The most common fossil dating techniques are radiometric dating techniques. Radiometric dating uses knowledge of the decay rates of unstable (radioactive) nuclei to determine, by comparison with the proportion of stable nuclei in a fossil sample, the date of the introduction of the radioactive material into the live organism, especially if the isotope was ingested while the organism was alive.
Carbon dating is the commonly known dating technique, but only dates accurately back to 60,000 years. Isotopes with longer half-lives must be employed to date older fossils. Therefore potassium-argon is the most used technique for dating fossils.
Carbon dating
The radioactive isotope carbon-14 is renewed in the upper atmosphere by cosmic neutrons striking nitrogen gas. Living organisms ingest the atmospheric ratio of carbon-12 to carbon-14, and the ratio of C-14 to C-12 drops within fossils as the C-14 decays radioactively.
The atmospheric ratio of carbon-12 to carbon-14 has varied over the millennia; therefore, calibration is required. Layers of ice in glacial ice sheet samples contain bubbles of ancient atmosphere dating back thousands of years. Using the gas bubbles to determine the isotopic ratios in ancient atmospheres, one can carbon-date organic remains, including fossils, as far back as 60,000 years.
Potassium-argon dating
Since the half-life of carbon-14 is only 5,730 years, carbon dating can be used on only the youngest fossils. The half-life of potassium-40 is 1.3 million years. And since potassium is an essential mineral ingested by organisms, it exists in animals' fossil remains.
Potassium-40 decays to argon-40, a gas, but fortunately the argon gas remains trapped in the rock until heated tremendously, e.g. by vulcanism. The ratio of the parent nuclide to daughter nuclide indicates how long ago the potassium has been solidified in rock.
Unfortunately, only 11 of 100 decayed K-40 atoms become argon-40, and only one of every 10,000 potassium atoms is the K-40 isotope; fortunately, potassium is one of the most abundant minerals on the Earth's surface.
Argon-argon dating
Argon contamination can reduce the accuracy of potassium-argon dating. Argon-39 is a decay element of potassium-39, and the ratio of potassium isotopes is known. Therefore, an alternative method is to find the relative amounts of argon-40 and argon-39, to determine and subtract out exogenous argon.
Furthermore, having to determine absolute amounts of argon entails heating to over a thousand degrees, making accuracy difficult to achieve. Equipment determining the isotope ratios has avoided the problem, via laser heating and mass spectrometry.
Tags: atmospheric ratio, atmospheric ratio carbon-12, back years, Carbon dating, carbon-12 carbon-14
