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Carbon 14 Dating: Past, Present and Future A simplified explanation

by Jim Cherry, Fayetteville, Arkansas

Originally Published in the Central States Archaeological Journal, Vol.56, No.1, pg.28

Used tens of thousands of times, car­bon-14 (C-14) dating continues to be an es­sential tool for archeology. But how did we get this tool and how does it work? I will attempt to explain this in an archeological (down to earth) way.

There are 3 varieties of carbon, called iso­topes. These are carbon-12 (the most plenti­ful), carbon-13, and carbon-14. Only C-14 is radioactive, the other two are called “stable isotopes.” C-14 forms from nitrogen-14 in the upper atmosphere by cosmic radiation from the sun. This radioactive form of carbon reacts with oxygen to form carbon dioxide, a gas in our atmosphere. Plants use carbon dioxide in photosynthesis. Animals eat plants, and some animals eat other animals, so a very small part of living bodies is made of radioactive C-14.

While living, they bring in C-14 and also get rid of it as part of waste products. But when they die, a clock starts ticking, which is the radioactive decay of the C-14. The clock begins at death because no more C-14 is com­ing in.

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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