Carbon 14 and potassium argon dating methods
Both plants and animals exchange carbon with their environment until they die.Afterward, the amount of the radioactive isotope carbon-14 in their remains decreases.Paleontologists still commonly use biostratigraphy to date fossils, often in combination with paleomagnetism and tephrochronology.A submethod within biostratigraphy is faunal association: Sometimes researchers can determine a rough age for a fossil based on established ages of other fauna from the same layer — especially microfauna, which evolve faster, creating shorter spans in the fossil record for each species.Researchers can first apply an absolute dating method to the layer.They then use that absolute date to establish a relative age for fossils and artifacts in relation to that layer. Anything below the Taupo tephra is earlier than 232; anything above it is later.Egyptologists, for example, created a relative chronology of pre-pharaonic Egypt based on increasing complexity in ceramics found at burial sites.Whenever possible, researchers use one or more absolute dating methods, which provide an age for the actual fossil or artifact.
Over time, certain kinds of rocks and organic material, such as coral and teeth, are very good at trapping electrons from sunlight and cosmic rays pummeling Earth.
Measuring carbon-14 in bones or a piece of wood provides an accurate date, but only within a limited range.
Says Shea: “Beyond 40,000 years old, the sample is so small, and the contamination risk so great, that the margin of error is thousands of years.
Paleomagnetism is often used as a rough check of results from another dating method.
Tephrochronology: Within hours or days of a volcanic eruption, tephra — fragments of rock and other material hurled into the atmosphere by the event — is deposited in a single layer with a unique geochemical fingerprint.