Carbon dating the history of life on earth
The changing ratio of C-12 to C-14 indicates the length of time since the tree stopped absorbing carbon, i.e., the time of its death.
In theory, it might be useful to archaeology, but not to geology or paleontology.Perhaps no concept in science is as misunderstood as "carbon dating." Almost everyone thinks carbon dating speaks of millions or billions of years.But, carbon dating can't be used to date either rocks or fossils.It is only useful for once-living things which still contain carbon, like flesh or bone or wood.Rocks and fossils, consisting only of inorganic minerals, cannot be dated by this scheme.Furthermore, the assumptions on which it is based and the conditions which must be satisfied are questionable, and in practice, no one trusts it beyond about 3,000 or 4,000 years, and then only if it can be checked by some historical means.
The method assumes, among other things, that the earth's age exceeds the time it would take for C-14 production to be in equilibrium with C-14 decay.
Since it would only take less than 50,000 years to reach equilibrium from a world with no C-14 at the start, this always seemed like a good assumption.
Carbon normally occurs as Carbon-12, but radioactive Carbon-14 may sometimes be formed in the outer atmosphere as Nitrogen-14 undergoes cosmic ray bombardment.
The resulting C-14 is unstable and decays back to N-14 with a measured half-life of approximately 5,730 years.
Thus the ratio of stable C-12 to unstable C-14, which is known in today's open environment, changes over time in an isolated specimen. As long as the tree lives, it absorbs carbon from the atmosphere in the form of carbon dioxide, both C-12 and C-14.
Once the tree dies, it ceases to take in new carbon, and any C-14 present begins to decay.