Radiocarbon dating

Radiocarbon dating is a means of determining the approximate time at which biological processes ceased in a once-living organism or in related organic substances. It allows scientists to estimate the ages of organic materials and the formations in which they occur.

Radioactive Carbon-14

Carbon compounds are among the most abundant in nature. Carbon, in the form of atmospheric carbon dioxide, is continuously moved through major environmental systems, a process known as the carbon cycle. The photosynthesis of carbon dioxide establishes carbon in various compounds in plants, which in turn may be consumed by animals. Most carbon is absorbed as carbon dioxide by the oceans or appears there as dissolved carbonate or bicarbonate compounds. Carbon occurs naturally in three isotopes: carbon-12, the most common; carbon-13, also a stable isotope; and the radioactive isotope carbon-14, which exists in minute quantities. Radiocarbon dating draws on several assumptions concerning the natural production of carbon-14, its presence in various environmental cycles, and its rate of decay, or half-life.

Radioactive carbon-14 is formed in the upper atmosphere as the result of bombardment by energetic cosmic radiation emanating from deep space. Statistically, it is most likely that free neutrons, which result from collisions between proton cosmic radiation and atmospheric gases, will shortly collide with molecules of stable nitrogen, nitrogen-14, by far the most abundant gaseous element in the atmosphere. The resulting reaction normally expels a proton from the nitrogen nucleus, producing an atom which now behaves chemically like carbon but is heavier than the stable carbon isotopes. Free carbon does not remain long in the upper atmosphere. It quickly combines with oxygen molecules to form carbon dioxide, whereupon it enters into various geological and biological processes.

Beta Decay

The half-life of carbon-14 usually is expressed as 5,568 ± 30 years, though it is more likely to be on the order of 5,730 ± 30 years. (Once large numbers of dates had been calculated on the basis of the former figure, leading scientific journals generally preferred to stay with it.) The particular process of radioactive decay of carbon-14 is called beta decay, in which the "extra" neutron in the nucleus emits a beta particle (essentially an electron) and a neutrino (an uncharged particle), thus changing itself, in effect, from a neutron into a proton. The result is once again an atom of stable nitrogen.

As long as an organism is alive and normal biological processes are occurring, the rate of accumulation of carbon-14 is in approximate equilibrium with the rate of radioactive decay of the carbon-14 already in the organism. This level of equilibrium is extremely small, on the order of one atom of carbon-14 to every 1 trillion atoms of carbon-12. The moment that biological processes cease, however, the equilibrium is broken, and the quantity of carbon-14 in the once-living organism begins to decrease at the predictable rate of radioactive decay. By measuring the rate of beta radiation from the residual carbon-14, one may estimate the age of a material, or, more precisely, when the biological processes of the organism from which the material derives ceased to operate. In general, the age of material which gives off only half as much beta radiation as a living organism would be 5,730 years; the age of material emitting only 25 percent as much radiation would be 11,460 (5,730 × 2) years; and so on.

Evolution of Measurement Techniques

After fifteen years of research, Willard F. Libby, an American physicist, introduced the radiocarbon dating method in 1949. Since Libby's pioneering work, radiocarbon dating has evolved to include several counting techniques. Normally the substance under study must be destroyed by combustion or other processes to produce gaseous carbon dioxide or a hydrocarbon gas, whose carbon-14 content is measured by a gas counter. Liquid counting techniques have also been developed, in which the carbon dioxide from the substance under study is synthesized into more complex liquid hydrocarbons, such as benzene. After 1980, direct measurement techniques, using particle accelerators and mass spectrometers, increasingly replaced gas and liquid counting.

The evolution of measurement techniques has greatly enhanced the usefulness of radiocarbon dating. The minimum acceptable mass of a sample substance is much smaller than that required in the years immediately following Libby's introduction of the method. Refined techniques and improved instrumentation also have extended the effective chronological limits of the method. At first, only materials less than about 20,000 years old could be dated with any confidence, but by the fortieth anniversary of Libby's initial dating experiments, there was wide agreement that the effective limit was about 40,000 years and, when supporting data could be gleaned from other dating methods, possibly 70,000 years.

