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March 27, 2007 > How Does Carbon Dating Work?

How Does Carbon Dating Work?

A team of archaeologists excavates a dugout canoe that is dated to 4200 B.C. Remains of a prehistoric campfire are found and dated to 6500 B.C. A frozen mummy is discovered in glacial ice and is determined to have lived some 12,300 years ago. How can scientists be sure about these dates? They have probably used a technique called carbon-14 dating or radiocarbon dating.

To understand how carbon dating works, a brief excursion to the realm of atomic physics might be helpful. The nucleus or core of an atom is composed of particles called neutrons and protons. The number of protons in an atom determines its atomic number or element, i.e., what type of atom it is. The sum of protons and neutrons determines the atomic weight. For example, carbon has six protons and six neutrons (atomic number 6, atomic weight 12); nitrogen has seven protons and seven neutrons (atomic number 7, atomic weight 14).

Sometimes, an external, high-energy neutron enters the nucleus of a nitrogen atom and ejects one of the protons, replacing it. This leaves eight neutrons and only six protons in the atom. So the atom has been transformed into a carbon atom (six protons). But unlike stable carbon with six neutrons, it has eight. This is carbon-14 or C-14, the "14" signifying the atomic weight.

How does this happen? X-rays and other high-energy radiation originate on the sun and in outer space. These cosmic rays are constantly bombarding the earth. When they interact with atoms high in the atmosphere, they can cause those atoms to emit energetic neutrons, as well as other particles and radiation. Nitrogen is the most plentiful element in the atmosphere, and when these solitary neutrons displace protons in nitrogen atoms, C-14 is created in the upper atmosphere and spreads over the earth. This is a natural process that has been occurring since the earth was young.

C-14 is a radioactive isotope. This means it is unstable, and like all radioactive materials, it decays to a stable element over time. In the case of C-14, it reverts to nitrogen by a process called beta decay. This process occurs at a very predictable rate. It takes 5730 years for half of the C-14 in any material to become nitrogen. After another 5730 years, one-quarter of the original amount remains, and so on. This 5730-year period is called the half-life of C-14.
Understanding half-life is important to understanding carbon dating. To emphasize the concept, if 80 atoms of C-14 were placed in a jar, there would be 40 remaining after one half-life, 20 after two half-lives (11,460 years), and only 10 after three half-lives (17,190 years).

Now let's turn from physics to biology. All living things, plants and animals, are carbonaceous; carbon forms the basis of their living tissues. Plants obtain carbon by absorbing carbon dioxide from the atmosphere. Animals eat plants, from which carbon is integrated into the cells of their bodies. While alive, all plants and animals exist in equilibrium with the atmospheric concentrations of carbon. Another way to state this is the ratio of radioactive C-14 to stable carbon, or C-12, is the same in the tissues of living beings as in the natural environment from which they draw their sustenance. By the way, that ratio is very small: about one C-14 atom to one trillion C-12 atoms.

When an organism dies it ceases to absorb carbon. As the C-14 decays to nitrogen and the C-12 remains unchanged, the ratio of C-14 to C-12 diminishes over time at a rate determined precisely by the 5730-year half-life of C-14. Thus by counting the number of C-12 and C-14 atoms in a sample, a technician can pinpoint the date of the death of that organism. For example, if the ratio is one C-14 atom to two trillion C-12 atoms, half the C-14 has decayed, so the tissue died about 5730 years ago.

This technique works for all once-living things. Examples of items that are routinely carbon dated are wood, charcoal, bones, seeds, pollen, leather, hair, textiles, paper, shells, insects, and horns. Because most soils are rich with organic (once living) material, carbon dating has been used for soil samples, pottery, and peat as well.

So to return to the examples in the first paragraph, the canoe can be dated because carbon dating can determine when the tree used to make it was cut down. Similarly, the charcoal can be dated to the death of the plant that was burned to fuel that ancient campfire. The mummy's date of death can be fixed by analyzing a small amount of its tissue.

Recent advances in mass spectrometer technology, used to "count" the carbon atoms, have allowed scientists to perform accurate carbon dating with samples as small as one milligram. This is important when only tiny samples exist, or when the material being dated is of such historical or iconic value that destroying a large amount would not be acceptable. The Shroud of Turin and the Dead Sea Scrolls are in this latter category; both have been carbon dated in recent years.

Carbon dating was invented by Willard Libby of the University of Chicago. Libby first published his research in 1949. His subsequent work refining the technique garnered the Nobel Prize in Chemistry in 1960. It would be difficult to overstate the impact of carbon dating on the fields of archaeology and geology. What had been largely guesswork and speculation has become a matter of scientific measurement. There are currently more than 100 carbon dating laboratories operating worldwide.

While the value and precision of carbon dating is widely accepted in the scientific community, it does have several important limitations. First, it is useful in dating materials only up to about 60,000 years old. After 10 or more half-lives, there is so little C-14 remaining that it cannot be accurately measured.

The accuracy of carbon dating has been verified by calibrating results against samples of known age. For example, archaeological materials such as Egyptian mummies and scrolls, whose histories have been well documented and dated by other means, have been used for this purpose. In this way, the technique has been validated for samples of up to about 10,000 years old.

However, it is possible that the rate of C-14 production in the atmosphere was not strictly constant between 10,000-60,000 years ago, and if so, the results from that period may not be precisely accurate. There is no convincing evidence that this is the case. In fact, there is evidence that, apart from some known small variations, the rate of C-14 production has been fairly constant over this period. But pending more research this is still a possible source of inaccuracy.

Carbon dating is somewhat problematic in the case of marine life. Some aquatic organisms may ingest carbon from non-atmospheric sources, such as materials in ocean sediments. As any C-14 in such material would already have experienced some decay, dating these specimens must be undertaken with this in mind, and less certainty can be attached to the results than to those for land-based life.

Scientists in the far future will have more difficulty in applying carbon dating. In the 1950s and 1960s, the amount of radioactive C-14 in the atmosphere increased due to aboveground testing of nuclear weapons and other nuclear research. On the other hand, the burning of fossil fuels on a massive scale in the last 50-100 years has decreased the ratio of C-14 to C-12, since these materials contain no appreciable C-14, but spew tremendous quantities of C-12 into the atmosphere, primarily in the form of carbon dioxide. Whatever their ultimate impact on atmospheric C-14 concentrations, these changes will have no effect on dating organisms that perished prior to the 1900s.




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