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Define absolute dating of fossils

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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.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.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.

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.Researchers can measure the amount of these trapped electrons to establish an age.But to use any trapped charge method, experts first need to calculate the rate at which the electrons were trapped.When it comes to determining the age of stuff scientists dig out of the ground, whether fossil or artifact, “there are good dates and bad dates and ugly dates,” says paleoanthropologist John Shea of Stony Brook University.The good dates are confirmed using at least two different methods, ideally involving multiple independent labs for each method to cross-check results.Biostratigraphy: One of the first and most basic scientific dating methods is also one of the easiest to understand.