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Organisms at the base of the food chain that photosynthesize – for example, plants and algae – use the carbon in Earth’s atmosphere.They have the same ratio of carbon-14 to carbon-12 as the atmosphere, and this same ratio is then carried up the food chain all the way to apex predators, like sharks.At high geomagnetic latitudes, the carbon-14 spreads evenly throughout the atmosphere and reacts with oxygen to form carbon dioxide.Carbon dioxide also permeates the oceans, dissolving in the water.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.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.Since atmospheric carbon 14 arises at about the same rate that the atom decays, the Earth's levels of carbon 14 have remained constant.In living organisms, which are always taking in carbon, the levels of carbon 14 likewise stay constant.

Here’s an example using the simplest atom, hydrogen. Carbon-14 is an unstable isotope of carbon that will eventually decay at a known rate to become carbon-12.

Along with hydrogen, nitrogen, oxygen, phosphorus, and sulfur, carbon is a building block of biochemical molecules ranging from fats, proteins, and carbohydrates to active substances such as hormones.

All carbon atoms have a nucleus containing six protons.

But in a dead organism, no new carbon is coming in, and its carbon 14 gradually begins to decay.

So by measuring carbon 14 levels in an organism that died long ago, researchers can figure out when it died.