Nuclear tests, nuclear reactors and the use of nuclear weapons have also changed the composition of radioisotopes in the air over the last few decades.This human nuclear activity will make precise dating of fossils from our lifetime very difficult due to contamination of the normal radioisotope composition of the earth with addition artificially produced radioactive atoms.Also, many fossils are contaminated with carbon from the environment during collection or preservation proceedures.
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.
Scientists use a technique called radiometric dating to estimate the ages of rocks, fossils, and the earth.
Many people have been led to believe that radiometric dating methods have proved the earth to be billions of years old.
However, the carbon-14 already in the organism's body continues to decay at a constant rate.
Therefore, the amount of carbon-14 in an artifact decreases at a predictable rate while the amount of carbon-12 remains constant.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.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.This rules out carbon dating for most aquatic organisms, because they often obtain at least some of their carbon from dissolved carbonate rock.