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Techniques for determining the ages of objects at archaeological sites are critical to understanding the evolution of human societies. Archaeologists use relative dating methods (such as a fossil’s position within layers of soil), to determine the sequence of events and absolute dating methods (such as the radioactive decay of a carbon isotope), to determine the actual age of an artifact.

Archaeologists employ a broad array of techniques collectively called dating methods to determine when “things happened” in history. These methods are applied directly to objects in order to determine their actual age or to situate them accurately in a chronological sequence. The dates on individual items may then be associated with and applied to the archaeological sites from which they were recovered. By a further extrapolation, the dates of these sites are then used to answer general questions concerning the timing and tempo of significant events or developments in the history of humanity: the evolution of upright walking, the earliest tool making, the controlled use of fire, humanity’s geographic expansion beyond its African evolutionary nursery, the origins of agriculture, the development of metallurgy, the invention of writing, the appearance of urban civilizations. Our understanding of the origins of these and myriad other important steps in the evolution of human societies often is based, ultimately, on our ability to accurately and precisely date objects associated with these developments.

Dating methods employed in archaeological analysis can be divided into two general categories: relative methods and absolute (sometimes called chronometric) procedures. Relative dating involves placing objects, sites, or events in a chronological sequence, without reference to an actual date, year, or even to a broad range of years. Absolute dating techniques produce actual dates or age ranges.

Relative Dating Methods

Among the relative dating procedures employed by archaeologists, the ones most commonly used involve sequencing objects, occupations, and sites on the basis of the soil levels in which cultural remains have been found.

As long ago as the late eighteenth century, the British geologist William Smith recognized that Earth’s history is written as a metaphorical book whose pages are represented as a series of soil layers, one laid down upon another in a sequence across a vast expanse of time. Smith’s analysis of these layers— the stratigraphy of buried geological deposits— provided scientists with a sequential natural history in which each layer contained fossil evidence of the types of life forms that dominated during the period in which the layer was deposited.

Archaeologists soon realized that stratigraphic analysis could similarly be applied to human history. Human-made objects and the remains of other materials whose context in the soil was the result of human activity were, like fossils of plants and animals, deposited in soil layers being laid down during the period in which those objects were lost, discarded, or deposited for safekeeping. Archaeologists recognized that cultural materials, like fossils, could themselves be sequenced on the basis of their relative position in a succession of strata.

Further, because human activity itself can produce layers of deposit, cultural stratigraphy can be read to determine a relative chronological sequence of occupations of the same spot. For example, so-called tells in the Middle East are artificial hills produced by the frequent reoccupation by human beings of the same locations. Each occupation results in the deposition of materials lost and discarded and, in so doing, contributes to the formation of the mound, with the materials left behind by each subsequent occupation located on top of previous deposits. For example, the tell of Hazor (Tel Hazor) in Israel is an artificial hill produced by the remnants of some twenty-one separate occupations of the same location, with the remains of each successive occupation overlying each previous one.

Absolute Dating Methods

In most cases, the application of a relative dating procedure like stratigraphic analysis represents a preliminary step in archaeological dating. The hope always is to go beyond merely sequencing and to determine a site’s age in a chronometric sense. One of the most important chronometric procedures archaeologists employ is radiocarbon dating (also called carbon dating or C-14 dating), one of a series of so-called radiometric techniques. Radiometric dating methods, including carbon dating, potassium-argon dating, argon–argon dating, and uranium-series dating all rely on the existence of what amounts to natural clocks or calendars in archaeological and geological specimens. These natural calendars are produced by the fixed and measurable pace of the decay of radioactive (unstable) isotopes (varieties) of elements that occurs naturally in raw materials used by ancient human beings.

Radiocarbon Dating

In 1946, the University of Chicago chemist Willard Libby predicted the existence of carbon 14 in living matter, and by 1949 he had measured contemporary levels of this unstable variety of carbon and assessed its half-life—in essence he began the process of calibrating this natural clock. Soon thereafter, he began to apply the method to dating archaeological specimens. With radiocarbon dating, the known rate of decay of a radioactive variety of carbon, called C-14 (carbon 14) for the fourteen particles (six protons and eight neutrons) in its nucleus (as compared to the twelve particles—six protons and six neutrons— present in the stable and abundant variety of carbon), provides the archaeologist with a natural calendar.

