Bristlecone Pine high in the mountains of California.
photos on this page courtesy of J. Scott Bovitz. Copyright
J. Scott Bovitz.)
Until the middle of the twentieth century archaeologists
had a real problem. If they found a bit of pottery, an old coin,
or another object while digging up a site, just how old was
it? How could they tell if the object had been dropped on the
ground thirty years ago or thirty centuries ago?
In general, archaeologists knew that the farther
they had to dig down to find an object, the older it probably
was and that objects found at the same level, or stratum, were
close in age. That was helpful when comparing the relative ages
of two items, but how could they get the actual age? And how
could they compare the ages of items found at sites fifty miles
If a society had written records the archaeologists
could compare the history against the type of objects they were
finding in the ground. A coin bearing the likeness of Augustus
Caesar certainly couldn't have minted before Caesar started
his reign. Likewise an Egyptian piece of pottery with a design
depicting horse and chariot couldn't have been made before the
horse was introduced to Egypt.
Using these types of clues archaeologists were
able to construct a firm history for some societies. By matching
pottery between a culture with a known chronology against one
without, they could even hazard a guess about historical dates
when there were no records. This still left scientists wondering
about the age of sites where neither cross-matching of pottery
or written records could be used. That was, until the invention
of radiocarbon dating.
The development of atomic physics in the beginning
of the 20th century allowed scientists to understand the phenomenon
of radioactivity. Radioactivity results when an atom has a combination
of neutrons and protons in its nucleus which is unstable. The
atom will expel particles or energy in an attempt to become
stable. The expelled energy or particles are the radioactivity.
Eventually the atom will change itself into a different substance
which may no longer be radioactive. The time it takes for half
of the atoms in a sample of radioactive material to decay into
another form is known as the "half-life" of the radioactive
material. Different radioactive materials may have half-lives
that range from a few seconds to hundreds of thousands of years.
One naturally-occurring radioactive material found
in the atmosphere is carbon-14. As plants and animals use the
air, their tissues absorb some of the carbon-14. After they
die, though, they no longer absorb the carbon-14 and the material
in their tissues starts to decay.
In 1949 it occurred to a scientist named Willard
Libby that the amount of carbon-14 decay found in an animal
or plant could be used as a gauge of how long it had been dead.
Carbon-14 has a half-life of 5730 years. That meant if Libby
found a sample where the amount of carbon-14 was only half the
amount that might be expected in a living creature, he knew
the age of it would be about 5730 years.
Though archaeologists could not directly use radiocarbon
dating on objects such as coins, they could often find organic
material (like charcoal from a fire) on the same stratum at
a site as the object. In this way the age of the coin, or any
other non-organic item, might be inferred from the age of the
When scientists began to use this method to find
the age of sites they ran into a problem. The new radiocarbon
numbers didn't seem to jibe with the written records. In some
cases they seemed to be hundreds of years off.
In an attempt to understand the differences, scientists
looked towards the bristlecone pines of California. These trees
are thought to be the oldest living things on Earth and some
have ages of almost 5,000 years. Fortunately the bristlecone
pine, like other trees, lays down a growth ring every year.
Each growth ring turned out to be a measure of the amount of
carbon-14 in the atmosphere during that year. What scientists
hadn't understood was that the amount of carbon-14 in the atmosphere
wasn't constant, but changed. This meant that the living organisms
had different levels of carbon-14 in their tissues depending
on the year in which they died. This explained why the early
radiocarbon dates didn't match up with the written records.
With a record of the amount of carbon-14 found
in the atmosphere available through the pines, scientists were
then able to calibrate the test and get dates that matched their
written records. The bristlecone pine is such a tough tree that
its wood can survive intact for a long time after it is dead.
By comparing the growth rings of living bristlecone pines with
ones dead for many years, scientists have been able to extend
the calibration chart back about 11,000 years.
The radiocarbon dating method has been invaluable
in helping scientists date archaeological sites where no other
method was available and confirm dates at other locations. Some
of these sites include Stonehenge,
in England, Mystery Hill, in the
United States, and Easter Island in
bristlecone pine trunks like this one hold the key to getting
accurate radiocarbon dates as far back as 11,000 years. (Photo
by J. Scott Bovitz)
Krystek 1997. All Rights Reserved.