SciTech

How Things Work: Carbon-14 Dating

While some college students might have trouble figuring out how long that Pad Thai’s been sitting in the refrigerator, scientists can use carbon-14 dating to determine the age of materials as old as 60,000 years.

Carbon-14 dating, or radiocarbon dating, uses the amount of carbon-14 in a substance to calculate its age.

Scientists can detect the amount of carbon in a material using a process called mass spectronomy, where a magnetic field is used to direct particles toward a sheet of metal with a tiny slit. The magnetic field will curve the paths of the less massive particles to a greater extent than those of the more massive particles, and the slit is placed so that only the particles with the desired mass (that of carbon-14) will go through it.

Carbon-14 dating was developed in 1949 by Willard Libby, who later won the Nobel Prize in chemistry for the discovery and advancement of the procedure.

Radiocarbon ages — those determined by carbon-14 dating — are often labeled as “years BP” (Before Present), which means years before 1950. This scale reflects the level of carbon-14 in the atmosphere in 1950, as determined by Libby and his team.

Currently, the maximum ages scientists are able to determine using carbon-14 dating are within the range of 50,000 to 62,000 years.

Carbon-14 dating is commonly used to determine the ages of old artifacts such as mummies. For example, ages determined by carbon-14 dating were included in a traveling exhibit called The Mysterious Bog People, which visited the Carnegie Museum of Natural History July 2005 through January 2006.

The exhibit featured seven “bog bodies,” mummified for thousands of years within the peat bogs of Northern Europe. The airless bog environment is a natural mummifier, preserving both skin and hair. In fact, when people inadvertently uncover a mummified body in peat, they often call the police, mistaking a well-preserved, thousand-year-old body for a modern murder victim.

One of the mummies from the Bog People exhibit, a man named “Red Franz,” was discovered in 1900. Scientists used carbon-14 dating to determine Franz’s date of death, approximately 1800 years ago.

As for the science behind this phenomenon, carbon-14 is an isotope of the element carbon. Isotopes are all forms of a given element; each has the same amount of protons and electrons (in the case of carbon, six of each) but a different number of neutrons.

Once created, carbon-14 atoms combine with oxygen atoms to produce carbon dioxide.
As part of the carbon cycle, plants absorb carbon dioxide during photosynthesis. Carbon dioxide returns to the atmosphere when plants (and the animals that eat plants) break down glucose and similar molecules to produce both carbon dioxide and water.

Thus, all living organisms are constantly gaining and losing carbon dioxide as they interact with their environment. However, this exchange is only present in living things; once an organism dies, it ceases to gain new carbon dioxide and loses remaining carbon dioxide through a process called beta decay.

Beta decay, more formally radioactive beta decay, is a process by which carbon-14 produces nitrogen-14 by emitting an electron and anti-neutrino (a neutral particle produced during the decay). Here, carbon-14 is called a “parent” isotope, which decays to produce a “daughter” isotope, nitrogren-14.

When dealing with radioactive decay, scientists often speak of half-lives, the amount of time it takes for a substance to decrease to half its original amount. The half-life of carbon-14 is 5730 years, plus or minus 40 years.

If you know how much carbon-14 was originally present in a given substance, and how much there is left, you can plug these values into an equation and solve for time. Essentially, this is how scientists use carbon-14 to determine the age of a material.

The amount of carbon-14 in the Earth’s atmosphere, however, is a fluctuating quantity. Solar storms can affect the intensity of cosmic rays, which affect the rate of carbon-14 production.

Also, following the carbon cycle, carbon-14 is present in the ocean and sedimentary rocks. When the climate changes, it sometimes affects the transfer of carbon between these reservoirs and the atmosphere.
Humans can affect these changes as well. In the ’50s and ’60s, atomic bomb tests nearly doubled the amount of carbon-14 present in the atmosphere. Recently, however, the quantity has been reduced as a result of fossil fuels, which do not release carbon-14.

With all of these influences, it is necessary to calibrate radiocarbon ages. Scientists use the growth rings of trees, samples of coral, cave deposits, and other sources to determine the amount of carbon-14 present in the atmosphere at a given time.