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, porn ideally involving multiple independent labs for each method to cross-check results. Sometimes only one method is possible, reducing the confidence researchers have in the results. Kidding aside, dating a find is crucial for understanding its significance and love relation to other fossils or artifacts. Methods fall into one of two categories: relative or absolute. Before more precise absolute dating tools were possible, researchers used a variety of comparative approaches called relative dating. 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. 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 oral you can reasonably assume it’s older than anything above it. 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. Earth’s magnetic polarity flip-flops about every 100,000 to 600,000 years. The polarity is recorded by the orientation of magnetic crystals in specific kinds of rock, and researchers have established a timeline of normal and reversed periods of polarity. Paleomagnetism is often used as a rough check of results from another dating method. 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. They then use that absolute date to establish a relative age for fossils and artifacts in relation to that layer. For example, oral New Zealand’s massive Taupo volcano erupted in A.D. 232. Anything below the Taupo tephra is earlier than 232; anything above it is later. 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. Generally speaking, the more complex a poem or piece of pottery is, the more advanced it is and the later it falls in the chronology. Egyptologists, sex 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.
Unlike observation-based relative dating, most absolute methods require some of the find to be destroyed by heat or other means. This family of dating methods, some more than a century old, takes advantage of the environment’s natural radioactivity. Certain unstable isotopes of trace radioactive elements in both organic and inorganic materials decay into stable isotopes. This happens at known rates. By measuring the proportion of different isotopes present, researchers can figure out how old the material is. Sometimes called carbon-14 dating, this method works on organic material. 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. 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. 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. As potassium decays, it turns into argon. 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. While K-Ar dating requires destroying large samples to measure potassium and argon levels separately, Ar-Ar dating can analyze both at once with a single, smaller sample. U-series dating includes a number of methods, each based on different uranium isotopes’ decay rates. 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. 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. Art icle h as been created by GSA Conten t Gene ra tor DEMO!