By Kamil Erkan
Libby’s discovery, now known as the carbon-14 (or radiocarbon) technique, was a method that could be used to determine the age of organic remains. In the following years, archeologists used this technique extensively and determined exact dates for pre-historic settlements in the ancient world.
Some Neolithic (later stone age) remains were dated back to fifty thousand years in Russia and Africa. The city of Eriha in Palestine was dated back to eleven thousand years, and was designated as the first permanent human settlement. Today, archeologists and paleontologists employ this technique to determine the age of organic materials (bones, teeth, wood, etc.) that are less than fifty thousand years in age.
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The theory is simple: Cosmic particles coming from outer space continuously collide with stable carbon-12 atoms in CO2 molecules, which are widespread in the atmosphere. Each carbon-12 atom takes up two neutrons and is converted into a radioactive carbon-14 atom. Radioactive carbon-14 atoms rapidly mix and become uniform throughout the atmosphere.
Deep oceans, the biosphere, and carbonate rocks are giant reservoirs of carbon and with the addition of the atmosphere they constitute the carbon cycle of the Earth. Within this cycle, radioactive carbon-14 is continuously created and disintegrated.
Both processes are in equilibrium. Since the total amount of carbon on the Earth is constant, a constant ratio is established between the amount of stable and radioactive carbon. This same ratio is valid in all the reservoirs of carbon in this giant cycle.
In the biosphere, both carbon-14 and carbon-12 atoms are added to the food chain via assimilation; first by plants through photosynthesis and then by animals through consumption of the plants. For an animal or a plant, a carbon-14 atom is no different from a carbon-12 atom in assimilation.
Living beings continuously take up both atoms, so the ratio of both atoms in their bodies remains constant throughout their life. When an organism dies, the uptake of exogenous carbon is terminated. After this point, although the amount of carbon-12 remains constant, carbon-14 continues to disintegrate and the ratio starts to decrease after the body dies.
Because the ratio after death is related to the time that has passed since death, it is possible to determine the date of death by measuring the amount of radiocarbon present.
The half-life of radiocarbon is 5,730 years. This means that after 5,730 years half of the total amount of radiocarbon in a dead body disintegrates. The remaining half decays in the following 5,730 years and only a quarter of the first amount remains. This goes on until a very minuscule, undetectable amount remains.
In bodies less than 50,000 years in age the amount of radiocarbon can be detected. For an older body, the amount of radiocarbon is so small that the instruments would be unable to measure the amount of radiocarbon present. In addition, such a test obviously works only on the remains of things that were once alive, such as bones or wooden parts of an old structure.
But how accurate is an age determined by this method? How dependable is this technique for enlightening us about the past? Although the theory seems quite consistent from a general outlook, one can see it is not the case when analyzed more rigorously.
Archeologists have tried different ways to test the accuracy of the method. The results have revealed long-term and short-term variations from the actual ages. Long-term variations show systematic deviations of the radiocarbon age from the real age; that is as the date of the sample gets older the deviation increases.On the other hand, short-term variations show irregular fluctuations in the radiocarbon age from the real age.
These deviations apparently reveal that the assumptions made concerning the radiocarbon technique were not accurate. The results of these important abnormal conclusions in radiocarbon dating were summarized in the Introduction to Prehistoric Archaeology as follows: “for years, it was thought that possible errors could have minor effects, however, recent research shows that the natural concentration of carbon-14 deviates at some certain periods, significantly affecting the calculated ages.”
The method is based on two assumptions that should be examined carefully: Firstly, the method assumes that the ratio of carbon-14 to carbon-12 has remained constant in the atmosphere from the time the body died to the present. However, recent scientific research has proven that this ratio has not remained constant during geological time.
Secondly, the method also assumes that the carbon supply to the organism was made only by the global carbon cycle and no other source of carbon has affected the system.
