Most everyone has heard of Carbon dating on the news or elsewhere sometime in the past years. Ever wonder what “Carbon dating” means and why it is so important? In this article I hope to explain the theoretical and physical science behind Carbon dating, and discuss how it affects our lives and the validity of the process.
Scientists use Carbon dating for telling the age of an old object, whose origin and age cannot be determined exactly by normal means. Because of this method Chemistry has become intertwined with History, Archeology, Anthropology, and Geology. (Poole) Many items that have been thought to come from one time have been tested and found out to actually come from a few thousands years beforehand. Places where historians believed that human civilization came to exit say, only 2,000 years ago, have actually been proven to have had some form of human civilization more than 4,000 years ago. (Poole) Fine art collectors have used Carbon dating to determine if a piece of antique art is actually genuine. Some have saved themselves several thousands of dollars by testing the piece before they bought it and finding out that it is not the original, but a very clever modern copy. (Poole) But how is this done? What are the ides behind carbon dating?
Atoms of given elements have different Isotopes. Isotopes are atoms of the same element, i.e. they have the same number of Protons and Electrons in the atom, but they have a different number of Neutrons in the nucleus, so they have different atomic masses. (Jones & Atkins)
The element Carbon is in all living things, it is a basic building block for the construction of organic material. The normal molar mass of Carbon is around 12, however there are a few Carbon atoms that have a molar mass of about 13, and even fewer that have a molar mass of about 14. These atoms have one or two more neutrons in the nucleus than most Carbon atoms. Scientists call the isotope with molar mass around 14, Carbon-14. Carbon-14 is manufactured in the upper atmosphere by the action of cosmic rays. (Ham, Snelling, & Wieland) Ordinary nitrogen is converted into Carbon-14; however it is not a stable element. It turns out to be radioactive and decays over time.
All organic material has decaying Carbon-14 in it. However, plants and animals that are still alive constantly replace the supply of carbon in their systems and so the amount of Carbon-14 in the system stays almost constant. Once a plant or animal dies the Carbon is no longer being regenerated and so the Carbon-14 starts to decay. In this way, by measuring the amount of Carbon-14 in the body of a prehistoric animal or plant, a scientist can deduce when the plant or animal died.
All radioactive materials have a half-life. If you have a certain amount of a radioactive material, its half-life is the time it takes for half of the material you started out with to decay.
Carbon-14 decays back into nitrogen. This is a first order reaction equation and the rate at which it the reaction proceeds over time can be modeled by the equations:
[C14 at t=t] = [C14 at t=0](e^-kt)
Ln[C14 at t=0] – Ln[C14 at t=t] = kt
Ln([C14 at t=0] / [C14 at t=t]) = kt
To get the half-live of Carbon-14 we let [C14 at t=t] = .5[C14 at t=0] and the equation simplifies to:
Ln2 = k(half-life) , where k is the rate constant fro C14
“The half-life of a first-order reaction is characteristic of the reaction and independent of the initial concentration. A reaction with a large rate constant has a short half-life.” (Jones & Atkins)
Willard F. Libby Was the man who first developed the idea and procedure for Carbon dating. He measured the half-life of Carbon-14 to be about 5,568 years. However after about 50,000 years there is so little Carbon-14 left in the specimen that it is very hard, almost impossible, to calculate its age. (Van Der Merwe) Libby ran many tests on items where the age was known, or estimated by other means. His test results came rather close, to within plus or minus a few hundred years. (Poole)
In the laboratory, samples must be processed and cleaned so that there is no material on them that might throw off the age reading. Then the sample is burned and passes through a completely sterile vacuum system as Carbon dioxide gas. The gas is then subjected to more purifying procedures. Afterward the gas is stored in a tube insulated by Mercury and Lead, so as to minimize the chances of the sample being affected by radiations from the atmosphere. When a Carbon-14 atom disintegrates fine instruments detect the action, a light flashes on a control panel, and a counter records the number of decaying atoms. By this method the scientist can keep track of how many atoms are decomposing per minute and per second. (Poole)
This sounds great! We are now ably to date anything we want, even that something at the back of the fridge, and know how old it is within a few hundred years, but are there any problems with the Carbon dating method?
