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
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 b