Alternative dating systems

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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.

Both methods date rock instead of organic material. But unlike radiocarbon dating, the older the sample, the more accurate the dating — researchers typically use these methods on finds at least 500,000 years old.

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.

The uranium-thorium method is often helpful for dating finds in the 40,000- to 500,000-year-old range, too old for radiocarbon but too young for K-Ar or Ar-Ar.

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.

Biostratigraphy: One of the first and most basic scientific dating methods is also one of the easiest to understand.

Layers of rock build one atop another — find a fossil or artifact in one layer, and you can reasonably assume it’s older than anything above it.

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This family of dating methods, some more than a century old, takes advantage of the environment’s natural radioactivity.

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.

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.

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