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Sometime between 1,237 and 1,238 years ago, the earth was inundated by a massive blast of high-energy radiation greater than any known to have occurred either before or since – but no one knows its source.

This startling fact was uncovered by tree-ring analysis – the same technique that has proved so useful to climate scientists as they attempt to reconstruct temperatures before the narrow band of time for which we have accurate instrumental readings.

The tree-ring analysis and its results are described in a paper published on Sunday in the online version of the journal Nature, written by three Japanese scientists from Nagoya University's Center for Chronological Research, and one from that same school's Solar-Terrestrial Environment Laboratory.

"We argue that neither a solar flare nor a local supernova is likely to have been responsible" for the burst, say the paper's authors, citing the two most likely sources for such an immense surge in high-energy radiation.

The authors' skepticism is well-founded. The increase in ¹⁴C detected during that growing season's tree rings is about 20 times that which is normally found in such sampling. A supernova of that magnitude would surely have been noted by contemporary writers – and none was* – and a solar flare creating that much radiation would likely have wiped out the Earth's ozone layer, "with devastating ecological consequences," according to Nature.com.

For those Reg readers whose remembrances of their college astrophysics is a wee bit rusty, ¹⁴C is created when energy from space hits high-atmospheric atoms and produces neutrons, which then smash into nitrogen-14, which in turn decays into ¹⁴C. This is an ongoing process – in fact, as Nature.com reminds us, the presence of ¹⁴C is what allows us to perform the now-prosaic task of radiocarbon dating.

That normal, steady, day-to-day ¹⁴C creation spiked by 1.2 per cent during the 775 A.D. growing season – a massive increase in what is otherwise an exceptionally stable system – and now appears in the tree rings studied by the Nagoya researchers.

"The work looks pretty solid," space physicist Daniel Baker of the University of Colorado's Laboratory for Atmospheric and Space Physics told Nature.com of the researchers' paper. "Some very energetic event occurred in about AD 775."

The question, of course, is what exactly was that "very energetic event." A supernova that would have been capable of producing the energy required for such a jump would likely have been so bright that it would have been able to be seen during the day. An equally energetic solar flare would've had to have been massively larger than any storm detected since such observations began a century and a half or so ago – not to mention that it would have produced auroral displays of such brilliance that they almost certainly would have been commented upon by contemporary observers.

The University of Colorado's Baker, however, told Nature.com that he wouldn't rule out solar flares entirely, noting that some of those events are accompanied by coronal mass ejections (CMEs) – ludicrously humongous plasma explosions that fire high-energy magnetically charged particles at the earth.

"We know much more these days about how important proton acceleration is at the shock fronts that precede CME structures as they propagate towards Earth," Baker said. "I would like to think about whether a strong CME moving directly towards Earth could have produced the intense proton population that impacted Earth's atmosphere" and led to the spike in ¹⁴C.

Not being bound solely by the strictures of scientific analysis, we at The Reg would like to postulate a third possibility. Perhaps – just perhaps – some hot-rodding alien flashed by the earth at near light speed during the 775 A.D. growing season, and we caught a whiff of his high-energy exhaust.

Or not. Other ideas – equally ludicrous or perceptively analytical – are solicited in Comments.

* Bootnote

Speaking of comments, one reader of the Nature.com announcement of the researchers' paper noted that there was, indeed, a contemporary mention of a celestial display during the time in question. The Anglo-Saxon Chronicles note that in 774 A.D., "This year also appeared in the heavens a red crucifix, after sunset."

Another points out that the Cyclopaedia of Biblical, theological, and ecclesiastical literature, Volume 9 notes that the hidden, martyred body of the rather obscure Irish saint, Rumoldus, was discovered when "Celestial lights marked the place where it lay." Unfortunately, a scholarly 1922 paper on that miraculous event better defines the legend as noting that said display was merely "a mysterious flame of light" that hovered over the river where Rumoldus had been dumped, alerting some fishermen.

The ¹⁴C tree-ring mystery continues.

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