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It’s not absolutely constant due to several variables that affect the levels of cosmic rays reaching the atmosphere, such as the fluctuating strength of the Earth’s magnetic field, solar cycles that influence the amount of cosmic rays entering the solar system, climatic changes and human activities.Among the significant events that caused a temporary but significant spike in the atmospheric carbon-14 to carbon-12 ratio were above-ground nuclear test detonations in the two decades following World War II.Another problem derives from the “reservoir effect” in which old material, limestone or graphite, has contaminated the samples. This is particularly true of marine samples and contemporary shells may seem to be hundreds of years old. The explanation was that the physicists had assumed that the amount of C-14 in the atmosphere had been constant, when in fact it had varied over time.The solution came using dendrochronology (tree ring dating).Because the cosmic ray bombardment is fairly constant, there’s a near-constant level of carbon-14 to carbon-12 ratio in Earth’s atmosphere.Organisms at the base of the food chain that photosynthesize – for example, plants and algae – use the carbon in Earth’s atmosphere.
Radiocarbon dating uses isotopes of the element carbon. Cosmic rays – high-energy particles from beyond the solar system – bombard Earth’s upper atmosphere continually, in the process creating the unstable carbon-14. Because it’s unstable, carbon-14 will eventually decay back to carbon-12 isotopes.Each radiocarbon date has a statistical probability shown by the ± number.This number is called a standard deviation and is a measure of the spread of measurements around the mean (average).One standard deviation has a 68% probability and two standard deviations have a 95% probability.Radiocarbon dating has had an enormous impact on archaeology around the world since it made it possible to date carbon and wood could be directly without dependence on characteristic artifacts or written historical records.