Dating

Use of dating techniques to determine the age of an artifact or dates of occupation for a site or dates for a sequence of events is one of the most fundamental archaeometric activities conducted by archaeologists and is essential to organizing archaeological evidence. Prior to the twentieth century, archaeologists were limited to the use of relative dating techniques such as stratigraphy and seriation. However, the twentieth century saw the development of a number of absolute dating techniques, many of which have improved to the point that it is now possible to determine dates from fewer to hundred years back to several million years.

The first absolute dating technique was developed in the 1920s by Andrew Ellicot Douglas who discovered that the widths of annual rings in trees from the American Southwest were related to the climatic variation during antiquity. Douglas applied his new technique to pine beams from a twelfth century building at Pueblo Bonito, thus giving rise to dendrochronology. Use of dendrochronology has been extended to other regions of the world, but each region is somewhat different due to climatic differences (see Dendrochronology).

The development of radiocarbon dating by Willard Libby in the late 1940s has probably had the greatest impact on archaeological science. The technique is based on the premise that all living organisms have a continuous uptake of radioactive carbon-14 until they die (see Carbon-14 Dating). By measuring the amount of radiocarbon present in their remains, it is possible to estimate the time elapsed since the organism died. For the first time, chronologies for different parts of the world could be compared back to about 50 000 years. The impact of radiocarbon dating on archaeology cannot be overestimated. Libby was awarded the Nobel prize for chemistry in 1960, and radiocarbon labs were established around the world.

Throughout the remainder of the twentieth century, a number of other absolute dating methods were developed, including accelerator mass dating of carbon-14 which requires less sample and extends the range to about 100 000 years, thermoluminescence dating and archaeomagnetic dating which can be applied to materials that have been heated in antiquity such as pottery or stone, and radiometric methods such as uranium series, fission-track, and potassium-argon dating which can be used to date much older archaeological materials such as those associated with the formation of corals and stalactites or the occurrence of volcanic events, etc. (see Electron Spin Resonance Dating; Luminescence Dating). The suitability of an individual dating technique for a particular application depends upon the types of archaeological material available for dating, the range of ages, and other factors.