Carbon 14 Dating

QUESTION: How does carbon 14 dating work?

ANSWER:

To understand how carbon 14 dating works, you first have to understand what carbon-14 is and what part it plays in our biosphere. All living creatures are made in part of carbon. As they live and grow and interact with their environments they consume more and more carbon. By far, the most abundant form of carbon is carbon-12. Carbon-12 is a stable isotope; that is, it doesn't decay naturally. Carbon-14 on the other hand is an unstable isotope; that is, it decays naturally over time. Carbon-14 is also relatively rare. It is produced naturally in the atmosphere and mixes with carbon-12 becoming part of our planet's biosphere.

As creatures consume carbon from their environment and incorporate it into their bodies, they consume both carbon-12 and carbon-14. When a creature dies, it ceases to consume more carbon. Since carbon-12 doesn't naturally decay while carbon-14 does, once a creature stops incorporating more carbon into its body, the ratio of carbon-12 to carbon-14 in its body begins to change, with less carbon-14 per carbon-12 as time passes.

Since creatures incorporate carbon-12 and carbon-14 into their bodies at about the same ratio as it occurs in the atmosphere during their lifetimes, by looking at the ratio in the atmosphere today and by comparing it to the ratio as it is found in the specimen we are examining, we are able to determine when the specimen stopped consuming more carbon-14 (i.e. when it died). This is, of course, assuming that we know how long it takes for carbon-14 to decay and that we know that the ratio of carbon-12 to carbon-14 in the atmosphere today is about the same as if was during the creature's lifetime. If either assumption is wrong carbon-14 dating doesn't work.

The first assumption is fairly reasonable, though some scientists have recently called into question whether or not nuclear decay rates were accelerated in the past (this might change the rate of carbon-14 decay). The second assumption isn't as reasonable seeing as we know that the carbon-12 to carbon-14 ratio changes. Scientists have tried to minimize the uncertainties by using other dating techniques like dendrochronology (tree-ring dating) but their success is questionable and carbon-14 dating remains a controversial dating technique.