Risk of Contamination

The extremely small quantities of carbon-14, even in living organisms, demand that substances undergoing counts be prepared most carefully and that every possible source of contamination be considered. For example, fallout from atmospheric testing of thermonuclear weapons in the 1950s interfered with some of Libby's early experiments. Numerous chemical processes affect archaeological artifacts and other substances which might be candidates for radiocarbon analysis. Carbon compounds from associated soil layers find their way into dating samples. Carbonate encrustations may develop around other samples, especially in locations with abundant groundwater. In the 1980s, some radiocarbon laboratories had to consider their local environments, where, in many cases, the buildup of carbon dioxide and other carbon compounds in the atmosphere from fossil fuel combustion threatened to interfere with sample integrity and analysis. (Fossil fuels, because of their great geological age, contain virtually no carbon-14.)

Some substances are at a greater risk of contamination than others. Woody plant remains, such as charcoal or building materials carbonized by fire, produce the best results. Less dependable are textiles, fibers, and the remains of nonwoody plants, which do not live as long as woody species and, as a result, may reflect short-term or local carbon-14 anomalies. Bone is notorious for its ability to absorb carbon compounds from its surroundings. Inorganic substances which contain carbon compounds, such as eggshell and marine shell, also present serious problems of carbonate contamination. Of these substances, marine shell is preferred, since the extraneous carbonate levels can be determined on the basis of relatively constant values present in ocean water.

Adjusting for Variability

In reality, radiocarbon dating is subject to numerous variable factors and is not nearly so accurate. Radiocarbon dates are expressed as years b.p. or "before present." For example, increases in atmospheric carbon dioxide since the Industrial Revolution have diluted the concentration of carbon-14 in what is known as the Suess effect, named after Austrian chemist Hans Suess. Observations by the THEMIS spacecraft fleet have shown that twenty times more solar particles cross the earth's magnetic shield when it is aligned with the sun's magnetic field, compared to when the two magnetic fields are oppositely directed. "Present" is actually a zero base date of 1950. Scientists chose this year because it was close to the first experimental application of the method and because the buildup of atmospheric radioactivity from nuclear weapons tests in later years introduced complications into the measurement process. Each date is expressed with a standard error, or standard deviation—in essence, a "confidence level"—indicated by a plus-or-minus sign. A typical radiocarbon date of, say, 2750 b.p. ± 60 indicates that there is a 66 percent probability that the true date falls within sixty years of 2750 b.p. and a 95 percent probability that the true date falls within twice the standard error—in this case, 120 years.

Another source of variability in test results was only dimly suspected before about 1970. Libby assumed that the formation rate of carbon-14 in the upper atmosphere had a constant value over time, since the process depended on cosmic radiation. Scientists now know that the formation rate varies over time and space, in part according to the strength and contours of the earth's magnetic field. The field deflects a certain portion of cosmic radiation, so some less energetic particles never reach the atmosphere. At any time, for example, more carbon-14 is formed in the atmosphere near the poles, where the magnetic field is relatively weak, than near the equator, where it is stronger.

The strength of the magnetic field itself is affected by cyclical changes in solar activity. The sun has roughly an eleven-year cycle of sunspot activity; the earth's magnetic field is most energetic when sunspots are most numerous. Longer-term and even more significant changes apparently have occurred in more recent decades of solar history, as in the case of the so-called Maunder minimum (1645-1715), when sunspot activity may have been almost nonexistent. During such a period, the earth's magnetic field would be less energetic, carbon-14 formation levels would be abnormally high, and materials from the era would appear "younger" in radiocarbon terms than they otherwise would.

Dendrochronology (the study of tree rings) provides a means of identifying these anomalies and thus calibrating radiocarbon dates more closely with absolute chronology. In principle, each growth ring in a tree lives only for one year. By comparing the known age of tree rings with their radiocarbon ages, scientists can locate anomalies in the carbon-14 absorption rate and, therefore, the formation rate. Wood samples from the extremely long-lived bristlecone pine provide invaluable data. The oldest living bristlecone is more than 4,500 years old, and the remains of dead specimens have yielded a calibration matrix extending almost nine thousand years into the past.

Several other dating systems have been developed which utilize the half-life of radioactive isotopes, and some provide corroboration of radiocarbon chronologies. Further support comes from closely related techniques, such as thermoluminescence. These methods, as well as the established practices of classical archaeology, strengthen the credibility of the radiocarbon technique.

Application to Archaeology

Through radiocarbon dating applications, scientists have amassed a wealth of information on such matters as sea-level fluctuation, climatic change, glaciation, habits of marine life, volcanic activity, and early forms of atmospheric pollution from the combustion of fossil fuels. From time to time, results of carbon-14 analysis also attract attention by establishing the authenticity of artistic works or the ages of religious relics.

