Cosmic rays showered Earth 1,230 years ago, but where from?

Radioactive carbon-14 found in rings of Japanese cedar trees shows that Earth had a major influx of cosmic rays about 1,230 years ago, but researchers can find no obvious source for the increase. The most likely sources would have been either a major eruption on the surface of the sun or a nearby supernova, either of which could have showered the Earth’s atmosphere with highly energetic radiation, but historical records show no evidence of such an event. For the time being, its cause will have to remain a mystery, Japanese researchers reported Sunday in the online version of the journal Nature.

Carbon-14 has a half-life of 5,730 years, so it eventually disappears from Earth. But new carbon-14 is continually produced when energetic cosmic rays strike the upper atmosphere, striking nitrogen-14 atoms, which then decay to produce carbon-14. Despite that replenishment, the radioactive isotope accounts for only about one-trillionth of 1% of the carbon in the atmosphere. That carbon-14 is taken up by plants in proportion to the amount that is in the atmosphere. When the plant stops growing, no new carbon-14 is incorporated and the isotope that is already there begins to decay at a steady rate. Scientists can then measure the amount of carbon-14 in a specimen and determine how old it is -- up to a limit of about 50,000 years.

Cosmic ray physicist Fusa Miyake of Nagoya University in Japan and his colleagues measured the amount of carbon-14 in tree rings from two Japanese cedars. They found that the amount of the isotope in the ring produced during the growing season of AD 775 was about 1.2% more than in the previous year. Variability between years is normal, but usually only in the range of about 0.05%. The results indicate that some cosmic event between AD 774 and 775 produced a major influx of radiation that created an excessive amount of carbon-14.

A massive supernova nearby might have been powerful enough to produce the influx, but anything powerful enough to produce so much radiation would have been extremely bright, highly visible even during daytime. Records show no such event. Similarly, a massive solar flare might have produced such an influx, but there are no records of such a flare; moreover, a flare powerful enough to produce that much radiation probably would have destroyed the Earth’s protective ozone layer, which would have had disastrous ecological consequences.

Other data supports their finding. Unpublished data obtained by tree-ring researcher Mike Baillie of Queen’s University Belfast in the United Kingdom shows a similar increase in carbon-14 concentrations in AD 775. Data on beryllium-10 -- another isotope produced by cosmic rays -- from Antarctic ice cores also shows a spike during the period.