Just like carbon 12, carbon 14 combines with oxygen in the atmosphere to produce carbon dioxide. Plants respire—in essence, they breathe—and, through photosynthesis, break apart the bonds between the carbon and oxygen atoms in carbon dioxide. Plants exhale the oxygen and retain the carbon, from which they manufacture their leaves, stems, roots, fruits, bark, and so forth. Carbon 14 is incorporated into plant materials in the same proportion as it appears in the atmosphere. When animals ingest plant materials, their bodies use the ingested carbon in the plant to produce bone, muscle, sinew, and so on, and the carbon 14 provided by the plant is incorporated into the animal’s body in the same proportion as was present in the plant. When a carnivore eats a plant-eating animal, it incorporates carbon 14 into its body in the same proportion as was present in its prey. In this way, all livings things, as long as they are alive, breathing, and eating, are in radiocarbon equilibrium with the atmosphere.

Because carbon 14 is unstable, it constantly undergoes radioactive decay at a steady and measurable rate. Radioactive decay rates are expressed as half-lives, the length of time it takes for half of the radioactive material present in a substance to decay to a stable form. The half-life of carbon 14 has been measured at 5,730 years.

As long as an organism is alive, the carbon 14 that is slowly disappearing through that process of decay is immediately replenished (by plants as they breath and by animals as they eat). Death breaks that cycle. After death, the decaying carbon 14 is no longer replenished, and its loss becomes calculable. It is the constant, steady, and measurable decay of carbon 14 that provides a natural chronometer in substances containing carbon—essentially, anything that was once alive.

Scientists can precisely calculate the amount of carbon present in a specimen about to be dated; they also know how much carbon 14 would be present in the material if it were entirely modern—in other words, if it were alive today. If the amount actually present in the object is less than would be expected in a modern material, the amount of time it must have taken for the proportion of C-14 to decline to its current measure can be calculated, providing an age for the material.

Radiocarbon dating, by its nature, can be applied only to organic materials, such as bone, wood, charcoal, seeds, fruit pits, roots, hair, and hide. The half-life of radiocarbon also places restrictions on the application of carbon dating. Any specimen less than a few hundred years old usually is too young to date, as not enough of the radioactive isotope has decayed for an accurate age determination. In its current application, any specimen much older than about seven half-lives (approximately forty thousand years) has so little of its original carbon 14 remaining that dating accuracy diminishes dramatically, and the technique becomes unreliable. Ways of extending the outer boundary of radiocarbon dating to fifty- or even sixty thousand years are currently being developed.

Finally, it should be added that the accuracy of carbon dating depends on there being a constant amount of carbon 14 in Earth’s atmosphere through time. This turns out not to be precisely the case. Adjusting for this depends on another dating procedure— dendrochronology—which permits carbon dates to be calibrated and better reflect the actual age of an object.

Other methods based on radioactive decay are employed by archaeologists. Perhaps the most important is the potassium-argon technique (a more accurate version of the method is called the argon-argon method) in which argon-gas buildup is measured in volcanic rock (the gas results from the decay of radioactive potassium). The argon gas accumulates at a known rate, and therefore the age of the volcanic rock (meaning when the molten material solidified) can be determined. This technique is applicable to material millions and even billions of years old and so is most useful in dating the very ancient sites of our human ancestors. In one of its most interesting applications, the volcanic ash in which ancient human ancestors left their very modern-looking footprints at Laetoli, in Kenya, has been directly dated to about 3.5 million years ago.

Radiation Damage Techniques

Another class of radiometric techniques relies on the statistically predictable nature of radioactive decay in an indirect way. These procedures fall into a subcategory of radiometric dating called radiation damage techniques. These techniques, which first were used in the 1950s, take advantage of the fact that radioactive decay may leave measurable damage on archaeological specimens, damage that accumulates regularly through time. The amount of damage, therefore, is proportional to the amount of time that has passed. For example, the fired clay in a ceramic object deposited in the ground is buffeted by radioactive decay that originates in the surrounding soil. The energy released through that decay is captured by the ceramic object, trapped in its atomic lattice in an amount proportional to the amount of time the object has been in the soil. This trapped energy can be released by heat in a procedure called thermoluminescence, or by light in the application called optically stimulated luminescence; the amount of energy released can then be measured. Once the rate at which the radioactive material in the soil is releasing energy—the so-called background radiation—is calibrated, the rate at which energy is trapped by a ceramic buried in that soil can be calculated, and, by inference, the age of the artifact can be determined. Dating techniques based on the rate of energy capture in archaeological materials are applicable to raw materials that possess a structure that, in fact, acts to trap such energy. These materials include ceramics, as well as some minerals, coral, teeth, and shell. Luminescence dating had a wide range of applicability; it has been used successfully to date ceramic objects just a few hundred years old, calcite deposits more than 250,000 years old, and heated flints up to 500,000 years old.