Unfortunately there are. In order to know how long a sample of radioactive material had been decomposing we need three variables defined, how much of the sample we have left now, what the half-life of the sample is, and how much of the sample we started out with. For Carbon dating we have already experimentally measured the amount of Carbon-14 left, and Libby has already measured the half-life of Carbon-14 to an acceptable exactness, however how much Carbon-14 was there in the specimen at the time of death. The amount of Carbon-14 in an organic body is constant with the amount of Carbon-14 in the atmosphere. Thus specimens have the same amount of carbon-14 in them as the rest of the atmosphere at the time that the specimen lived. However, if we could measure the amount of Carbon-14 in the atmosphere when they lived, we would be living during the time and there would be no reason for dating. (Ham, Snelling, & Wieland)
We know for a fact that the amount of Carbon in the atmosphere has not stayed the same in the past century. A recent proof of that would be the Industrial revolution. Factories put out massive amounts of Carbon, and during that time the concentration of Carbon-14 in the atmosphere increased significantly. Fortunately, Libby was a smart guy and accounted for this discrepancy. He measured the amount of Carbon in the inner layers of trees that were older than the Industrial revolution. He was able to calculate the amount of Carbon-14 in the atmosphere, before the industrial revolution, and adjust his equation accordingly. (Ham, Snelling, & Wieland)
However, Libby then assumed that the amount of Carbon in the atmosphere was relatively constant for a very long time up until the Industrial revolution. Can this be assumed to be correct? In the atmosphere the amount of Carbon-14 decaying over time increases with the greater concentration of Carbon-14 in the atmosphere. Eventually the reaction would reach some equilibrium and the amount of Carbon-14 in the atmosphere would remain constant. Scientists have calculated that the amount Carbon-14 in the atmosphere would become stable after 30,000 years from the beginning of the reaction. The reaction must have started when the Earth was formed, and thus the reaction would reach equilibrium after the Earth was 30,000 years old. Scientists have assumed that the Earth is many millions of years old, however, no one was living when the earth was formed, and no one has concrete proof as to when the Earth was formed and therefore no one can say exactly how old it is. (Ham, Snelling, & Wieland)
Today the rate of production of Carbon-14 is greater than the rate of disintegration. This would seem to indicate a reaction that is not yet in equilibrium. Libby’s tests showed that the amount of Carbon-14 entering the atmosphere was 12% greater than the amount decaying, before the industrial revolution. These results were within his error margins and thus were ignored. If we do not ignore them, and I think that 12% is a very large error margin, they would seem to suggest that the reaction was in progress less than 30,000 years and a younger Earth, however, again no one has concrete proof. (Ham, Snelling, & Wieland)
There are also some tests that have been done that don’t quite match up. For instance, bones of a sabre-toothed tiger, theorized to be between 100,000 and one million years old, gave a Carbon date of 28,000 years. A freshly killed seal, dated using Carbon-14, showed it had died 1300 years ago. Living mollusk shells were dated at up to 2,300 years old. Some very unusual evidence is that living snails' shells showed that they had died 27,000 years ago. (Ham, Snelling, & Wieland)
“"The troubles of the radiocarbon dating method are
undeniably deep and serious ... It should be no surprise, then,
that fully half of the dates are rejected. The wonder is, surely,
that the remaining half come to be accepted." (Lee, R. E.,
Radiocarbon, "Ages in Error", Anthropological Journal of
Canada, 1981, vol. 19, No. 3, p. 9)” (Ham, Snelling, & Wieland)
Carbon dating is used now for almost everything old that people want to date. It is taken as fact and used as evidence to gather information on the world and past civilizations. However, Carbon dating is at best a good theory, and that is all it is, a theory. Too many people forget the definition of a theory. Theory is not fact; it is a hypothesis that is supported by some experimental evidence. There have been many theories in the past that have been disproved. I am not saying that Carbon dating is a bad idea. Willard F. Libby was a very brilliant scientist and had some wonderful ideas. We just need to keep it in perspective and not take a theory for a fact. I wonder if I dated that bowl of something or other in my fridge, what age it would be from. Then again, I am not completely sure it’s dead.
Chemistry: Molecules, Matter, and Change
By Loretta Jones and Peter Atkins
W.H. Freeman and Company, New York, 2000
By Lynn Poole
Whittlesey House, 1961
The Carbon-14 Dating of Iron
By Nikolaas J. Van Der Merwe
The University of Chicago Press, Chicago, 1969
Excerpt from “The Answers Book”, Revised Edition, by Ken
Ham, Andrew Snelling and Carl Wieland
Master Books, 1992.