False carbon dating reads
This raises questions about the accuracy of carbon dating for very old objects.
Beck and colleagues tested slices of a half-metre long stalagmite that grew between 45 000 and 11 000 years ago in a cave in the Bahamas.
C14 is continually being created and decaying, leading to an equilibrium state in the atmosphere.
When the carbon dioxide, containing C14 as well as stable C12 and C13, is taken in by plants it is no longer exposed to the intense cosmic ray bombardment in the upper atmosphere, so the carbon 14 isotope decays without being replenished.
Uranium 235 decay to lead has a half-life of 713 million years, so it is well suited to dating the universe.
Living organisms and some geological features absorb stable carbon-12 and radioactive carbon-14, which are present in the air in a well-known ratio.
This is part of the carbon cycle - the recirculation of carbon through the oceans, atmosphere, plants and animals.
other isotope pairs cover intermediate time periods between the spans for carbon 14 and uranium.
Some radiometric dating methods depend upon knowing the initial amount of the isotope subject to decay.