This family of dating methods, some more than a century old, takes advantage of the environment’s natural radioactivity.
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.
It would be like having a watch that told you day and night.” Single crystal fusion: Also called single crystal argon or argon-argon (Ar-Ar) dating, this method is a refinement of an older approach known as potassium-argon (K-Ar) dating, which is still sometimes used.
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.
Researchers can first apply an absolute dating method to the layer.
Afterward, the amount of the radioactive isotope carbon-14 in their remains decreases.
Measuring carbon-14 in bones or a piece of wood provides an accurate date, but only within a limited range.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.For example, New Zealand’s massive Taupo volcano erupted in A. Relative chronology: Researchers have often constructed timelines of a culture or civilization based on the stylistic evolution of its decorative or dramatic arts — that’s why the method is also sometimes called stylistic seriation.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.Paleomagnetism: Earth’s magnetic polarity flip-flops about every 100,000 to 600,000 years.These methods — some of which are still used today — provide only an approximate spot within a previously established sequence: Think of it as ordering rather than dating.