The radiocarbon emits an electron and changes back into nitrogen.
As far as anyone can tell, ¹⁴C was produced at a constant rate above the
earth for at least 50,000 years before the first atomic bomb was
exploded. In other words, the ¹⁴C cycle is like an hourglass in which
the sand in the upper part is replenished as fast as it runs out through
the hole in the waist. A process of this sort, where production equals
decay, is called a SECULAR EQUILIBRIUM.
The newly produced ¹⁴C soon is evenly mixed with the carbon dioxide in
the air, is taken up by all living plants, and then finds its way into
all living animals. In effect, all carbon in living organisms contains a
constant proportion of ¹⁴C. If any of this carbon is taken out of
circulation—when a tree branch is broken off, for instance, or when a
shellfish dies in the ocean—no more new ¹⁴C is added to that particular
system, but the old ¹⁴C continues to run out. In effect it now starts
measuring time as an hourglass should.
[Illustration: _To illustrate secular equilibrium, one must imagine an
hourglass in which the sand in the top bulb is continuously
replenished—as fast as it runs out through the hole in the waist and
disappears._]
When we find a piece of charcoal in a cave or a piece of wood in some
ancient structure, for example, we can measure the amount of carbon in
it, determine how much of it is ¹⁴C, and then calculate back to the time
when the radioactivity from the ¹⁴C was the same as we now find in
living wood. In other words, if we assume that we know from the observed
secular equilibrium how much ¹⁴C originally was present in living
material, then we can calculate the time of death of any similar but
ancient material. That is the basis of the ¹⁴C method of age
determination.
[Illustration: _Dinosaur tracks imprinted in rock in Navajo Canyon,
Arizona, arouse the professional interest of this scientist. Fossil
traces of extinct prehistoric creatures were for a long time the best
clues to the age of rock formations._]
[Illustration: _A scientist using liquid nitrogen to freeze carbon
dioxide gas made from a sample of ancient material that he is preparing
for age determination by the carbon-14 technique._]
For example, a bit of a rafter from a prehistoric cliff-dwelling or a
remnant of charcoal from an ancient fire may be analyzed for its
remaining ¹⁴C content, and its age determined accurately within the
margin of a few hundred years. This fixes the time at which the wood for
the rafter or the firewood was broken or cut from the living tree, and
hence the period in which the men lived who used the wood.
[Illustration: _The most useful samples for carbon-14 age determination
are charcoal, wood, and shells._]