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.