Carbon-14 Dating

Carbon is an important in nature and is one of the basic elements found in all forms of life. It has three isotopes; two of these, carbon 12 and 13, are stable, whereas carbon 14 is radioactive. Carbon 14 has a half-life of 5,730 years plus or minus 30 years. Carbon 14 dating is based on the ratio of carbon 14 to carbon 12 and is generally used to date once-living material.

The short half-life of carbon 14 makes this dating technique practical only for specimens younger than about 70,000 years. Consequently, the carbon 14 dating method is especially useful in archaeology and has greatly aided in unraveling the events of the Late Pleistocene Epoch.

Carbon 14 is constantly formed in the upper atmosphere through the activity of cosmic rays, which are high-energy particles (mostly protons). These high-energy particles strike the atoms of upper-atmospheric gases, splitting their nuclei into protons and neutrons. When a neutron strikes the nucleus of a nitrogen atom (atomic number 7, atomic mass number 14), it may be absorbed into the nucleus and a proton emitted. Thus, the atomic number of the atom decreases by one, while the atomic mass number stays the same. Because the atomic number has changed, a new element, carbon 14 (atomic number 6, atomic mass number 14), is formed. The newly formed carbon 14 is quickly assimilated into the carbon cycle and, along with carbon 12 and 13, is absorbed in a nearly constant ratio in all living organisms.

Because carbon 14 is continually created in the upper atmosphere and distributed throughout the carbon cycle, the ratio of carbon 14 to carbon 12 remains constant in all living things. When an organism dies, however, carbon 14 is not replenished, and hence the ratio of carbon 14 to carbon 12 decreases as the carbon 14 decays.

Currently, the ratio of carbon 14 to carbon 12 is remarkably constant in both the atmosphere and living organisms. There is good evidence, however, that the production of carbon 14, and thus the ratio of carbon 14 to carbon 12, has varied somewhat over the past several thousand years. This was determined by comparing ages established by carbon 14 dating of wood samples against those established by counting annual tree-rings in the same samples. As a result, carbon 14 ages have been corrected to reflect such variations in the past.


How it is formed...
(1) high-energy cosmic rays strike...upper atmospheric gases splitting their nuclei into protons and neutrons (2) free neutron strikes N-14 atom forming a release of P⁺ thus converting N-14 to C-14. C-14 is produced in upper atm mix with rest of atm...the belief is that the ratio of C-14 & C-12 has remained constant over time (3) newly formed C-14 enters the carbon cycle by reacting with oxygen to form CO₂ (4) C-14 (in same ratio as in ATM) enters the food chain when CO₂ is absorbed by plants during photosynthesis (5) when organism dies carbon is no longer incorporated into its tissue and the C-14 that was present at the time of death decays to N-14 and % of C-14 in the remains declines in relation to C-12 with time.

The ratio of C-14 & C-12 can be used to determine when death occured.