Electron spin resonance, which measures the amount of energy trapped in shells, corals, volcanic rock, and tooth enamel, is another radiation damage technique. Here again, the amount of trapped energy present in an archaeological specimen is a function of time; once the amount of trapped energy is measured and the background radiation calculated, the amount of time since the rock, shell, coral, or tooth formed can be computed. Electron spin resonance has been applied successfully to objects as young as one thousand years and as old as one million years of age.

Fission track dating, a third radiation damage technique, makes use of the fact that radioactive decay may leave discernible, microscopic tracks in a material. These tracks may accumulate at a regular rate through time. When this rate can be estimated and the number of tracks counted, an estimate of the age of an object can be calculated. Fission track dating has been used to date materials as young as a few thousand years old; from a human standpoint, there is no effective upper limit to the applicability of this procedure.

Dendrochronology

Biology also supplies the archaeologist with an important natural calendar in the form of the annual growth rings of trees. It is commonly the case that trees add a single growth ring for each year that they are alive. In many cases, the thickness of a growth ring is a factor of a measurable aspect of the environment during the year the ring was added; for example, mean temperature or amount of precipitation. This correlation between tree ring width and yearly climate fluctuations actually was recognized in the fifteenth century by Leonardo da Vinci. In the first three decades of the twentieth century, the University of Arizona astronomer A. E. Douglass (1867–1962), attempting to assess a possible correlation between sunspot activity and climate change, examined fluctuating tree ring width and is credited with recognizing the potential of tree ring analysis in archaeological dating. In the American Southwest, where tree ring dating, or dendrochronology, has been particularly useful in archaeological application, the width of an annual tree ring is proportional to the amount of rainfall that fell in the year the ring grew; thick rings are added in years when precipitation is abundant, rings of intermediate width are added when rainfall amounts are intermediate, and thin rings develop during years when rainfall amounts are meager.

This yearly variation in tree ring width allows researchers to extend a tree ring sequence back in time. Dendrochronologists accomplish this by developing what is called a master sequence for a region by beginning with a living tree. That tree, like all trees, exhibits a nonrepeating, more or less random succession of thick, medium, and thin rings reflecting the nonrepeating pattern of high, medium, and low yearly rainfall amounts during its lifetime. This tree’s sequence of ring widths will be a close match to that exhibited by all the other trees of the same species growing in the region since they were all subjected to the same rainfall amounts as these varied from year to year. The living tree anchors the master sequence in time, as the dendrochronologist knows the precise year in which each of its rings were created simply by counting backward from the outermost, most recently grown ring, which represents the current year.

Next the sequences of tree rings exhibited by a number of dead trees are compared with the rings of the living tree in the search for a substantial, matching series of tree ring widths. If the lives of the living and any of the dead trees overlapped in time, their ring width sequences of thick and thin rings will match for that period of overlap. For example, if the succession of varying widths of the innermost ten rings of the living tree match in size and order the outermost ten ring widths of a dead tree, in all likelihood those two trees were both alive during that same ten-year period. Because we know the precise year each of the living tree’s rings correspond to, we can now fix the dead tree in time; its final ring was added in the tenth year of the living tree’s existence, and we know what year that was. By repeating the same procedure numerous times with successively older trees overlapping in time, dendrochronologists have developed master tree ring width sequences that reach back more than twelve thousand years. The discovery of well-preserved tree ring sequences in ancient Kauri trees excavated in New Zealand may enable the extension of a master sequence for the region where the tree remains were found back to nearly sixty thousand years.

