by Donald Collier
The commonest and usually the first question asked by visitors to archaeological digs and museums is “How old is it?” This general curiosity about the age of things made by man in the past is shared by archaeologists, for it is impossible to reconstruct the history of ancient civilizations without chronology. In the absence of written records, like those left by the ancient Egyptians and Mayas, archaeologists have had to depend on indirect methods for determining time sequences in the past. The single exception is that of the American Southwest where, thanks to the tree-ring method of dating, it has been possible to trace with great accuracy the history of Indian cultures during the past 2,000 years. The indirect methods, such as stratigraphy, typological cross-dating, and correlation of human remains and artifacts with geological events and climatic changes, are laborious and inaccurate and do not yield dates in years but only relative sequences.
The new method of radiocarbon dating, developed by Dr. Willard F. Libby at the Institute for Nuclear Studies of the University of Chicago, promises to revolutionize dating problems in archaeology. This method determines the age of things that lived during the past 20,000 years by measuring the amount of carbon 14 they contain.
Carbon 14 is an unstable (radioactive) heavy form of carbon with an atomic weight of 14. Normal, stable carbon has an atomic weight of 12. The half-life of carbon 14 is about 5,500 years. This means that an ounce of carbon 14 is reduced by decay to half an ounce in 5,500 years, that half the remainder decays during the next 5,500 years, leaving a quarter of an ounce, and so on.
Carbon 14 is constantly being formed in the earth’s upper atmosphere as the result of the bombardment of nitrogen-14 atoms by cosmic rays (neutrons). The carbon-14 atoms thus created combine with oxygen to form carbon dioxide, which becomes mixed in the earth’s atmosphere with the vastly greater proportion of carbon dioxide containing ordinary carbon atoms. The carbon 14 then enters all living things, which, through the life process, are in exchange with the atmosphere. This exchange is carried out through photosynthesis in plants. Dr. Libby has shown experimentally that all living matter contains a constant proportion of carbon 14, which is about one trillionth of a gram of carbon 14 to each gram of carbon 12. This constant proportion results from the equilibrium between the rate of formation of carbon 14 and the rate of disintegration of the carbon 14 contained in the atmosphere, in the ocean, and in all living things.
When a plant or an animal dies, it ceases to be in exchange with the atmosphere and hence there is no further intake of carbon 14. But the carbon 14 contained at death goes on disintegrating at a constant rate, so that the amount of carbon 14 remaining is proportional to the time elapsed since death. Given the carbon-14 content of contemporary living matter and the disintegration rate of carbon 14 (the half-life), it is possible to calculate the age of an ancient organic sample from the amount of carbon 14 it contains.
Samples are Burned
The laboratory procedure consists of burning the sample to be dated, reducing it to pure carbon, and measuring its radioactivity (rate of atomic disintegration) in a specially constructed, extremely sensitive radiation counter (a form of Geiger counter). The measurement is expressed in terms of the number of carbon-14 disintegrations per minute per gram of carbon. This value is 15.3 for contemporary living samples, 7.65 for samples 5,568 years old, and 3.83 for samples 11,136 years old. Very old samples contain such a small amount of carbon 14 that the error in counting becomes very large, so that the effective range of the method with present techniques is something less than 20,000 years. But there exists a method for enriching or concentrating the carbon 14 in a sample that may make it possible to obtain useful dates back to 30,000 years. At present the year error in dating samples ranges from 5 to 10 per cent.
Although carbon 14 is present in all organic matter, certain kinds of material have been found to be most useful for dating. These are plant material and wood, charcoal, shell, antler, burned bone, dung, and peat. Unburned bone appears to be unreliable because it is more easily altered chemically than these other materials and hence may lose or gain carbon-14 atoms by exchange during the time between death of the animal and the present.
This method has the disadvantage that the sample to be dated must be destroyed by burning. However, in most cases this is not serious because the size of the sample needed is relatively small. The minimum amount of pure carbon necessary for a single counting run (the amount of carbon placed in the Geiger counter) is 8 grams (about a third of an ounce). Since the carbon content of different organic materials varies, the size of the sample needed to yield this much pure carbon also varies. In general, it is necessary to have about 2 ounces of plant material or wood, 1 to 3 ounces of charcoal, 4 ounces of shell, 5 to 10 ounces of dung or peat, and one to several pounds of antler or burned bone. For the greater accuracy obtained by making two independent counting runs these quantities would need to be doubled.
