Page 139 - Berkshire Encyclopedia Of World History Vol Two

488 berkshire encyclopedia of world history

each subsequent occupation located on top of previous respire—in essence, they breathe—and, through photo-
deposits. For example, the tell of Hazor (Tel Hazor) in synthesis, break apart the bonds between the carbon and
Israel is an artificial hill produced by the remnants of oxygen atoms in carbon dioxide. Plants exhale the oxy-
some twenty-one separate occupations of the same loca- gen and retain the carbon, from which they manufacture
tion, with the remains of each successive occupation over- their leaves, stems, roots, fruits, bark, and so forth. Car-
lying each previous one. bon 14 is incorporated into plant materials in the same
proportion as it appears in the atmosphere. When ani-
Absolute Dating Methods mals ingest plant materials, their bodies use the ingested
In most cases, the application of a relative dating proce- carbon in the plant to produce bone, muscle, sinew, and
dure like stratigraphic analysis represents a preliminary so on, and the carbon 14 provided by the plant is incor-
step in archaeological dating. The hope always is to go porated into the animal’s body in the same proportion as
beyond merely sequencing and to determine a site’s age was present in the plant.When a carnivore eats a plant-
in a chronometric sense. One of the most important eating animal, it incorporates carbon 14 into its body in
chronometric procedures archaeologists employ is radio- the same proportion as was present in its prey. In this
carbon dating (also called carbon dating or C-14 dating), way, all livings things, as long as they are alive, breathing,
one of a series of so-called radiometric techniques. and eating, are in equilibrium with the atmosphere.
Radiometric dating methods, including carbon dating, Because carbon 14 is unstable, it constantly undergoes
potassium-argon dating, argon-argon dating, and ura- radioactive decay at a steady and measurable rate. Ra-
nium series dating all rely on the existence of what dioactive decay rates are expressed as half-lives, the length
amounts to natural clocks or calendars in archaeological of time it takes for half of the radioactive material pres-
and geological specimens. These natural calendars are ent in a substance to decay to a stable form.The half-life
produced by the fixed and measurable pace of the decay of carbon 14 has been measured at 5,730 years.
of radioactive (unstable) isotopes (varieties) of elements As long as an organism is alive, the carbon 14 that is
that occurs naturally in raw materials used by ancient disappearing through that process of decay is constantly
human beings. being replenished (by plants as they breath and by ani-
mals as they eat). Death breaks that cycle.After death, the
Radiocarbon Dating decaying carbon 14 is no longer replenished, and its loss
In 1946, University of Chicago chemist Willard Libby becomes calculable. It is the constant, steady, and mea-
predicted the existence of carbon 14 in living matter and surable decay of carbon 14 that provides a natural chro-
by 1949 had measured contemporary levels of this unsta- nometer in substances containing carbon—essentially,
ble variety of carbon and assessed its half-life—in essence anything that was once alive.
he began the process of calibrating this natural clock. Scientists can precisely calculate the amount of carbon
Soon thereafter, he began to apply the method to dating present in a specimen about to be dated; they also know
archaeological specimens. With radiocarbon dating, the how much carbon 14 would be present in the material if
known rate of decay of a radioactive variety of carbon it were entirely modern—in other words, if it were alive
called C-14 (carbon 14) for the fourteen particles (six pro- today. If the amount actually present in the object is less
tons and eight neutrons) in its nucleus (as compared to than would be expected in a modern material, the
the twelve particles—six protons and six neutrons— amount of time it must have taken for the proportion of
present in the stable and abundant variety of carbon) pro- C-14 to decline to its current measure can be calculated,
vides the archaeologist with a natural calendar. providing an age for the material.
Just like carbon 12, carbon 14 combines with oxygen Radiocarbon dating, by its nature, can be applied
in the atmosphere to produce carbon dioxide. Plants only to organic materials such as bone, wood, charcoal,