In practice, when an archaeological specimen of wood is found—for example, a log beam in an ancient pueblo or a fragment of log used in a prehistoric palisade—its ring width sequence is examined by computer, and its fit along the master sequence is determined. In that way, the actual year in which the archaeological specimen died or was cut down can be established. If we assume that the log was used soon after it died or was cut, that year can be associated with the archaeological site in which it was found. Different master sequences have been worked out and are constantly being expanded and refined for various reasons all over the world.

Calibrating Radiocarbon Dating

As mentioned previously, the accuracy of radiocarbon dates depends on a constant proportion of radioactive carbon in the Earth’s atmosphere, and this, unfortunately, is not always the case. Tree ring dating, along with a number of other methods, can help control for changes in the atmospheric radiocarbon content. Once laid down in a tree, no additional carbon is incorporated into a ring. Therefore, a radiocarbon date determined for a particular tree ring represents the “radiocarbon year” for that ring’s “calendar year” as represented on the dendrochonological master sequence. An enormous number of individual tree rings have been radiocarbon dated, and a calibration curve has been developed. Combining dendrochronology with other methods, notably varve analysis (varves are individual strata deposited in lakes on a yearly basis, one varve for each year), the radiocarbon calibration curve now reaches back approximately twenty-six thousand years. This means that within that period, a given radiocarbon date can be accurately adjusted to a more realistic actual or calendar age by reference to that calibration curve.

Dating with Style

New technologies are constantly replacing old ones, and styles come into fashion and then fall out of favor. Most of us are familiar with the progression of technology and the vicissitudes of fashion in our own culture. Most would have no trouble guessing the approximate date of an automobile with lots of chrome and sharp fins (styles popular in the 1950s) or a photograph in a college catalogue showing male students with long hair and faded, torn jeans (the late 1960s and early 1970s). Even a casual examination of the history of the now nearly ubiquitous iPod shows a series of technological and stylistic developments since the MP3 player’s introduction in 2001. If you know when Apple introduced subsequent iPods with new technologies (for example, when they replaced the scroll wheel with the touch-sensitive wheel or when they introduced color displays), you can tell the manufacture date of an iPod just by looking at it.

Archaeologists may accurately determine an ancient artifact’s age in much the same way. If a unique style of making spear tips or pottery has previously and consistently been dated to a particular point in time, then when another spear tip or potsherd is found that matches the known object’s form and style, archaeologists propose that the newly found artifact dates to a similar period. For example, when confronted with a long, narrow spear point with a shallow channel running up less than half the length of each face of the point, a New World archaeologist will confidently conclude that the spear point in question is between 10,000 and 11,500 years of age. This confidence results from the fact that whenever points looking like that have been found previously, they have been associated with dates in that range (for example, by being found embedded in an animal bone that produced a radiocarbon date that matches that time period).

Other Methods of Dating

There are other dating techniques used by archaeologists. Archaeomagnetic dating is an absolute technique that has been used especially in the American Southwest for more than thirty years. Magnetic particles in, for example, the bricks in an ancient kiln will point to what was magnetic north at the time the kiln was in use. Noting where they point and knowing when that location corresponded to magnetic north allows archaeologists to date the kiln. The Colorado State University scientist Jeff Eighmy and his colleagues have worked out a master curve of the locations of magnetic north that extends from the present back to approximately 600 CE. Seriation, a relative technique developed in the late nineteenth century, is based on a typical pattern of change in the popularity of artifact styles from their introduction into a culture, through their growth in popularity, to their decline and eventual replacement by new styles.

Whatever the procedure used, accurate sequencing and dating allows the archaeologist and historian to construct a temporal framework from which the trajectory of human cultural development can be calculated. Indeed, being able to answer the question “when?” provides researchers with a context within which the others whys and wherefores of human history can be answered.

Biliography:

Bowman, S. (1990). Radiocarbon dating. Berkeley and Los Angeles: University of California Press.
Feder, K. L. (2008). Linking to the past. New York: Oxford University Press.
Harris, E. C. (Ed.). (1997). Principles of archaeological stratigraphy. San Diego, CA: Academic Press.
Michels, J. W. (1973). Dating methods in archaeology. New York: Seminar Press.
Schweingruber, F. H. (1988). Tree rings: Basics and applications of dendrochronology. Boston: D. Reidel Publishing.

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