The discipline of archaeology can open up the past to us in a way in which we can enter into a dialogue with our ancestors. There are many ways it can do this: stratigraphic excavation, pottery typology, socio-historic interpretation, etc. However, one thread running through this increasingly focused pursuit is that of dating the physical findings to a particular cultural timeline. This is crucial if we are to know, with as much certainty as is allowed, who we are speaking with. Indeed, dating may be the proverbial thread that holds entire pursuit together, without which the individual pieces of the puzzle might be compared to the children of Israel, who the author of Judges describes as each going off in their own direction. Like the king Judges so wistfully imagines, dating brings cohesiveness and direction to a potentially (or real?) chaotic situation.
In this paper I will explore the particulars of radiocarbon dating; from its background and origin, to discussion of samples, method, results, and calibration.
With the explosions of the first atomic bombs echoing in his thoughts, Willard F. Libby, working with A.V. Grosse, Ernest Anderson, and several students, pioneered the work on a theory that natural C14 not only existed, but that it could also be used as a method for dating certain artifacts of antiquity. Working under the premise of his 1946 paper that suggested C14 might exist in all living organisms, a team including Libby and Grosse collected readings from the Baltimore sewer system. It was discovered that methane collected from these sewers contained radiocar...
bon activity, whereas methane derived from petroleum did not. These findings gave strength to the theory suggested in his paper, but several years of additional research were needed. After their sewer experiments, global samples of wood were gathered in which the researchers discovered a consistent level of radiocarbon deposits. From that point on, the theory developed until it became widely accepted within the scientific community, culminating with Libbys acceptance of the Nobel prize for chemistry in 1960.
The Origin of C14
Natural C14 is formed in the upper atmosphere when nitrogen reacts with neutrons. Neutrons are produced by cosmic rays bombarding the earth, and are thus dependent upon the level of cosmic ray bombardment, as well as the earths natural ability to receive the rays from space. The only known deterrent to cosmic ray absorption is the earths magnetic field. The stronger it is, the less the cosmic rays reach the earth. Once the C14 is produced, it exists in a very small quantity in carbon dioxide, a product of oxygen and carbon. From here, all living organisms take in C14 by either photosynthesis (plant life) or the food chain (breathing life). Since C14 is radioactive, and therefore destructive to life, it is necessary for all organisms to release C14 at basically the same rate they consume it. Those that do not release radioactive carbon as quickly as they absorb it, it is assumed, would not survive as a species. Consequently, in principle, there exists an equilibrium between the levels of C14 in the atmosphere and that which exists in all living organisms. When an organis
ceases to live, it ceases also to take in C14, and the rate of release (radioactive decay) can then be measured and compared to values pertaining to the half-life of the isotope. At present, the best estimate for the half life of C14 is 5730! 40 years, thus making it an almost ideal determinant to archaeologists, and in particular archaeologists concerned with excavations in the middle east.
Because of the very nature of radiocarbon dating, the ideal sample pool is limited. Preferably the sample is organic in composition, and is free of any contamination, such as exposure to other organic material that would skew the readings. This is particularly important in terms of packing and shipping the sample to a radiocarbon laboratory. Below is a list of some potential samples and concerns for each.
Charcoal and Wood are predominant among samples found at archaeological sites. Both are preferred because there is little chance of contamination. However, the possibility of underground water causing a change in C14 saturation needs to be considered with the charcoal. And, while not considered contamination per se, precut growth in wood samples needs to also be taken into account. For instance, if the wood being sampled was cut from the center of a tree, the date read from the analysis would be the date the tree started to grow, not the date the tree was felled for use in construction. Most of the time this would account for an error on the line of 100-200 years. However, it could potentially be greater than that considering the life span of some trees like the Bristle cone Pine is 4000+ years.
“Short-lived” Samples include items such as seeds, hides, paper, cloth, grass, and grains. These are usually preferred to the wood samples because of the potential problem of precut growth. They are called “short-lived” due to the fact that they are, or are made from, items with a relatively short life span. Like the wood and charcoal listed above, there is little chance of contamination.
Ivory is an excellent candidate for dating because it is rare that the specimen will be contaminated. This is due to compaction in its structure and the fact that it, for the most part, remains in a state of preservation comparable to that of contemporary samples. There have been tusks found over 40,000 years old that were first suspected to have come from modern-day elephants. Like trees though, care needs to be taken in ascertaining where in the tusk the sample originated. Whereas in trees, the center is the oldest, it is just the opposite for tusks.
Bones are often found at archaeological sites, but they present a unique problem for carbon dating. First, the carbon found in bones is mostly inorganic and unsuitable for testing. Second, bones are very porous. Unless a good seal is established around the sample, it is very likely the sample will be contaminated by ground water. This can throw a date off considerably. The best bones to use then, are ones that have been preserved within a heavy layer of charring from a fire or volcanic activity. Collagen in bones can also
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