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Dating Techniques

Dating Techniques

For as long as time travel remains the stuff of dreams and science fiction, will continue to provide the most direct means to gain access to the remote past. Historical sources (such as written texts) and ethnoarchaeology provide abundant and invaluable insights concerning ancient times, but even these approaches have significant limitations. For example, the earliest written sources date from only about 5,000 years ago, and such evidence is restricted to certain regions and found in a limited number of literate cultures. Thus, all of prehistory stands beyond the reach of historical research, strictly defined, and the radiometric (science-based) dating techniques, including the so-called radiocar­bon revolution, are all the more valuable. The avail­ability of written evidence (e.g., inscriptions in stone, clay tablets, coins, papyri) can provide firm dates for archaeological strata and artifacts by historical asso­ciation with, for example, references to astronomi­cal events or names of rulers. In principle, however, archaeological excavation, armed with a wide array of dating techniques, allows access to any period of prehistory and history in all parts of the world, even when and where no documentary evidence exists.

Of course, archaeology (the systematic recovery and analysis of the material remains from past cultures) faces its own limitations: theoretical, logistical, financial, and methodological. Archaeo­logists desire to bring order to, and understand, the various types of data that survive in the ground. Therefore, the search for better techniques by which to link sites and artifacts to particular peri­ods and the need to understand them within a relatively narrow chronological framework remain important. Archaeology contributes to, and bene­fits from, chronometry (the scientific measurement of time). Many well-known scholars have contrib­uted to a host of procedures used in archaeological dating (e.g., Jacques Boucher de Perthes, Christian Jürgensen Thomsen, Charles Lyell, William M. F. Petrie, Pitt Rivers, Andrew Ellicott Douglass, Willard F. Libby); the steady advance of chrono­metric techniques, especially those developed by physicists and chemists, promises even more preci­sion for scholars who will examine the past in generations to come.

The field of archaeology emerged from a back­ground that included several (e.g., geology, anthropology); its early practition­ers included antiquarians from the Age of Enlightenment, whose views about the past ranged from naive to sophisticated. Once these early scholars accepted the idea of a past that dif­fered from their own day, they began to develop means by which to date their discoveries and impose order on the past. As in all the sciences, progress depended on the development of reason­able methods of research and the invention of instruments to measure and analyze data. Because archaeology focuses on cultural changes through time, archaeologists have always sought methods that offer reliability and precision in dating mate­rials from the past. Nowadays, archaeologists have access to many different dating tech­niques; these are the conceptual tools and instruments used to reach back in time to create a chronological framework for the prehistoric and historic eras.

In the broadest terms, these dating techniques fall into two categories: (1) relative dating, which identifies the sequence of archaeological materi­als (e.g., stratigraphy, ceramic typology), and (2) absolute dating, which assigns actual dates (how­ever approximate) to sites, artifacts, and events (e.g., dendrochronology, [i.e., various techniques that determine age by mea­suring radioactive decay]). These approaches include some older, well-established methods that have demonstrated their value and a number of more recent scientific techniques. All of these dating methods have significant limitations (e.g., cost, kinds of materials that can be tested, span of time for which reliable results can be expected), and scholars who use these techniques must acknowledge problems where they exist (e.g., identification of variables, danger of contamina­tion). Because dating is a pivotal aspect of archaeological analysis, archaeologists and scien­tists who work with multidisciplinary research teams must constantly work to refine dating techniques, improving the old standbys and per­fecting the newer scientific methods, most of which were developed during the latter half of the 20th century.

As noted earlier, many different dating tech­niques are available, and scientists continue to invent new methods and improve the old ones. These different approaches focus on a variety of recognizable and measurable data, all of which allow archaeologists to date materials in relative or absolute terms. Sheridan Bowman has conveniently categorized the more science-based dating tech­niques and identified significant examples of each. The first category includes methods that are based on radioactive decay (e.g., fission track dating, potassium-argon dating, radiocarbon [carbon-14] dating, and uranium-series dating). Bowman’s sec­ond category, dating techniques based on climatic change, includes calcite banding, dendrochronol­ogy, ice cores, oxygen isotope dating, and varves. The third type of dating method is based on special properties of materials (e.g., amino acid racemiza­tion, archaeomagnetic dating, electron spin reso­nance, fluorine uptake, thermoluminescence). Techniques based on diffusion processes, Bowman’s fourth category, include obsidian hydration dating and nitrogen and sodium profiling.

