- From Nitrogen to Carbon 14 - Cheri Lasswell
In 1947, Dr. Willard Libby first proposed the carbon-14 dating method. Carbon dating (also known as carbon-14, radiocarbon, and radiometric dating) uses radioactivity to determine the age of biological artifacts (wood, bone, cloth, wood and plant fibers) up to 50,000 years old. Carbon dating can’t be used to date non-biological artifacts such as rock or stone.
Understanding the Chemistry Behind Carbon Dating
Elements are a substance made up entirely of one type of atom. Of the 115 known elements, each has a specific atomic number equal to its number of protons. Protons define the element and if the number of protons is altered, the element is altered accordingly. Atoms with a set number of protons but varying neutron counts are called isotopes. Carbon has an atomic number of 6 but can have 6, 7, or 8 neutrons. Nearly all carbon isotopes are the C-12 variety; 6 protons and 6 neutrons. But there’s also C-13 and C-14 isotopes with 7 and 8 neutrons respectively. Since all have 6 protons they are still carbon.
Carbon-14 is produced in the upper atmosphere when a nitrogen-14 isotope (7 protons, 7 neutrons) is bombarded by cosmic radiation. A neutron displaces one of its protons, lowering its atomic number from 7 to 6 and raising its neutron count from 7 to 8 – a carbon-14 isotope. The single displaced proton becomes hydrogen while the new carbon isotope is radioactive and naturally unstable so it decays back into nitrogen after a period of time.
The carbon isotopes combine with oxygen to form carbon dioxide which is absorbed by plants during photosynthesis. Animals and people eat plants and absorb the carbon as a result. The ratio of carbon-12 to carbon-14 in the air and all living things is the same. One in a trillion carbon atoms are carbon-14.
Carbon Dating Premise
When an organism dies, it stops taking in new carbon; the level of carbon-12 remains the same while the carbon-14 begins to decay back into nitrogen. As organic material ages, the ratio (concentration) of C-12 to C-14 decreases. Other factors taken into consideration include the decay rate (-.0693) and the half life (5730 years) of carbon. Half life is the time for half the atoms in a radioactive substance to disintegrate. For example, you have a hundred carbon-14 isotopes and the half life is 5730 years. In 5730 years you’ll have fifty isotopes. In another 5730 years you’ll have twenty-five isotopes, etc. By comparing the ratio of carbon-12 to carbon-14, taking into account the decay rate and half life for carbon, it’s possible to determine the age of a former living thing fairly accurately.
Carbon-14 dating is only reliable up to about 60,000 years and is based on the assumption that the ratio of C-12 to C-14 in the atmosphere has remained relatively unchanged in the past.
Other Methods of Dating
Other radioisotopes used for dating include uranium-235 (half life of 713 million years), potassium-40 (half-life of 1.3 billion years), uranium-238 (half-life of 4.5 billion years), thorium-232 (half-life of 14 billion years) and rubidium-87 (half life of 47 billion years). All can be used to date uranium ores and granitic rocks except potassium-40 and rubidium-87. Potassium-40 is used to date potassium-bearing minerals and rubidium-87 is used for some granite rocks, sandstones, and igneous, sedimentary and metamorphic rocks older than 10 million years.
Known Problems with Carbon Dating
As mentioned, carbon dating relies on the premise that the ratio of carbon-12 to carbon-14 has remained constant over the ages. But fluctuations can and do alter the ratio between the two.
- Experts say radiocarbon is forming 28 to 37% faster than it is decaying therefore the ratio of today reflects a larger store of C-14 than in the past.
- Regional areas of intense volcanic activity spew C-14 depleted carbon dioxide into the atmosphere increasing the levels of carbon-12. On a more intense scale, the industrial revolution had a similar effect caused by the burning of C-14 depleted coal. In both cases, carbon dating would make organic material appear older than its true age by the decreased radioactive concentration.
- Precise measurements taken over the last 140 years have shown a steady decay in the strength of the earth's magnetic field which deflects cosmic rays. The more cosmic rays present in the upper atmosphere, the more carbon-14 is generated.
- Some plants take in less carbon-14 than others, testing older than they really are.
- Carbon dating can't be applied to aquatic life because water is shielded from cosmic radiation so carbon-14 doesn't enter the ocean at an amount substantial enough for proper analysis. This in turn causes problems carbon dating animals that eat mostly seafood.
- Nuclear testing in the 50’s and 60’s significantly increased the radioactive concentration by producing artificial radiocarbon so that anything carbon dated from that time would appear younger than its true age because of the increased radioactive concentration.
Changes in global radiocarbon levels necessitated the use of a reference standard for carbon-14 dating. What was needed was an organic material not contaminated by carbon variations from fossil fuel burning or nuclear weapons testing. Oxalic acid is now the standard for carbon-14 dating. Its radiocarbon content is theoretically the same as a wood sample grown in AD 1950 so that year is the zero point of the radiocarbon timescale used to quote carbon dating results.
References:
All About Archaeology. “Carbon Dating - The Premise.” All About Archaeology.org Accessed May 2010
BETA: The World’s Largest Professional Radiocarbon Dating Service. “Radiocarbon Dating and Bomb Carbon.” Radiocarbon.eu Accessed May 2010
Scientists and Discovery. “Radiometric Dating.” Museumvictoria.com.au Accessed May 2010
Cheri Lasswell - A bookkeeper by profession, writing has always been a passion of mine. I've had two short fiction stories published, await word on a ...
