Carbon-13 and carbon-14 are thus isotopes of carbon-12.Isotopes participate in the same chemical reactions but often at differing rates.Since this rate is slow relative to the movement of carbon through food chains (from plants to animals to bacteria) all carbon in biomass at earth's surface contains atmospheric levels of C is present at atmospheric levels, the molecule must derive from a recent plant product.The pathway from the plant to the molecule may have been indirect or lengthy, involving multiple physical, chemical, and biological processes.For example, every person is hit by about half a million cosmic rays every hour.It is not uncommon for a cosmic ray to collide with an atom in the atmosphere, creating a secondary cosmic ray in the form of an energetic neutron, and for these energetic neutrons to collide with nitrogen atoms.
The ratio of normal carbon (carbon-12) to carbon-14 in the air and in all living things at any given time is nearly constant.
A hydrocarbon found in beach sediments, for example, might derive from an oil spill or from waxes produced by plants.
Cosmic rays enter the earth's atmosphere in large numbers every day.
Levels of C can represent either mixtures of modern and dead carbon or carbon that was fixed from the atmosphere less than 50,000 years ago.
Signals of this kind are often used by chemists studying natural environments.