By far the most significant application of radiocarbon dating, however, has been in providing archaeologists with the means of constructing chronological relationships independently of traditional archaeological assumptions about cultural processes. The technique is especially valuable in prehistoric archaeology, for which little or no documentation is available and artifact interpretation previously could allow only relative chronological sequences. In the Western Hemisphere, Africa south of the Sahara, and other parts of the world where "prehistory"—in the sense of absence of documentation—extends nearly into the modern era and chronologies had been mere guesswork, carbon-14 dating was a revolutionary technological breakthrough.

The results obtained from applying carbon-14 dating to prehistoric materials, however, made the technique immediately controversial. For example, early dates for the origins of agriculture in the Near East—generally referred to as the Neolithic Revolution—were dramatically earlier than archaeologists had expected. At Jericho, one of the first sites to provide such material, scholars previously had placed the start of the Neolithic Revolution at around 4000 BCE, but carbon-14 dates were on the order of four thousand years earlier. Further research in the Near East has established a chronological frontier for the Neolithic Revolution at 10,000 BCE. or even earlier.

A second major achievement of radiocarbon dating was the establishment of the first outlines of an absolute chronology for developments similar to the Neolithic Revolution in Mexico and Central America. Supported by pollen analysis, carbon-14 dating of the remains of early domesticated maize suggested an astonishingly long continuum for agriculture in the region extending back to 5000 BCE. This finding was the first real indication of the depth and complexity of Mesoamerican cultural heritage.

In sub-Saharan Africa, the immense potential of charcoal for providing carbon-14 dates led to reconstruction of the prehistory of a region which, before 1950, was unknown. Charcoal is especially plentiful at the large numbers of iron-smelting sites in Africa. Slag from the smelting operation, sometimes even iron remains, may be dated using carbon-14; carbon migrates as an impurity into the iron. Before 1950, conventional wisdom attributed the presence of iron-smelting technology in sub-Saharan Africa to cultural borrowing, either from Phoenician traders or from the Egyptians. Carbon-14 dates clearly established a thriving iron technology in West Africa as early as 1000 BCE, several hundred years before the earliest known incidence of smelting in Egypt itself.

Controversial Results

Perhaps the most disturbing revelation of the first decade or so of carbon-14 dating was its support for an extended Neolithic chronology in Europe. Although archaeologists already were debating the merits of a "long" versus "short" European Neolithic, most scholars and cultural opinion favored the latter, since it accommodated the notion of cultural "diffusion" from the ancient Near East, relegating Europe to a state of barbarism until stimulated by classical civilizations. The famous megalithic structure of Stonehenge in southern England, for example, was thought to have been inspired by Mycenaean influence and was therefore dated to around 1400 BCE Yet, carbon-14 results obtained from reindeer bone fragments at the site—the bones probably were digging implements used in construction—suggested dates for vital parts of the Stonehenge complex that were several centuries earlier, making it, in effect, pre-Mycenaean. These findings threatened to sever the "diffusionist" link with the Near East and forced a reassessment of the state of prehistoric European culture.

The most severe test of the radiocarbon technique occurred when it was used to corroborate dates for Egyptian historical artifacts that already had been fairly accurately dated with conventional archaeological methods. Early results were not reassuring. The method repeatedly generated dates for materials from the second millennium BCE that were several centuries too recent. Since these discrepancies were more or less consistent in scope and did not alter the sequences of Egyptian history, they raised serious questions about the integrity of the whole chronological framework of ancient history patiently constructed over decades by archaeological research. Conversely, many traditional scholars, confident of their work and suspicious of a technique derived from nuclear physics, preferred to reject the radiocarbon system out of hand.

By the early 1970s, these difficulties, together with the developing precision of sample handling techniques and instrumentation, had led to the realization that there were variables in the carbon-14 cycle unaccounted for by Libby's earlier formulations—principally the magnetic field fluctuations, which, as noted earlier, result in periodic changes in the carbon-14 formation rate. Once radiocarbon dates could be calibrated on the basis of tree-ring data, they agreed very closely with previously established dates for Egyptian artifacts and therefore confirmed the work of traditional archaeologists. With that, the last serious barrier to acceptance of the technique disappeared. Some of the prehistoric dates for Europe, however, when corrected according to tree-ring data, actually pushed the Neolithic horizon even further into the past. The earliest portions of Stonehenge are now believed to be separated from what once were thought to be their Mycenaean origins by what one archaeologist has called a "yawning millennium."

