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Death starts the stopwatch

Q: I understand that radiometric dating measures the time that it takes for a radioactive parent isotope to decay. How do scientists determine the initial percentage of parent isotope in a dating sample? -Terry B., Albuquerque, New Mexico

[SARC, Univ of Oslo] A prehistoric bow

A: It depends on the dating method. A method useful in dating rocks ( potassium-argon dating) assumes a zero initial percentage of the stable daughter atom (argon) and measures the accumulation of argon. A rock starts off as a molten or hot mass that crystallizes into a rock. The molten mass may contain potassium but cannot contain argon since argon, an inert gas, cannot combine chemically with other elements. Any potassium present in the newly formed rock also contains a radioactive isotope. The radioactive isotope of potassium decays into argon and the stopwatch starts. Consequently, you measure the accumulation of argon in a rock sample to estimate the time when the rock formed.

Carbon-14 dating, on the other hand, measures the absence of the radioactive isotope. We estimate the initial percentage by assuming it's the same as it is now. In nature, an element is almost always found with its isotopes mixed together in the same proportions. Then we compare radioactivity. For example, consider dating an ancient bow.

The half-life of carbon-14 is 5,730 years. So, half the carbon-14 atoms in the bow decays into stable nitrogen in that length of time. Moreover, the radioactivity also decreases by half since only half the number of carbon-14 atoms is left to radiate.

The atmosphere contains carbon, including a millionth of one percent carbon-14 and the rest mostly carbon-12. Both of these carbon atoms combine with oxygen to form carbon dioxide. When the bow was alive-existing as a tree--it took in carbon dioxide from the air. Consequently, a tiny percentage of carbon-14 enters the tree.

Throughout the tree's life, it brings in new carbon-14 to replace that which decays and the level of carbon-14 stays fairly constant at a known value. Then, a prehistoric hunter comes along, chops a branch off the tree to make the bow, and that wood dies. The branch that becomes a bow no longer replaces the carbon-14 in its cells.

The stopwatch starts. The number of carbon-14 atoms in the bow steadily dwindles. We determine the time elapsed since death by measuring the radioactivity of the bow and comparing it with that of a living tree. If, for example, we find that a tiny sample of carbon from the bow has half the radioactivity as an equal sample taken from a living tree, then we figure the bow has aged a half-life: 5,730 years.

That's how it's done: a comparison of radioactivity that assumes the same percentage of carbon-14 when the wood died as a living tree has now.

(Answered by April Holladay, science correspondent, February 13, 2002)

Further Surfing:

emuseum: potassium-argon dating

U of Waikato, New Zealand: The carbon-14 method

webmuseum: how death starts the stop watch

USATODAY.com: Egypt's oldest art discovered

Conceptual Physics by Paul Hewitt, Addison-Wesley, 1998, p. 605