Foundational Approaches

The most fundamental principle of archaeological excavation, borrowed from geology’s focus on stratigraphy, is the law of superposition. On a complex archaeological site, this law provides the basis for the main concept of relative dating by noting that one normally finds older remains in the lower strata of a site, as they are covered by more recently deposited materials. The careful delineation of stratigraphic relationships provides invaluable information concerning site history (sequence of site occupation and use) and the rela­tionships among all deposits, features, and arti­facts in the site. As excavators soon discover, a variety of factors can complicate the picture and turn the site into something other than a picture­perfect sequence of easily defined strata. Nevertheless, the primary objective of excavation is the removal of layers (and their contents) in the reverse order in which they were deposited. More sophisticated approaches, like the Harris Matrix, lend precision to this process by forcing excava­tors to identify sequential, chronological links. Along the way, more precision becomes possible as researchers isolate and assign dates to features, materials, and objects through one, or more, of the scientifically based techniques, comparison with datable items from other sites (cross-dating/ synchronisms), or through historical associations. Occasionally, evidence of well-known seismic or volcanic activity helps pinpoint a site’s history, as in the case of Vesuvius’s eruption in 79 CE.

Another foundational approach in archaeologi­cal dating is known as typology. Typology takes advantage of archaeologists’ attention to detail and to their interest in imposing order on materials recovered from a site. The typological approach, applied to the full range of artifacts or even archi­tectural features (e.g., stone or metal tools, gate plans, pottery), arranges archaeological materials into an order that reflects change and moves toward a hypothetical (relative) chronological order. Once a specific object, like a type of pottery lamp (distinctive in form, decoration, and manner of production), becomes linked with a fixed date at one site, the pottery assemblage of which that lamp is a part assumes a place in the optional dating tools available to the archaeologist (at least for a particular region). Though a ceramic typology can often provide fairly close dates, pottery and other materials arranged into typologies provide only relative dates (such as Thomsen’s “three-age sys­tem” [stone, bronze, and iron], with elaborate appli­cations to deal with cultures around the world ). In fact, archaeologists make frequent use of the terms terminus ante quem and terminus post quem to distinguish between types of chronological evi­dence and to highlight the central role that relative dating continues to plays in interpretation.

Another type of relative dating is called seria- tion, a method that helps archaeologists arrange artifacts into chronological sequences by noting the “life cycles” of changes (in which typological fea­tures appear, experience extensive use, and fall into disuse). William M. F. Petrie first developed this approach in Egypt to provide a sequence for some 900 predynastic graves. He gave special attention to changes in ceramic forms, including wavy ledge handles that featured prominently on jars in the early ceramic repertoire. Petrie’s use of the “con­centration principle” makes this one of the earliest applications of statistics to archaeological research. In a period of research in southern Palestine, espe­cially through observations he made during his brief excavations at Tell el-Hesi, in 1890, Petrie also recognized the nature of a stratified mound and laid foundations for the development of stra­tigraphy and typology.

Decades later, William F. Albright developed a significant ceramic typology (“ceramic index”) on the basis of his work at Tell Beit Mirsim. Many scholars subsequently improved upon the percep­tive observations made by Petrie, Albright, and others, but their contributions represent similar breakthroughs made around the world by many other pioneers in archaeological dating. Though it is not unique in this regard, ceramics has played a special role in providing a chronological frame­work for thousands of historic and late prehistoric sites in the Middle East.

Dendrochronology

Dendrochronology, which offers a genuinely abso­lute date, measures and links the annual tree rings of wood samples that comprise master chronolo­gies for particular regions of the world. This tech­nique developed out of climatic research, but anthropologists quickly recognized its potential in determining the age of ruins in the southwestern United States. Now species of trees known for their longevity, like the bristlecone pine and the European oak, provide a nearly continuous timescale that reaches beyond 10,000 years ago and serves as the primary component of the all-important “calibra­tion curve” in . As with the radiocarbon analysis of wood samples, archaeolo­gists must always consider the possible gap between the date assigned to, say, a roof beam and the actual use of that timber in an archaeological con­text (i.e., distinguish between the age of the wood and the date of the archaeological “event” itself).

