© Eden, Janine and Jim, FlickrAn ice core from Greenland.
Ice cores and tree rings around the world show mysterious increases in the concentrations of certain elements around 994 CE. The newest evidence originates in Antarctic ice, validating the prior observations and suggesting that the cosmic rays came from the Sun.
The Japanese scientists behind the new research analysed ice cores from Greenland as well as from two locations in Antarctica. They looked specifically at the concentration of beryllium-10, a radioactive but long-lived form of beryllium with one more neutron than the most common beryllium atoms. This element is typically produced when high-energy particles from space called cosmic rays strike certain atmospheric atoms, like oxygen. Ice cores can store a record of these atoms and their concentrations over time in layers, kind of like tree rings.
The researchers found a 50 per cent increase in the concentration of beryllium-10 in the Antarctic ice cores around 992 CE, according to the paper published recently in Geophysical Research Letters. They point out that there isn't much data elsewhere on the beryllium-10 concentrations, likely because the signal is so small that it could be hard to pick out of background noise or other terrestrial processes that could form beryllium-10.
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