reading this section you will be able to do the following:
why carbon can be found in all living organisms.
how carbon can help determine the age of some objects.
As you learned in
the previous page, carbon dating uses the half-life of
Carbon-14 to find the approximate age of certain objects that are 40,000
years old or younger. In the following section we are going to
go more in-depth about carbon dating in order to help you get
a better understanding of how it works.
exactly is radiocarbon dating?
Radiocarbon dating is
a method of estimating the age of organic material. It was developed
right after World War II by Willard F. Libby and coworkers, and
it has provided a way to determine the ages of different materials
in archeology, geology, geophysics, and other branches of science.
Some examples of the types of material that radiocarbon can determine
the ages of are wood, charcoal, marine and freshwater shell, bone
and antler, and peat and organic-bearing sediments. Age determinations
can also be obtained from carbonate deposits such as calcite,
dissolved carbon dioxide, and carbonates in ocean, lake, and groundwater
How is carbon-14
Cosmic rays enter the earth's atmosphere
in large numbers every day and when one collides with an atom
in the atmosphere, it can create a secondary cosmic ray in the
form of an energetic neutron. When these energetic neutrons collide
with a nitrogen-14 (seven protons, seven neutrons) atom it turns
into a carbon-14 atom (six protons, eight neutrons) and a hydrogen
atom (one proton, zero neutrons). Since Nitrogen gas makes up
about 78 percent of the Earth's air, by volume, a considerable
amount of Carbon-14 is produced. The carbon-14 atoms combine with
oxygen to form carbon dioxide, which plants absorb naturally and
incorporate into plant fibers by photosynthesis. Animals and people
take in carbon-14 by eating the plants.
The ratio of normal carbon (carbon-12)
to carbon-14 in the air and in all living things at any given
time is nearly constant. Maybe one in a trillion carbon atoms
are carbon-14. Both Carbon-12 and Carbon-13 are stable, but Carbon-14
decays by very weak beta decay to nitrogen-14 with a half-life
of approximately 5,730 years. After the organism dies it stops
taking in new carbon.
How do scientist
use Carbon-14 to determine the age of an artifact?
To measure the amount of radiocarbon
left in a artifact, scientists burn a small piece to convert it
into carbon dioxide gas. Radiation counters are used to detect
the electrons given off by decaying Carbon-14 as it turns into
nitrogen. In order to date the artifact, the amount of Carbon-14
is compared to the amount of Carbon-12 (the stable form of carbon)
to determine how much radiocarbon has decayed. The ratio of carbon-12
to carbon-14 is the same in all living things. However, at the
moment of death, the amount of carbon-14 begins to decrease because
it is unstable, while the amount of carbon-12 remains constant
in the sample. Half of the carbon-14 degrades every 5,730 years
as indicated by its half-life. By measuring the ratio of carbon-12
to carbon-14 in the sample and comparing it to the ratio in a
living organism, it is possible to determine the age of the artifact.
dating can determine the age of an artifact that is up to 40,000
organisms absorb carbon my eating and breathing.
burning a small piece of an artifact, scientists compare the amount
of Carbon-14 to the amount of Carbon-12 to determine the age
of the object.