774–775 event. The 774–775 CE carbon-14 spike is one of the several Miyake events and it produced the largest and most rapid rise in carbon-14 ever recorded. 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. The signal exhibits a sharp increase of around 1.2% followed by a slow decline (see Figure 1), which is typical for an instant production of carbon-14 in the atmosphere, indicating that the event was short in duration. The globally averaged production of carbon-14 for this event is (1.3 ± 0.2) × 108 atoms/cm2. Several possible causes of the event have been considered. Annus Domini (the year of the Lord) 774. 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. The "red crucifix" recorded by the Anglo-Saxon Chronicle has been variously hypothesised to have been a supernova or the aurora borealis. In China, there is only one clear reference to an aurora in the mid-770s, namely the one on 12 January 776. Instead, an anomalous "thunderstorm" was recorded for 775. The common paradigm is that the event was caused by a solar particle event (SPE), or a consequence of events as often happen, from a very strong solar flare, perhaps the strongest ever known but still within the Sun's abilities.