A simplified explanation
by Jim Cherry, Fayetteville, Arkansas
Originally Published in the Central States Archaeological Journal, Vol.56, No.1, pg.28
Radioactive elements decay at fixed rates, some very slow, some very fast. Scientists call this rate or speed of decay a "half-life." One half-life equals the length of time for half of the isotope to decay. The half-life of C-14 is 5,730 years. So if we start with 1 pound of C-14, after 5,730 years we should have % pound of C-14. What happened to the other '/2 pound? It decayed back to the stable, nonradioactive nitrogen-14. After another 5,730 years, we have 1/4 pound of C-14, etc. After 8 to 10 half-lives, radioactivity decreases so greatly that effectively, the clock stops. The oldest materials C-14 can date are about 40 to 50 thousand years.
Willard Libby developed C-14 dating in 1947, beginning with a $5,000 research grant. During WWII, he had worked on the Manhattan Project as part of the team that developed the atomic bomb. After WWII, he became interested in the effect of solar radiation on earth's atmosphere. He predicted that he should be able to find cosmic generated C-14 in living things, but instruments for detection were very crude. After developing better instruments, he detected C-14 in methane gas obtained from the Baltimore, Maryland sewerage disposal plant. C-14 was not found in methane gas derived from petroleum. Because of its great age, the original C-14 had long ago decayed.
The first step was to find the C-14. The second step was to see if C-14 could be used to date archeological materials that at one time had been alive (no, we cannot directly date arrowheads, unless they are made of bone or antler).
To check C-14's dating accuracy, materials of known or closely known ages were tested, such as a loaf of bread excavated at Pompeii, the Roman city destroyed by the eruption of Mt. Vesuvius in A.D. 79. Wood from an Egyptian first Dynasty tomb was the oldest test sample. By historical records it was known to be about 4,900 years old.
Before the 1950s, archeology had a huge problem. By stratigraphy, we could tell that one artifact should be older than others because it was found in lower, deeper strata than other artifacts. This is called relative dating. But putting absolute dates on prehistoric artifacts was largely creative guess work. That all changed with C-14. For example, Libby determined the last ice age ended about 10,000 years ago, and not 25,000 years as previously thought.
In 1960, he received the Nobel Prize in chemistry for the revolutionary changes C-14 dating brought to the field of archeology.
Early use of the method required large samples which limited its usefulness. For example, a large portion of a basket had to be sacrificed in order to date it.
Fortunately, AMS (accelerator mass spec-trometry) came along in the 1970s. Now extremely small samples could be dated. Even small scrapings from the charcoal pigment used in European Neolithic cave paintings have been dated. Why is AMS a more sensitive test? Because we no longer have to wait for C-14 atoms to decay in order to detect them. AMS can literally sort out and count the atoms of C-14 from C-12 and C-13.
However, life is not always as simple as we would like. Early on, small discrepancies were found between C-14 dates and dates obtained by other methods. Libby assumed solar radiation and the production of C-14 has been constant over the millennia. However, we now know that the sun's production of solar radiation increases slightly during periods of sun spot activity, which is about an 11 1/2 year cycle.
Also, the earth's magnetic field varies in strength slightly over time. The surrounding magnetic field deflects harmful solar radiation particles from the earth's surface. A stronger magnetic field gives more protection from solar radiation and less production of C-14. A weaker magnetic field results in a little more C-14 production.
Atomic testing from the 1950s and 60s increased the amount of C-14. This "new C-14" can be a troublesome source of contamination for the laboratory, especially with very old samples. The burning of fossil fuels has added ancient carbon to our atmosphere, which also complicates things.
The date of ancient wood, bone, antler, etc. is calculated by comparing the amount of radioactive to stable carbon in the sample. We will ignore the mathematics, but age is calculated from the C-14 radioactive decay curve. Because of the mentioned problems, the real curve has several little wiggles that vary a few percentage points from the expected, or theoretical, curve. The variations make a small difference in young objects, but big differences in old objects. For example, a 5% variation gives a date that is 50 years off on an object that is really 1,000 years old. The date will be 1,000 years off on an object that is actually 20,000 years old. Early on, C-14 proved its usefulness, but it had these irritating variations from the expected decay curve. Could C-14 dating be fine tuned to correct for these problems? Enter dendrochronology.
Dendrochronology means telling time by trees. Even in a pre-scientific age, Leonardo Da Vinci explored the idea of trying to reconstruct the "nature of past seasons" by examining the relative widths of tree rings.
Tree rings consist of a light and dark band corresponding to the warm and cold seasons of the year. The width of tree rings reflects the good or bad growing conditions for that year. The most useful trees are the giant sequoia which can live more than 2,500 years, and the bristle cone pine that may live more than 4,500 years. Beginning with living trees, and overlapping with tree ring patterns of dead trees, a chronology can be built. In North America, a history of 12,500 years of tree rings has been compiled.
What new work is being done? Hopefully, a tree ring chronology going back 60,000 years can eventually be put together using the New Zealand Kauri tree. These trees live for at least 2,200 years and many ancient specimens have been preserved in swamps on New Zealand's North Island.
Because the rings are progressively older toward the center of the tree, C-12, 13 and 14 values can be directly determined for any particular year in the past 12,500 years. So the fine tuning (calibrated) C-14 curve goes back 12,500 years, and hopefully it will be extended further in the future. Though not as ideal as tree rings, cave formations have been used to calibrate the C-14 curve back to 45,000 years ago.
Can we get some weird results from C-14 dating? Yes, absolutely! For example in the early 1960s, campfire carbon remains from a Clovis site near Lewisville, Texas, 41DN72, gave a date of "greater than 37,000 years" (the upper limit for the test at the time). The archeologists were surprised by this early date and controversy raged for years over the accuracy of the age because it was inconsistent with dates obtained from other Clovis sites. In 1978-80, a drought lowered lake levels so the site could be re-examined. Further excavations of hearths revealed that in addition to mesquite wood, the Indians were burning lignite, a soft form of coal that outcrops in the area. The lignite, of course, is much older than the date of the campfire, giving the unexpected old date.
Accuracy depends on the sample obtaining its carbon from the atmosphere, either directly, as with plants, or indirectly as with animals. The shells of living mollusks were once dated as being 2,300 years old! However, tests showed they were absorbing ancient carbon from the local limestone (calcium carbonate) in their habitat. Therefore, archeologists tend to avoid using shells for dating purposes.
Because of these avoidable pitfalls, occasional lab error, or contaminated samples, some people with a particular religious viewpoint argue that C-14 dating is unreliable and should be discarded. By using that reasoning then, we should also conclude that because automobiles sometimes do not work properly, and occasional operator errors occur, (car accidents), no one should drive cars!
What else is new? Rather than working to wash and strip contaminates away from the carbon in the sample, some scientists are taking an opposite approach. They are developing techniques to directly extract specific material that fix its carbon from the atmosphere such as leaf waxes. This process leaves possible contamination behind, and should lead to even more precise and consistent results. Carbon-14 dating has undergone many refinements since 1947. It has been cross checked by other independent dating methods such as ice cores, corals, lake varves, and deep sea sediments, and it continues to be an important tool for archeology, anthropology, paleontology, and the study of past climate changes."Used by Permission of the Author"
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