Virtually all the nitrogen in the atmosphere is N-14 (only 0.36% is N-15). Occasionally, a fast-moving electron from the sun strikes a N-14 nucleus, combines with a proton to create a neutron, and creates an atom of Carbon-14. Carbon-14 is radioactive with a half-life of about 5700 years, but as C-14 atoms decay, new ones are created. The amount of C-14 in the atmosphere settles into an equilibrium where creation of new atoms is equal to the decay of old atoms. The C-14 combines with oxygen to form carbon dioxide, plants take in the radioactive carbon dioxide, animals eat the plants, and so all living things on earth have a constant level of C-14: roughly one atom in a trillion.
When an organism dies, it stops taking in carbon-14 and the carbon-14 it already contains starts to decay. So, in principle, it's a fairly straightforward matter to measure the concentration of C-14 in organic material and determine its age. The organic matter need not be a fossil. It can be cloth, paper, charcoal, lumber, or leather.
A gram of carbon contains 5 x 1022 atoms. One atom in a trillion means 5 x 1010 C-14 atoms. A half-life of 5700 years implies a decay constant of 0.693/5700 = 1.2 x 10-4 per year or 3.8 x 10-12 per second. In other words, our gram of carbon will give us four counts per second. Ten half-lives, or 57,000 years, will decrease the original abundance of C-14 by 1024. The sample will give us about a dozen counts an hour. Twenty half-lives, or 114,000 years, will reduce the original abundance of C-14 by about a million. There will only be 50,000 C-14 atoms left, or one count every two months! In fact, this is not how C-14 is measured - it is both too slow and too insensitive.
So we can see:
Several factors can introduce error into C-14 dating.
Contamination can be handled by selecting samples carefully and cleaning them with solvents to remove loose or surficial carbon. Nevertheless, contamination can occur.
One celebrated case where contamination with recent carbon has been alleged is the famous Shroud of Turin, purportedly the burial shroud of Christ. Radiocarbon dating using ultra-sensitive techniques has indicated that the Shroud is actually medieval. But the Shroud was damaged by a fire during the Middle Ages, during which the silver casket holding it melted, charring several large holes in the Shroud. So die-hard Shroud believers have argued that smoke from the fire contaminated the fibers used in dating, introduced medieval carbon into the samples, and resulted in an erroneously young age.
The problems with this scenario are many. Any smoke would have to have adhered to the surface of the fibers, meaning the total amount of carbon that could have been introduced would have to be very small. Whether significant amounts of carbon would have gotten deeply enough into the fibers to significantly affect the age is very questionable. Finally, with the Shroud itself being charred in a confined space, it would be far more likely to be "contaminated" with its own carbon, meaning the age should not change at all.
A real case of contamination with old carbon is that of modern sea shells that dated at 1500 years old. Critics of radiocarbon dating love to point to this case as proof that very ancient radiocarbon dates could be wildly in error, except that the shells in question were growing on limestone, and derived their carbon from the limestone rather than from the sea.
Does the level of solar activity fluctuate on a long time scale? Yes. Does the production rate of carbon-14 vary with time? Yes. Do these variations cause carbon-14 ages to be in error? Yes.
So to calibrate the scale, we use objects of known age. These include artifacts that can be historically dated, wood from ancient trees that can be dated by tree-ring chronology, and carbon from lake deposits with varves (annual rhythmic beds that permit a precise chronology to be established).
And guess what? For the last 10,000 years or so, radiocarbon ages are systematically too young. True ages are older than radiocarbon ages. One early application of this finding was to revise our understanding of prehistoric Europe. A melding of historical chronology with uncalibrated radiocarbon ages jibed nicely with the prejudice that agriculture and animal domestication had moved from the settled Middle East to backward Europe. While agriculture and animal domestication did travel that way (as shown by the species domesticated), calibrated radiocarbon dates show it happened a lot earlier than previously supposed.
You have to feel sorry for critics of radiocarbon dating. Variability of carbon-14 production is a perfectly valid concern. And it works in the wrong direction.
Between the desire to extend carbon-14 dating back in time, plus the need to test minute samples from valuable artifacts like the Shroud of Turin, counting decays is far too slow and insensitive. Most radiometric dating is done by physically separating individual atoms in a mass spectrometer. A sample is vaporized, the atoms ionized, and then propelled through a magnetic field. Atoms of different mass are deflected different amounts by the magnetic field and strike detectors in different locations.
Created February 21, 2002, Last Update 14 December 2009
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