Radioactive carbon-14 levels measure much higher than normal.
In the late eighth century, Earth was hit by an extremely intense burst of high-energy radiation of unknown cause, say Japanese scientists.
A team led by Fusa Miyake of Nagoya University found that radioactive carbon-14 levels in two cedar trees were much higher in 774 and 775 compared to other years, and was far bigger than any known flare from the Sun.
Earth is constantly battered by protons and other sub-atomic particles blasted across space by high-energy sources. The particles collide with nitrogen-14, which then decays to radioactive carbon-14 . This background radiation produces a continuous source of carbon-14 for radiocarbon dating.
But if not from the Sun, then where did that cosmic ray blast originate? asked Miyake, a cosmic-ray physicist.
The other possibility is a supernova, or a star that explodes at the end of its life in a welter of gamma radiation. But there is no documented record in the northern hemisphere of a supernova at around 775.
“With our present knowledge, we cannot specify the cause of this event,” Miyake admits.
The team intends to fine-tune their search by looking at telltale traces of beryllium and nitrate isotopes.
They also plan a wider search of historical documents to see if, 1,237 years ago, anyone noted a strange flare in the sky.
The researchers reported their finding in the journal Nature on Sunday.
The team found that carbon-14 levels in the two cedars were about 1.2 percent higher in 774 and 775 compared to other years.
Compare this to the Gothenburg magnetic reversal of 11,500 years ago, when carbon-14 levels shot upward some three to four times normal.
Or look at the Mono Lake magnetic reversal of 23,000 years ago, when carbon-14 levels soared to four to five times normal. (“Magnetic Reversals and Evolutionary Leaps,” p. 103)
Thanks to David Newton, Benjamin Napier and Steven Woodcock for these links
Miyake, F., Nagaya, K., Masuda, K. & Nakamura, T. Nature http://dx.doi.org/10.1038/nature11123 (2012).
Reader Duster sent this additional info:
“The article notes that an historic record from the time notes what sounds like profoundly increased aurora or something similar, and oddly, a separate source (The Daily Mail citing New Scientist):
“… the only contemporaneous record is from a 13th-century English chronicler, called Roger of Wendover, who, according to New Scientist, is quoted as saying: ‘In the Year of our Lord 776, fiery and fearful signs were seen in the heavens after sunset; and serpents appeared in Sussex, as if they were sprung out of the ground, to the astonishment of all.’
“My take would be a solar flare or flares,” says Duster. “If a single one, it would have to have been more powerful than a Carrington Event. Not sure where the snakes came from.”