Carbon-14 (¹⁴C) is a naturally occurring radioisotope that is created in the atmosphere by cosmic rays. This is a continuous process, so more ¹⁴C is always being created. As a living organism develops, the relative level of ¹⁴C in its body is equal to the concentration of ¹⁴C in the atmosphere. When an organism dies, it is no longer ingesting ¹⁴C, so the ratio will decline. ¹⁴C decays to ¹⁴N by a process called beta decay; it gives off energy in this slow process.

After approximately 5,730 years, only one-half of the starting concentration of ¹⁴C will have been converted to ¹⁴N. The time it takes for half of the original concentration of an isotope to decay to its more stable form is called its half-life. Because the half-life of ¹⁴C is long, it is used to age formerly living objects, such as fossils. Using the ratio of the ¹⁴C concentration found in an object to the amount of ¹⁴C detected in the atmosphere, the amount of the isotope that has not yet decayed can be determined. Based on this amount, the age of the fossil can be calculated to about 50,000 years. Isotopes with longer half-lives, such as potassium-40, are used to calculate the ages of older fossils. Through the use of carbon dating, scientists can reconstruct the ecology and biogeography of organisms living within the past 50,000 years.

Figure 2.4 The age of remains that contain carbon and are less than about 50,000 years old, such as this pygmy mammoth, can be determined using carbon dating. (credit: Bill Faulkner/NPS)