to the invention of calendrical systems and writing in Egypt and Mesopotamia and the keeping of historical records, absolute dates can, with varying degrees of reliability, be assigned to occupation levels beginning in the late fourth and especially the third millennium BC (see below). For earlier, pre-literate periods, the attribution of absolute dates remained a matter of guesswork until the development of the carbon 14 dating method just after the Second World War by Willard Libby of the University of Chicago. Researchers have continued to refine the calibrations of dates, notably through testing of dendrochronological samples from such long-lived trees as the bristlecone pines of the south-west United States.
All organic material contains a small amount of carbon 14, the radioactive isotope of carbon, in a fixed ratio to the amount of non-radioactive carbon (atomic weight: 12). But the radioactive carbon 14 is unstable, and little by little it disintegrates into nitrogen and a low energy radiation. Over the course of approximately 5,730 years, half the original amount disappears. After an additional 5,730 years, the remaining carbon 14 is once again reduced by half. The regular carbon 12, meanwhile, has remained constant. So the ratio between the quantity of carbon 14 and carbon 12 is continually changing, but at a predictable rate. Libby and his colleagues monitored the time span of the changing ratio, and promoted its application for the dating of materials from archaeological excavations. The archaeologist sends a sample of a find with high organic content, for example burnt wood or seeds (the burning helps preserve the item), to a laboratory equipped to measure its carbon 14 content. After comparing that content to the list of known ratios, an absolute date can be assigned. Radiocarbon date determinations can extend back 50,000 years, although earlier than 35,000 BC the precision of measurements decreases because of the increasingly minute amounts of carbon 14 remaining in a sample.
The method is not foolproof. A margin of error always accompanies any radiocarbon determination: for example, “3540 BC, plus or minus 80 years.” The plus or minus indicates the standard deviation: that is, there is a 66 percent probability that the date falls somewhere between 3460 and 3620 BC. If you double the deviation (to plus or minus 160 years, using our example), then the probability increases to 95 percent.
Furthermore, samples can be contaminated (say by modern ants who have set up housekeeping in an ancient deposit and carry in food and other items from outside) or difficult to interpret (what if beams from a 500-year-old tree were used in a new house or burned as firewood?). And scientists themselves have continued to debate such aspects as the actual length of the half-life of carbon 14 and whether or not carbon 14 always disintegrated at the same rate. Despite these difficulties, the radiocarbon method of dating remains invaluable for determining chronology in prehistoric and even early historic times.
World History
