A report in the Daily Telegraph of the 2nd Feburary 2013 anounced:
Black hole collision may have irradiated Earth in eighth century
"A powerful gamma ray burstcaused by a collision of two black holes or neutron stars may have irradiatedthe Earth in the eighth century, researchers claim."
"A study last year foundunusually high levels of the isotope carbon-14 in ancient rings of Japanesecedar trees and a corresponding spike in beryllium-10 in Antarctic ice.
The readings were tracedback to a point in AD 774 or 775, suggesting that during that period the Earthwas hit by an intense burst of radiation, but researchers were initially unableto determine its cause.
Now a separate team ofastronomers have suggested it could have been due to the collision of twocompact stellar remnants such as black holes, neutron stars or white dwarfs."
The isotope Carbon 14 is the base half-life measurement used in short period radiometric dating, I say short period because the C14 isotope is impossible to detect in material more than 60,000 years old. An irradiation of C14 would therefore replenish the isotope and make accepted calibration of the dating invalid.