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.

Production of Carbon-14

Carbon-14 is produced continuously in the Earthís 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 ® 14C + 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 + O2 ® 14CO2 (radioactive)

12C + O2 ® 12CO2 (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).

Decay of Carbon-14

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 (A0 / A)),

where A is the carbon-14 activity of the sample and A0 is the initial carbon-14 activity of the substance at age zero (Geyh p. 165).

Radiocarbon Dating

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).

Determining Radiocarbon Samples

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).

Radiocarbon Methods

Once a sample is obtained, there are several fundamental methods of radiocarbon dating that may be used including counting techniques and acceleration techniques. In one method, the sample is burned to convert it to carbon dioxide gas. This carbon dioxide is then purified and the amount of radiocarbon in the purified carbon dioxide is measured with radiation counters (Berger p. 2). On a similar approach, the sample is converted to methane gas and then the radioactivity is measured by a Geiger detector (Fleming p. 66). However, these counting techniques require large sample sizes and are less accurate than modern approaches (Geyh p.162). Another method of radiocarbon dating involves preparing the sample as a solvent, such as benzene. The original carbon sample yields a compact liquid in which the radioactivity is intensely concentrated. The isotopic composition of the final benzene product is very close to that of the sample material (Fleming p. 66).

Sample ® CO2 ® Li2C2 ® C2H2 ® C6H6

Yet, another method uses accelerator mass spectrometers instead of radiation counters. The accelerator mass spectrometer technique relies on ionizing the sample before passing it through the accelerator mass spectrometer; the isotopes are separated by a magnetic field and are directly counted (Wilbraham p. 576). This method works well on extremely small samples and often times produces a more precise date of the material (Berger p. 2).

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).

Radiocarbon Dates
The following ages of these artifacts were determined by radiocarbon dating:

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. Martinís, 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. 4th ed. New York: W.H. Freeman & Co., 2000.

Wilbrahm, Antony C. Chemistry 2nd ed. 1987. New York: Addison-Wesley, 1990. (574-576).