History of Process
A brief review of the history of the development of radiocarbon dating will help to make clear the nature of the method. In 1934 shortly after the discovery of artificial radioactivity, Dr. A. V. Grosse predicted the possible existence of radioactive elements produced by cosmic rays. In 1946 Dr. Libby predicted that natural or “cosmic” carbon 14 would be found in living matter. The following year he and Grosse checked this hypothesis by testing methane gas derived from sewage (an organic product) and found carbon 14 to be present in the expected amount.
The next step was to test the assumption that carbon 14 was present in the same concentration in all living matter. This research, called by Dr. Libby a “world-wide assay of natural radiocarbon,” consisted of measuring the carbon-14 content of contemporary living material from various parts of the world, various latitudes, altitudes, and geographical situations. This Museum contributed to this part of the research by furnishing from the botanical and anthropological collections wood samples from the Pacific Ocean, South America, Europe, Africa, and the Near East. These measurements, made by Dr. E. C. Anderson, confirmed the assumption and established the value for the carbon-14 content of present-day living matter.
Egyptian Boat Plank Used
The next phase, carried out by Dr. J. R. Arnold and Dr. Libby, consisted of testing the dating method by measuring some ancient samples of relatively accurately known age. These were wood from Egypt and Syria, a sample of wood dated by tree-rings, and a piece of old redwood. They ranged in known age from 1,300 to 4,600 years. One of the Egyptian samples, which was supplied by this Museum, was a piece of deck plank from the mortuary boat of King Sesostris III; who died about 1849 B.C. The dates obtained on this and the other samples agreed with the known ages within the calculated error of the method.
The final phase of this research has consisted of further checking of the method by dating selected archaeological and geological samples of unknown age. An effort was made to obtain from various parts of the world samples whose relative age had been established by the usual archaeological and geological methods. In some instances it was possible to secure several samples coming from different layers of a single stratified deposit. These stratified series were particularly valuable in testing the consistency of the carbon-14 dates obtained. Up to the present more than 150 samples of unknown age from North and South America, Europe, and the Near East have been dated. These range in age from a few centuries to more than 20,000 years, and relate to the problems of dating early man in North and South America; the Archaic Indian cultures of the eastern United States; the early cultures of the Southwest, Mexico, and Peru; the Late Paleolithic, Mesolithic, and Neolithic periods in Europe and the Near East; and the last glaciation in North America and Europe.
Test Museum Specimens
Among these samples were several furnished by this Museum. Two were portions of wooden implements from the Early Nazca culture on the south coast of Peru, excavated by a Museum expedition in 1926. Dr. Libby’s results show them to be about 2,000 years old, which is consistent with other carbon-14 dates obtained from Peruvian samples. Two other samples were charcoal from hearths belonging to the Chiricahua stage of the Cochise culture in western New Mexico. These hearths were discovered by the Museum’s 1950 Southwest Archaeological Expedition. The carbon-14 date indicates that these hearths were in use about 2500 B.C. This date is consistent with one obtained from a site of the Chiricahua stage in Arizona. Samples of charcoal, bark, and wood from the Hopewell culture of Ohio were also given to Dr. Libby. These turned out to be considerably older than archaeologists had believed.
Full assessment of the results of the final phase of Dr. Libby’s research on radiocarbon dating has not been made. But the general consistency of the results, including the dates on samples from periods older than 5,000 years, for which there are no absolute dates for checking, leaves little room for doubt that the method is sound and that dates accurate within the experimental error of the method can be obtained. Radiocarbon dating is destined to have a very important role in archaeology, both in increasing the accuracy of its findings and in reducing the amount of time and effort devoted to problems of dating. This will mean that, in the future, archaeologists can move on with greater facility to the syntheses and generalizations that are the ultimate aim of their work.
The method will be extremely useful also in the aspects of geology, paleontology, and paleobotany dealing with events and processes that occurred during the past 80,000 years. Dr. Libby has already obtained dates relating to the time of the last glaciation in North America and Europe. These dates indicate that the last glaciation was considerably more recent than accepted geological estimates. It will be possible with additional work to date the retreat of the glaciers quite accurately, and this information will have a crucial bearing on time estimates for the whole Pleistocene period, which is variously estimated to have lasted from 400,000 to 1,000,000 years. Radiocarbon dating will also give more reliable information on the time required for the formation of new species among plants and animals. In these and many other ways carbon-14 dating promises to be an extremely important scientific tool
(2001). Biblical Archaeologist 1-4, 14(electronic ed.).