Re: Terrestrial origin
Chain reactions can't happen in natural uranium on Earth as the level of fissile U235 is too low to sustain the reaction. Well it is now - if you go back in time, the proportion of U235 in uranium rises to a point that self-sustaining reactions could occur. So far one site is known where this happened, it was discovered at Oklo, Gabon in 1972 by French geologists mapping a uranium deposit. They discovered the level of U235 at Oklo was even lower than normal. About 1.7 billion years ago, the ore would have been about 3% U235 (compared to about 0.7% today), water circulating through a uranium ore acted as a natural moderator allowing a self-sustaining chain reaction to run at very low power for hundreds of thousands of years.
Personally I go with the idea that this C14 spike was caused by Camelot's nuclear testing programme.