Radiocarbon Dating
Carolyn Norquist
The need for discovery is an attribute still very characteristic of mankind in this modern age of life. This need for discovery is at the heart of the scientific branches of archaeology and geology as the earth unfolds new mysteries upon curious eyes. Radioactive dating, a notable application of chemistry, has played a primitive role in solving such mysteries in the genre of time. One particular radioactive element, carbon-14, has been extremely successful in the dating of organic materials; the process of radiocarbon dating is outlined in the following report.
Carbon-14 is produced continuously in the Earth’s upper atmosphere as a result of highly energized cosmic rays. These cosmic rays strike atoms, which disintegrate into electrons, protons, neutrons, and other particles. When a free neutron collides with a nitrogen atom and causes it to lose a proton, the radioactive product of carbon-14 is created (Wilbraham p. 575).
^{}14N + n ® ^{14}C + H
(Nitrogen-14 + Neutron ® Carbon-14 + Proton)
Carbon-14 Enters the Food Chain
Radioactive carbon-14 combines with oxygen, just as carbon-12 does to form uniformly mixed carbon dioxide molecules in the air. In the atmosphere there is about one radiocarbon atom for every trillion molecules of carbon dioxide gas (Berger p. 2).
^{}14C + O_{2} ® ^{14}CO_{2} (radioactive)
^{}12C + O_{2} ® ^{12}CO_{2} (nonradioactive)
By means of photosynthesis, plants use carbon dioxide to make carbon-containing compounds; animals proceed to eat plants containing these compounds and radiocarbon has effectively entered the food chain (Geyh p.164). All living organisms contain carbon-12 and carbon-14. Even though the carbon-14 slowly decays, it is continuously replaced so that the ratio of carbon-14 to carbon-12 is constant while the organism is living (Wilbraham p. 576). When the organism dies, carbon-14 is no longer replaced and the constant ratio of carbon 14 to carbon-12 decreases. This ratio, found experimentally in a dead tissue can be used to estimate the amount of time that has lapsed since the death of the organism (Jones p. 979).
Carbon-14 decays by low energy b ^{-} radiation emission to nitrogen 14 with a half-live of 5730 years. Therefore the age of organic matter which has lost carbon-14 by decay can be calculated in the following manner:
t= (5730yrs. / ln 2) (ln (A_{0} / A)),
where A is the carbon-14 activity of the sample and A_{0} is the initial carbon-14 activity of the substance at age zero (Geyh p. 165).
Radiocarbon dating is a process to estimate ages of organic material. American chemist, Willard F. Libby developed this method, immediately following World War II in the late 1940's; he received the Nobel Prize in chemistry for his work in 1960 (Berger p. 1). Radiocarbon dating has been widely applied in archaeology and geology for radiocarbon estimates can be derived from materials such as: wood, charcoal, marine and fresh water shell, bone and antler, peat and organic-bearing sediments (Berger p. 1).
The size of a sample for radiocarbon dating is determined by the carbon content, the degree of preservation, the degree of contamination and the method of carbon-14 analysis (Geyh p. 162). The table below illustrates the carbon content and sample sizes of a few common types of samples (Geyh p. 163):
Type of Sample |
C Content % |
Sample Size (Usual) |
Sample Size (Min.) |
Charcoal (dry) |
50-90 |
3-6g |
50mg-1g |
Wood, peat, grain, tissue (dry) |
10-50 |
3-50g |
2-25mg |
Sediment, soil |
0.2-5 |
50-1500g |
20mg-1g |
Bone, teeth |
1-5 |
60-300g |
20-300mg |
One of the largest problems with samples is contamination. The typical form of contamination results from the intrusion of younger materials in the sample (Geyh p. 173). Before the sample can be analyzed the contamination must be removed or particular fractions must be extracted, which can result in the loss of as much as 90% of the sample. Therefore with such a substantial loss of material it is important to obtain adequate amounts of the original sample (Geyh p. 163).
Sample ® CO_{2 }®_{ }Li_{2}C_{2 }® C_{2}H_{2 }®_{ }C_{6}H_{6}
Error in Radiocarbon Dating
When Willard F. Libby developed the radiocarbon dating method he assumed that the rate of carbon-14 production has been constant through the past 70,000 years (Fleming p. 58). However, the concentration of carbon-14 in the atmosphere has deviated, especially during the last 10,000 years (Geyh p. 167). Therefore, several correction factors have been determined based on the age of samples. The Suess effect can explain recent changes in the carbon-14 concentrations; the equilibrium of the natural carbon-14 cycle was disturbed by man with the onset of the industrial age that began around 1850 (Geyh p. 175). At most the modern samples aged within the last two hundred years will have an error factor of 25 years (Geyh p. 167). For samples within the last 2000 years, error factors range to 200 years; the factor exponentially climbs as the radiocarbon material ages. For the period from 2000 to 7300 years ago, the error factor reaches 800 years and for samples dating to 11,000 years estimates may be off by as much as 1,100 years (Geyh p. 168).
Sample |
Location |
Radiocarbon Age |
Error Factor |
Dead Sea scrolls (Wilbraham p. 576) |
Palestine |
1940 years |
+ 70 years |
Indian mummy (Fleming p. 77) |
Lake Winnemucca, Nevada |
2500 years |
+ 80 years |
Prehistoric sewn boat (Fleming p. 75) |
North Ferriby, Yorkshire |
2700 years |
+ 150 years |
Snail shells (Johnson p. 5) |
Jarmo, Iraq |
6707 years |
+ 320 years |
Deer antler (Johnson p. 8) |
Annis Mound, Kentucky |
4900 years |
+ 250 years |
Glacial wood (Johnson p. 12) |
Lake Butte, Wisconsin |
6864 years |
+ 300 years |
Lotus seeds (Johnson p. 19) |
South Manchuria |
1040 years |
+ 210 years |
Charcoal (Johnson p.18) |
Huaca Prieta, Peru |
4298 years |
+ 230 years |
The vast array of samples that have been collected from around the world to be successfully analyzed by radiocarbon dating prove its importance to the scientific world of discovery. With continued advancements in technology it is likely that more precise and accurate methods of radiocarbon analysis will be developed in the future. However, the basic concept will always remain that carbon-14 is a radioactive isotope that decays with a determined half-life of 5730 years.
Works Cited
Berger, Rainer. "Radiocarbon." CD-ROM. World Book. Chicago: World Book, Inc., 1999.
Fleming, Stuart. Dating in Archaeology: A Guide to Scientific Techniques. New York: St. Martin’s, 1976.
Geyh, Mebus A., and Helmut Schleicher. Absolute Age Determination: Physical and Chemical Dating Methods and Their Application. Trans. R. Clark Newcomb. Berlin: Springer-Verlag, 1990.
Johnson, Frederick. Radiocarbon Dating: A Report on the Program to Aid in the Development of the Method of Dating. Salt Lake City: Society for American Archaeology, 1951.
Jones, Loretta and Peter Atkins. Chemistry: Molecules, Matter, and Change. 4^{th} ed. New York: W.H. Freeman & Co., 2000.
Wilbrahm, Antony C. Chemistry 2^{nd} ed. 1987. New York: Addison-Wesley, 1990. (574-576).