Dating in Archaeology PDF Print E-mail
Written by Stephen Langfur
 
  
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Dating in Archaeology
Radiocarbon dating


Radiocarbon dating

Is there then no hope for getting absolute dates? What about the Carbon-14 method?

This too has problems, but they can be solved, and then the pottery fragments will be all the more relevant for dating.

 "Radiocarbon dating depends on a chain of natural events, some having taken place in deep space long ago. The sequence begins in various parts of the galaxy, where charged particles are accelerated to immense velocities, forming what are known as cosmic rays. A fraction of these particles eventually rain down on the earth and strike molecules of atmospheric gas, producing neutrons. Some of these neutrons in turn react with nitrogen," forming C-14, which combines with oxygen to form molecules of radioactive carbon dioxide. By the time this reaches the surface of the earth, "it has fully mixed with normal carbon dioxide and accounts for about one molecule in 10" (Taylor ).

Most of the radioactive carbon enters the oceans. But one or two percent goes into the terrestrial biosphere, because plants absorb carbon from the air during photosynthesis. The animals that feed on the plants get tagged as well with C-14. Living things maintain a C-14 content "about equal to the atmospheric concentration because the carbon atoms that undergo radioactive decay within their bodies are continually replaced. But once an organism dies and its metabolic processes cease, the amount of C-14 begins to diminish." A dead organism loses half its C-14 (relative to its non-radioactive carbon) through radioactive decay about every 5730 years.

Suppose, then, that we find the charred remains of wood in a layer of a tell. If we can measure the proportion of C-14 to its other carbon isotopes, then compare this to the concentration in a recently living thing, we should be able to determine when the tree died. For example, if the wood has only half the concentration of C-14 that a living tree has, then it must be about 5730 years old.

A scientist named Willard F. Libby grasped this principle and, in 1949, managed to determine the concentration of C-14 in dead organic matter by measuring the rate of radioactive decay. From the late 1970s, scientists learned to detect concentrations of the isotope directly by sorting out masses through spectrometry in an accelerator (cyclotron). This reduced the amount of organic material needed from grams to milligrams. It also reduced measurement-time from days to minutes. Tests on ancient wood had been problematic for dating a stratum, since wood may continue in use for thousands of years. Using the cyclotron, scientists could determine the C-14 in small amounts of organic materials that would not have been passed down, such as charred grain.

Yet C-14 tests did not always yield results corresponding to known historical dates. Soon it was realized that the amount of C-14 in the atmosphere had not been constant throughout the ages. Scientists found, however, a method of coping with this fact:

"For some species of trees growing in some climatic conditions, visually recognizable rings form annually with a climatically determined width pattern. This width pattern allows cross dating between trees felled in different periods (the pattern shown by the inner part of a recent tree can be matched with that shown by the outer part of a tree that was felled earlier.) In this way a  master chronology can be established for a region..." (Meyers, ) "Dating Techniques").

Such dendrochronology can be used directly to date pieces of wood found at a site. But more to our purpose, one can also measure the radiocarbon age of the wood in a tree trunk and check this against the number of rings. Using California giant sequoia and bristlecone pine, as well as European oaks, researchers have been able to document the amount of "offset" between "C-14 time" and "real time" for the past 11,800 years. On this basis they have been able to calibrate the radiocarbon method, calculating true age by adding or subtracting the appropriate offset.

"(I)nvestigators now have a good handle on the 'wiggles' in the calibration for the last twelve millennia. These variations are well documented, yet they cause considerable problems because they introduce built-in uncertainties in dating: Calendar ages can often be expressed only as broad ranges," even when the C-14 concentrations are known precisely (R. E. Taylor, op. cit.). Researchers express the range by adding a plus/minus figure: for example, 1468+/- 75 BC. This does not mean that the date necessarily lies within those ranges. The 75 years constitute what is called a "standard deviation" or "sigma." When a date is so given, it means that the tested item has a 66% probability of lying between 1468 + 75 BC and 1468 - 75 BC. It has a 95% probability of lying within the limits of two standard deviations, i.e., between 1468 + 150 BC and 1468 - 150 BC. It has a 99.5% probability of lying within three.

Calibration curves vary by region, and we do not yet have them for the Eastern Mediterranean. Scientists can trace the tree rings here from the present back to 362 AD . What's more, a Cornell University team has established a 1503-year 'floating sequence' of  rings for Bronze and Iron Age Anatolia, but this has yet to be connected to modern sequences in order to supply absolute dates. Until that happens, no C-14 results for the Middle East will be definitive. Once the Cornell work is completed, we should be able to date strata within the limits of probability mentioned above, and the pottery distinctive for a stratum so dated can then be used to date strata elsewhere, even when no organic material is found. The accumulation of data, along with further techniques, should solve the problem of absolute dating.

Given all the difficulties of Egyptian chronology and those of carbon dating, we may wonder how archaeologists in the Holy Land have been able to arrive at dates such as those given in this Website for periods before 763 BC. The answer is that they have assumed they were on firm ground with Egyptian chronology. Because Egyptology is vast and complex, they have not been able to check that chronology for themselves. "The dependence on Egyptian chronology is so strong that any change in the latter necessitates a parallel shift concerning Palestine" (Mazar, p. 29. ) Once the regional tree-ring sequence is all hooked up, there may be some rewriting to do, this Website being no exception.