Information needed for carbon dating
This carbon-14 labeled carbon dioxide is taken up by plants in their normal process of photosynthesis.Animals and people that eat these plants take the carbon-14 into their bodies as well.
Carbon dating cannot be used on most fossils, not only because they are almost always too old, but also because they rarely contain the original carbon of the organism.Scientists now realize that production of carbon-14 has not been constant over the last 10,000 years, but has changed as the radiation from the sun has changed.Carbon dates reported in the 1950s and 1960s should be questioned, because those studies were conducted before carbon dating was calibrated by comparision with other dating methods.At any given moment all living plants and animals have approximately the same percentage of carbon-14 in their bodies.
When a plant or animal dies it stops bringing in new carbon-14.
Although the half-life of carbon-14 makes it unreliable for dating fossils over about 50,000 years old, there are other isotopes scientists use to date older artifacts.