How can the amount of a radioactive carbon be used to determine the age of a fossil?
All known organisms on planet earth are carbon-based lifeforms – the molecules that we consist of are various combinations of carbon molecules and proteins. Now, while the majority of the carbon in our biomass is the stable isotope carbon-12, there are trace amounts of the isotope carbon-14 in all life. This carbon is regularly cycled through our systems – proteins are used and reformed, making the amount of carbon-14 in organisms pretty much constant. When an organism dies, however, the replenishment of new proteins (and carbon) stops. At this point, the carbon-14 will radioactively decay without replacement. Since we know the half life of carbon-14 (5730 +/- 40 years), we can measure the amount of carbon in a certain amount of biomass to determine its age. If there’s only half as much carbon-14 as there should be, then the organism is 5,730 years old.
