The reason is that, as long as the organism is alive, it replaces any carbon molecule that has decayed into nitrogen.After plants and animals perish, however, they no longer replace molecules damaged by radiocarbon decay.Instead, the radiocarbon atoms in their bodies slowly decay away, so the ratio of carbon-14 atoms to regular carbon atoms will steadily decrease over time (Figure 1c).
If we know what fraction of the carbon atoms are radioactive, we can also calculate how many radiocarbon atoms are in the lump.
Knowing the number of atoms that decayed in our sample over a month, we can calculate the radiocarbon decay rate.
By comparing the surviving amount of carbon-14 to the original amount, scientists can calculate how long ago the animal died.
Since the atmosphere is composed of about 78% nitrogen,2 a lot of radiocarbon atoms are produced—in total about 16.5 pounds (7.5 kg) per year.
We can measure in the laboratory how many carbon-14 atoms are still in the skull.
If we assume that the mammoth originally had the same number of carbon- 14 atoms in its bones as living animals do today (estimated at one carbon-14 atom for every trillion carbon-12 atoms), then, because we also know the radiocarbon decay rate, we can calculate how long ago the mammoth died. This dating method is similar to the principle behind an hourglass.6 The sand grains that originally filled the top bowl represent the carbon-14 atoms in the living mammoth just before it died.
Although many people think radiocarbon dating is used to date rocks, it is limited to dating things that contain the element carbon and were once alive (like fossils).
Rb)—are not being formed on earth, as far as we know.
When animals eat the plants, the carbon-14 enters their bodies.