By comparing how much carbon-14 there is in the dead organism with the amount in a living one, the age of the dead organism can be estimated.The half-life of uranium-238 is 4500 million years.When it decays it forms thorium-234 which is also unstable.Finally, after a series of radioactive isotopes are formed it becomes lead-206, which is stable.The starting ratio of radiocarbon to stable carbon is locked in at that point. The purpose in each of these methods is to determine the ratio of radiocarbon to stable carbon in the sample.From then on, the ratio of radiocarbon to stable carbon will decrease, because the unstable radiocarbon atoms will slowly decay. From this measurement the age in radiocarbon years is calculated. Modern radiocarbon dates are calibrated using long tree-ring chronologies.Plants obtain all their carbon atoms from the atmosphere.
When an organism dies (whether plant or animal) its intake of carbon atoms ceases.It provides an objective, absolute method of determining a sample's age with quantifiable precision.The foregoing article was primarily based on a discussion of radiocarbon dating found in The Biblical Chronologist Volume 5, Number 1. When an organism, eg a tree, dies it stops taking in carbon dioxide.The amount of carbon-14 in the wood decreases with time as it decays into nitrogen with a half-life of about 5700 years.