In the 1980s, investigators from England, Switzerland, and the United States applied radiocarbon dating techniques to fibers from the "Shroud of Turin." Held in reverence by many as the burial shroud of Jesus, this piece of linen retains the image of a bearded man with marks consistent with crucifixion. Measurements by separate laboratories agreed that the flax from which the linen was produced grew sometime in the thirteenth or fourteenth centuries—far too recent to have been the burial shroud of Jesus. Some scholars continue to dispute the findings, however, including criticizing technical aspects of the dating process.

Interface Between Science and Tradition

Radiocarbon dating, together with similar techniques using isotopes of other elements and a variety of methods drawn from the physical and life sciences since 1950, has elaborated a picture of human history and earth history to a degree that could not have been conceived by earlier scholars. Among the method's practical benefits is a much more sophisticated knowledge of the scope of natural climatic change, without which it would not be possible to make useful scientific or political decisions on matters that may affect future, human-induced climatic and environmental change. Radiocarbon dating also has shattered many long-standing notions about European prehistory, Europe's historical relationship with the ancient Near East, and the antiquity and complexity of non-European civilizations, thereby undermining fundamental assumptions about the centrality of Western civilization in the human saga.

The history of the application and results of carbon-14 dating require that one look carefully at the conditions and assumptions associated with certain dates and at the stage in the technique's development from which those dates derive. A prime example of how these matters can fuel controversy was the confusion generated by some carbon-14 dates which seemed to suggest that established dates for Egyptian artifacts of the second millennium BCE. were several centuries too early. Biblical scholars had long worried about an unexplained gap between the dates, derived from scientific genealogical and textual studies, for the Hebrew Exodus from Egypt and the establishment of the ancient kingdom of Israel. In terms of regarding the Old Testament as historically accurate, it would be convenient if several centuries of Egyptian history could be "erased" and earlier events moved up to fill the gap. Ironically, that is just what early carbon-14 dating of Egyptian artifacts suggested. Some biblical scholars were ecstatic that science seemed to verify the accounts of the Bible. However, later calibration of these Egyptian dates using tree-ring data reinstated traditional Egyptian chronology and nullified this temporary congruence. Clearly, the lay reader who encounters results of radiocarbon dating encounters a complex interface between modern science and the most fundamental issues in the Western religious and historical tradition.

Principal Terms

dendrochronology: the study of tree rings; it provides a means of calibrating radiocarbon dates with absolute chronology

half-life: the period required for half of any given quantity of a radioactive element to revert to a stable state

isotopes: forms of the same element with identical numbers of protons but different numbers of neutrons in their atoms' nuclei

Maunder minimum: the period from 1645 to 1715, when sunspot activity was almost nonexistent

photosynthesis: the process of fixing atmospheric carbon in organic compounds in plants with free oxygen as a by-product

radioactive isotope: an unstable isotope that decays into a stable isotope

thermoluminescence: the process by which some minerals trap electrons in their crystal structures at a fixed rate and release them when heated

Bibliography

Agrawal, D. P., and M. G. Yadava. Dating the Human Past. Pune: Indian Society for Prehistoric and Quaternary Studies. 1995. An interesting look at the use of radiocarbon dating technology in relation to anthropology. Suitable for the nonscientist. Illustrations.

Bard, E., F. Rostek, and Guillemette Menot-Combes. "A Better Radiocarbon Clock." Science 303 (2004): 178-179. A strong discussion of the need for a radiocarbon curve that extends beyond that of INTCAL98. This article is written in a manner that is accessible to undergraduates as well as graduate students.

Bard, Edouard, and Wallace S. Broecker, eds. The Last Deglaciation: Absolute and Radiocarbon Chronologies. Berlin: Springer-Verlag, 1992. This book deals with the radiocarbon dating processes and techniques that have been used to unravel the mysteries of glaciers and massive changes in climate. This book provides a simple explanation of radiocarbon dating and illustrates its applications.

Currie, L. A. "The Remarkable Metrological History of Radiocarbon Dating." Journal of Research of the National Institute of Standards and Technology 109 (2004): 185-217. This article accounts the discovery and progression of radiocarbon dating as a tool in geological and archaeological studies. It is extremely detailed and provides an excellent background for anyone using carbon dating techniques.