Radiometric Techniques

The second half of the 20th century witnessed the invention and development of a host of scientifi­cally based dating techniques, almost all of which derived from distinctive physical or chemical properties. Many of these newer methods are radiometric in nature; that is, they provide more- or-less absolute dates that are tied to known rates of radioactive decay and a precise measurement of isotopes. As indicated earlier, use of the term absolute requires some qualification, as labora­tory results on any tested sample will include a margin of error, even when scientists are working with known rates of decay and the nuclides func­tion somewhat like a clock. Scientists measure the deterioration of these radioactive substances in half-lives (e.g., the half-life of carbon-14 is 5,730 years). Precision is enhanced through multiple samples and cross-checking with other dating techniques, when that is possible.

Without a doubt, carbon-14 analysis of organic materials is still the best known radiometric dating technique; a by-product of research related to the Manhattan Project, this process was initially devel­oped in 1949 by W. F. Libby and colleagues. After nearly 6 decades, research on the application of carbon-14 to archaeological dating and the resul­tant “radiocarbon revolution” continue at full speed, with no indication that the latter will slow down. The major journal Radiocarbon (published at the University of Arizona) and a regular series of related conferences provide invaluable interna­tional venues for discussion about carbon-14 and other isotope dating. As a result of this constant research and refinement, scientists have a much better understanding of the factors that have caused, and still cause, fluctuations in the carbon- exchange reservoir in the atmosphere, ocean- sphere,and biosphere. The radiocarbon “calibration curve,” based on corrections provided by dendro­chronology and uranium-series dating, promises much greater reliability and precision. Today’s accelerator mass spectrometry (AMS) dating, which allows a direct counting of the carbon iso­tope ratios, has significant advantages over the conventional carbon-14 method, especially with respect to the small sample size that AMS requires. Even with carbon-14’s relatively short half-life and the possible sources of error, the radiocarbon time­scale gives good results for samples that date to 50,000 or even 70,000 years before the present. The recent application of Bayesian statistical anal­ysis also enhances the usefulness of radiocarbon, and the future undoubtedly holds other improve­ments for the carbon-14 method of dating.

Other radiometric dating techniques, which have timescales that reach further into the past than the radiocarbon method, include alpha recoil dating, fission track dating, potassium-argon dat­ing, radiocalcium dating, rubidium-strontium dating, and the uranium-series dating. The potas- sium-argon method achieved notoriety because of its early use in the study of hominid fossils at Olduvai Gorge (in research carried out by Louis Leakey, Mary Leakey, and colleagues) and other East African sites. By means of sensitive instru­mentation, this geochronological process measures the parent-to-daughter decay of potassium-40 to argon-40, isotopes that appear in volcanic lava. The half-life of this potassium isotope is 1.25 bil­lion years, and therefore this technique finds its primary application in geological dating.

Conclusion

Archaeology will continue to refine its array of dating techniques so that field research and labo­ratory analysis can provide more precise bench­marks in time and space, as well as offer a better understanding of past cultures. Better dating tech­niques also provide safeguards against fraudulent claims, as in disputes over pieces of art or even the infamous Piltdown hoax.

Gerald L. Mattingly

See also Anthropology; Archaeology; Boucher de Perthes, Jacques; Decay, Radioactive; K-T Boundary; Lyell, Charles

Further Readings

Aitken, M. J. (1990). Science-based dating in archaeology. New York: Longman.

Biers, W. R. (1992). Art, artifacts and chronology. New York: Routledge.

Bowman, S. (1990). Radiocarbon dating. Los Angeles: University of California Press.

Bowman, S. (2002). Dating techniques. In I. Shaw & R. Jameson (Eds.), A dictionary of archaeology. Oxford, UK: Blackwell.

Harris, E. C. (1989). Principles of archaeological stratigraphy. London: Academic Press.

Higham, T., Bronk Ramsey, C., & Owen, C. (Eds.). (2004). Radiocarbon and archaeology: Proceedings of the 4th symposium, Oxford 2002. Oxford, UK: Oxford University School of Archaeology, Institute of Archaeology.

Nash, T. (2000). It’s about time: A history of archaeological dating in North America. Salt Lake City: University of Utah Press.

O’Brien, M. J., & Lyman, R. L. (1999). , stratigraphy, and index fossils: The backbone of archaeological dating. New York: Springer.

Renfrew, C., & Bahn, P. (2008). Archaeology: Theories, methods and practice. New York: Thames & Hudson.

Taylor, R. E., & Aitken, M. J. (1997). Chronometric dating in archaeology. New York: Springer.

Wagner, G. A. (1998). Age determination of young rocks and artifacts: Physical and chemical clocks in quaternary geology and archaeology. New York: Springer.

Walker, M. (2005). Quaternary dating methods. New York: Wiley.

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