774–775 carbon-14 spike
The 774–775 Carbon-14 Spike was an increase of 1.2% in the carbon-14 content of tree rings during the years AD 774 or 775, which was about 20 times higher than the normal rate of variation. It was found during a study of Japanese cedar trees, with the year of occurrence determined through dendrochronology.  A surge in a specific isotope of beryllium (10Be), detected in ice cores of Antarctica, has been associated with the 774-75 event.
The event appears global with the same signal found in carbon-14 in tree rings from Germany, Russia, USA and New Zealand. There was also a "red crucifix" which the Anglo-Saxon Chronicle recorded in the skies of Britain for the year 774 AD; since no supernova remnant has been found for this year this is interpretable as an aurora borealis.
The increase has a shape with a sharp increase of ~1.2% followed by a slow decline (see Figure 1). This shape 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 Q= (1.1-1.5)×108 atoms/cm2
Several possible origins for the event have been considered.
The common paradigm is that the event was caused by a solar particle event (SPE) from a solar flare that was very strong, the strongest ever known, but still within the Sun's abilities. It was also proposed that the corresponding solar flare might have been triggered by a cometary impact on the Sun. The solar hypothesis is supported by the fact that other similar, although weaker, events were identified, such as the event of 994 AD  which makes an exotic scenario unlikely. Plus, the comet impact scenario could have been caused by the comet X/773 B1.
How frequent are such events?
The event of 774 AD is the strongest spike over the last 11 000 years in the record of cosmogenic isotopes, but it is not unique. Another event similar to that of 774 AD, but a factor 1.5 weaker, was discovered in another carbon-14 record for the year 994-995 AD. A list of other candidates for other events of the kind over the Holocene can be found in.
From this statistics one may expect that such strong events occur once per tens of millennia, while weaker events may occur once per a millennium or even a century. The event of 774 AD did not cause catastrophic consequences for the life on Earth, but if it had happened in modern times, it would produce catastrophic damage to the modern technology, including communication and navigation space-borne systems.
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