The 774–775 carbon-14 spike is an observed increase of 1.2% in the concentration of carbon-14 isotope in tree rings dated to 774 or 775, which is about 20 times as high as the normal background rate of variation. It was discovered during a study of Japanese cedar trees, with the year of occurrence determined through dendrochronology. A surge in beryllium isotope beryllium-10|, detected in Antarctic ice cores, has also been associated with the 774–775 event. The event appears to have been global, with the same carbon-14 signal found in tree rings from Germany, Russia, the United States, and New Zealand. spike around 774. The colored dots represent the measurements in Japanese and German trees, while the black lines represent the modeled profile corresponding to the instant production of carbon-14. The signal exhibits a sharp increase of around 1.2% followed by a slow decline, 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 calculated as.
Hypotheses
Several possible causes of the event have been considered. The "red crucifix" recorded by the Anglo-Saxon Chronicle was thought to have been a supernova. However, since no supernova remnant has been found for this year, it is interpreted as an 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, 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. Another discussed scenario of the event origin, involving a gamma-ray burst, appears unlikely, since the event was also observed in isotopes Beryllium-10| and Chlorine-36|.
Frequency of similar events
The event of 774 is the strongest spike over the last 11,000 years in the record of cosmogenic isotopes, but it is not unique. A similar event occurred in 993 or 994, but it was only 60% as strong as well as another event of. Several other events of the same kind are also suspected to have occurred during the Holocene epoch. From these statistics, one may expect that such strong events occur once per tens of millennia, while weaker events may occur once per millennium or even century. The event of 774 did not cause catastrophic consequences for life on Earth, but had it happened in modern times, it might have produced catastrophic damage to modern technology, particularly to communication and space-borne navigation systems. In addition, a solar flare capable of producing the observed isotopic effect would pose considerable risk to astronauts. As of 2017, there is "little understanding" of past variations because annual-resolution measurements are only available for a few periods. In 2017, another "extraordinarily large" increase has been associated with a 5480 BCE 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. In 2019, evidence of another "enormous solar storm" in around 660 BCE was published, comparable with the 774–775 and 993 events and likely relevant with the auroral records in Assyrian records. One of the conclusions of the work is that an exclusive reliance on records may lead to underestimation of the number of possibly catastrophic events. High-resolution measurements are needed for, and.
