From Gregg Brunskill
You claimed that plutonium does not occur naturally (12 July, p 36). Small amounts of plutonium-239 are found in uranium ores – about 1 part in 1011 of uranium – and plutonium-244 has been detected in the spectrum of the sun. All nuclear-reactor fission nuclides have also been found in the Earth’s geological record, especially in the ancient natural reactor at Oklos in Gabon, as reported by François Gauthier-Lafaye and colleagues in 1996 (Geochimica et Cosmochimica Acta, vol 60, p 4831).
Alligator Creek, Queensland, Australia
