How can carbon 14 dating can provide the age of a dead animal or plant?

Answer 1

Atmospheric carbon (as carbon dioxide) consists of both C-12 (mostly) and C-14 (actually there's a third but forget it for now). Plants use both isotopes in the formation of sugars. When the plant dies no more C-14 comes in and what C-14 is there (at death) slowly converts into C-12.

Note that animals eat plants (or animals that eat plants) until they die ... after that the same isotope decay occurres.

The half-life of C14 is about 5700 years, so dating of objects greater than 5 - 10 half-lifes becomes very difficult. So 40 - 60 000 years tops. Forget coal.

Answer 2

The atmosphere consists of a certain ratio of carbon 14 to carbon 12. Although this ratio has been in decline since the dawn of the industrial revolution, when humans began pumping depleted CO2 into the atmosphere (fossil carbon contains virtually no C14), the ratio is sufficiently constant that accurate ages can be obtained from any organic material within ten half lives of C14. One half-life of C14 is 5730 years.

Here is how it works. Plants absorb carbon dioxide from the atmosphere, break it apart and release the oxygen. Therefore any living plant would be age zero. Grasses and things that die back every year serve as excellent markers. Trees less so, because the ratio of C14 to C12 declines in the core of a tree as it accumulates carbon.

Animals eat the plant matter and incorporate the ratio of C14 to C12 in their bodies. Animals that eat animals (carnivores) will have a slightly lower proportion of C14 to C12 than herbivores.

When the plant or animal dies, the uptake of C14 stops. The older the carcass becomes, the lower the ratio of C14 to C12. After several hundred thousand years none of the original C14 will remain, and the remains can no longer be dated using radiocarbon.

C14 has been cross checked by tree rings, both in Europe and in North America, back ten to twenty thousand years. The science works pretty well.