Tree-ring mystery has no clear cause, but it wasn’t a comet

Physics arXiv Blog : In November 2012, while studying tree-ring records of Japanese cedars, Fusa Miyake of Nagoya University in Japan and her colleagues found a spike in atmospheric carbon-14 levels that dated to AD 773. While the date of the spike is certain, the cause is still unclear. Carbon-14 is normally produced by cosmic rays colliding with nitrogen atoms in the atmosphere, so the increased levels had to have been caused by an extraterrestrial source. One possibility is a comet impact, but the comet would have needed to be 100 km wide, according to Ilya Usoskin of the University of Oulu in Finland and Gennady Kovaltsov of the Ioffe Physical-Technical Institute in St. Petersburg, Russia, and no evidence exists of such a large impact. Miyake’s group has also suggested that a nearby supernova or a massive solar flare could have increased the amount of radiation hitting Earth. However, no documentation of a supernova occurring at that time has been found, and the solar flare would have been 1000 times as large as any that have been seen. Some research suggests that the Sun might produce flares on that scale on the order of once every 3000 years, but it isn’t conclusive.