Initial concerns about the possible sources of error were focused on the constant ratio assumption. So, why did the constant ratio assumption turn out to be incorrect? Actually, many reasons were found to refute the validity of this assumption. The most important ones are explained below:
Changes in the Earth’s magnetic field are believed to be responsible for long-term deviations in radiocarbon dating. By investigating the orientation of magnetic minerals in ancient rocks, geologists have proven that the magnetic field surrounding the Earth has not been constant throughout the time. Today, it is widely accepted that both the strength and direction of the Earth’s magnetic field has changed. Interestingly, these changes are appreciable even within a century.
Changes in the geomagnetism affect the radiocarbon production in the upper atmosphere; cosmic rays are deflected according to the strength of the Earth’s magnetic field. If the magnetic field is high, more cosmic rays are deflected away from the Earth and the production of radiocarbon falls. If it is low, production rises. When the production rate changes, a new equilibrium concentration in the carbon cycle as a whole can only be established after a considerable amount of time has passed.
The likely time scale for achieving the complete new equilibrium level is about 10,000 years. This is about the same as the age of the sample that is to be dated! The bottom line is that anything that affects the density of cosmic rays reaching the atmosphere will affect the rate of radiocarbon production, thus affecting the ratio.
Short-term changes might be the results of different factors. One of these is the variation in sunspot activity. Sunspots appear as dark places on the surface of the Sun for a short period of time and generate strong geomagnetic storms. Sunspot activity increases the Earth’s magnetic field and leads to a decrease in the radiocarbon production rate. Therefore, again, anything that causes a change in the Earth’s magnetic field will affect this ratio.
Other effects for short-term variations are the changes in the Earth’s climate. It is widely accepted that the amount of carbon in the atmosphere during geological time is strongly related to temperature changes on the Earth. This fact is also key in understanding the global greenhouse effect, which occurs with the release of high amounts of carbon dioxide to the atmosphere by hydrocarbon combustion.
The global sea level has also been affected by these climatic changes. During low temperature seasons (ice ages or glacial periods), large ice sheets covered most of the continents and as a result of this, the sea level dropped appreciably. During these periods, a high amount of carbon (as carbon-dioxide) was kept inside glaciers and they became C-14 depleted (dead carbon). By the end of the Ice Age, large amounts of dead carbon had been released into the system and they had decreased the global ratio of radiocarbon.
Actually, three more resources of dead carbon make a negative contribution to the ratio. One of them is the dead carbon that comes up from deep Earth through volcanic eruptions. Radiocarbon dating of an organism that lived in the vicinity of a volcano gives inaccurate results.
Because of the expulsion of dead carbon, samples found close to volcanoes have less radiocarbon in their body than others. Consequently, the age determination of these samples gives significantly incorrect results.
As is obvious from the previous examples, the main problem arises in the lack of knowledge about the history of the sample being dated by this method. Another example is when the sample being tested is wood from the inner part of a tree; the radiocarbon method gives an incorrect result in this case. The reason for this is that the innermost part of a tree finishes the carbon cycle before the tree dies. If a sample was made from this part of the tree (it is impossible to know which part of a tree is being used) then the date produced would be greater than its real age.
Even human activity is an important resource for dead carbon. Although only effective since the last century, a high amount of dead carbon in the carbon dioxide has been released into the atmosphere by the burning of fuel. So the ratio of radiocarbon has decreased. Actually, compared to the factors above, this effect has a more profound influence on the application of radiocarbon dating: No recent organic material can be used as a modern standard. Because of this, the zero point of the timescale chosen is to be 1950 AD, as determined by the US National Bureau of Standards for quoting radiocarbon results.
Consequently, the ages determined by the radiocarbon method are not taken seriously by archeologists because of the problems in the basic assumptions upon which the method was established. Occasionally, the radiocarbon method is used to roughly determine whether an object is modern or of considerable antiquity; in essence, it is used as an authenticity test. Even then the answer may not be clear-cut; for example, an old piece of timber could have been carved to produce an authentic looking sculpture!
Radiocarbon dating is an example of how scientific tools should be used carefully to unfold the reality around us. Scientific theories are only poor models of what is happening in reality. The history of science is full of such examples, which sometimes may be misleading if not handled carefully.
See Also Bad Dating Techniques?
Radiocarbon Dating, Sheridan Bowman, University of California Press, 1990