Fleming, Stuart. Dating in Archaeology: A Guide to Scientific Techniques. New York: St. Martin's Press, 1976. Places the methods and results of radiocarbon dating in the context of other dating techniques, such as dendrochronology, thermoluminescence, fission track dating, pollen analysis, and chemical methods. Discusses how several of these techniques may corroborate one another in establishing chronologies. Excellent bibliography.

Lowe, J. John, ed. Radiocarbon Dating: Recent Applications and Future Potential. Chichester, N.Y.: John Wiley and Sons, 1996. This college-level book offers a comprehensive overview of the techniques and protocols of radiocarbon dating. Several of the essays explore possible future usage and applications. Illustrations and maps help to clarify difficult concepts. Bibliographical references.

Reimer, Paula J., et al. "IntCal104 Terrestrial Radiocarbon Age Calibration, 0-26 cal kyr BP." Radiocarbon. 46 (2004): 1029-1058. This article provides a technical review of the past state of the radiocarbon calibration curve. More recent data are used to update and refine the curve. Written in a highly technical manner for practicing geologists and graduate students.

Renfrew, Colin. Before Civilization: The Radiocarbon Revolution and Prehistoric Europe, 2d ed. London: Penguin Books, 1990. A comprehensive discussion of the development of radiocarbon dating, the early discrepancies, and their correction using tree-ring data. Offers a thorough analysis of some alternative approaches to prehistory based on carbon-14 results from Europe.

Rick, T. C., R. L. Vellanoweth, and J. Erlandson. "Radiocarbon Dating and the ‘Old Shell’ Problem: Direct Dating of Artifacts and Cultural Chronologies in Coastal and Other Aquatic Regions." Journal of Archaeological Sciences 32 (2005): 1641-1648. An interesting and easy-to-follow discussion of a common problem in sample selection for carbon dating. This article addresses the problem of "old wood" and compares it to the sampling of shells at archaeological sites. Provides the basic principles of radiocarbon dating in a manner accessible to the layperson with some science background.

Scott, E. M., M. S. Baxter, and T. C. Aitchison. "A Comparison of the Treatment of Errors in Radiocarbon Dating Calibration Methods." Journal of Archaeological Science 11 (1984): 455-466. Discusses several methods of calibration with respect to the varying degrees of accuracy required in specific applications.

Taylor, R. E. "Fifty Years of Radiocarbon Dating." American Scientist 88 (January/February 2000): 60-67. This paper reviews the entire five decades of development of this remarkable technique. Taylor reviews the basic technique, deviation of carbon-14 dates from true dates due to variation in the magnetic field, means of correction, and attaining increased sensitivity by the application of mass spectrometry.

Taylor, R. E. Radiocarbon Dating: An Archaeological Perspective. New York: Academic Press, 1987. An excellent treatment of the procedures and complexities involved in measuring radiocarbon content. Discusses instrumentation, sources of contamination, case studies using various substances, and the historical development of radiocarbon methodology. The bibliography covers a broad range of sources.

Vita-Finzi, Claudio. Recent Earth History. New York: Halsted Press, 1974. An account of the physical changes undergone by the earth during the Holocene (modern) geologic era, presented in the form of a stratigraphical narrative based throughout on radiocarbon dates.

Wagner, Gunther A., and S. Schiegl. Age Determination of Young Rocks and Artifacts: Physical and Chemical Clocks in Quaternary Geology and Archaeology. New York: Springer, 2010. The authors cover various materials and dating methods. Well organized, accessible to advanced undergraduates and graduate students.

Walker, Mike. Quaternary Dating Methods. New York: Wiley, 2005. This text provides a detailed description of current dating methods, followed by content on the instrumentation, limitations, and applications of geological dating. Written for readers with some science background, but clear enough for those with no prior knowledge of dating methods.

Walther, John Victor. Essentials of Geochemistry, 2d ed. Jones & Bartlett Publishers, 2008. Contains chapters on radioisotope and stable isotope dating and radioactive decay. Geared more toward geology and geophysics than toward chemistry, this text provides content on thermodynamics, soil formation, and chemical kinetics.

"Willard Libby and Radiocarbon Dating." American Chemistry Society, 2016, www.acs.org/content/acs/en/education/whatischemistry/landmarks/radiocarbon-dating.html. Accessed 31 Aug. 2017.

Wilson, David. The New Archaeology. New York: New American Library, 1974. Perhaps the best account for the general reader of the development and subsequent refinement of radiocarbon dating techniques. Discusses their enormous impact on the field of prehistoric archaeology and perceptions of prehistory.