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.^[1] A surge in beryllium-10 (¹⁰Be), detected in Antarctic ice cores, has also been associated with the 774–775 event.^[2] The 774–775 CE carbon-14 spike is one of several Miyake events and it produced the largest and most rapid rise in carbon-14 ever recorded.^[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 carbon-14 spike around 774. Colored dots are measurements in Japanese (M12) and German (oak) trees; black lines are the modeled profile corresponding to the instant production of carbon-14.^[2]

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. The globally averaged production of carbon-14 for this event is (1.3 ± 0.2) × 10⁸ atoms/cm².^[2][7][8]

Hypotheses

Several possible causes of the event have been considered.

The Anglo-Saxon Chronicle recorded "a red crucifix, after sunset", which has been variously hypothesised to have been a supernova^[9] or the aurora borealis.^[2][10]

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.

— Anglo-Saxon Chronicle^[9]

In China, there is only one clear reference to an aurora in the mid-770s, on 12 January 776.^[11][12] However, an anomalous "thunderstorm" was recorded for 775.^[13]

As established by Ilya G. Usoskin and colleagues,^[14] the current scientific paradigm^[15] is that the event was caused by a solar particle event (SPE) from a very strong solar flare, perhaps the strongest known.^[16] Another proposed origin, involving a gamma-ray burst,^[8][17] is regarded as unlikely, because the event was also observed in isotopes ¹⁰Be and ³⁶Cl – a gamma-ray burst would not have produced significant ¹⁰Be, and cosmogenic radionuclides are concentrated near the poles, suggesting a flux of charged particles.^[16]

Frequency of similar events

The AD 774/75 event in view of ¹⁰Be, ¹⁴C, and ³⁶Cl

The event of 774 is the strongest spike over the last 11,000 years in the record of cosmogenic isotopes,^[18] but several other events of the same kind (Miyake events) have occurred during the Holocene epoch.^[18] The 993–994 carbon-14 spike was about 60% as strong;^[19] another event occurred in c. 660 BCE.^[20][21] In 2023 the strongest event yet discovered was reported, which occurred in 12,350-12,349 BC.^[22]

The event of 774 had no significant consequences for life on Earth,^[23][24] but had it happened in modern times, it might have produced catastrophic damage to modern technology, particularly to communication and space-borne navigation systems. Also, a solar flare able to produce the observed isotopic effect, would pose considerable risk to astronauts.^[25]

¹⁴C variations are poorly understood, because annual-resolution measurements are available for only a few periods (such as 774–775).^[26] In a 2017 study, a ¹⁴C increase of (2.0%) was associated with a 5480 BC event, but it is not associated with a solar event because of its long duration, but rather to an unusually fast grand minimum of solar activity.^[26]

See also

• Bomb pulse, a man-made C-14 spike
• Carrington Event
• List of solar storms