Before the 20th cent., archaeologists and geologists were largely limited to the use of relative dating techniques. Estimates of the absolute age of prehistoric and geological events and remains amounted to little more than inspired guesswork, as there was no scientific basis for testing such proposals. However, as the basic principles of relative dating progressed during the course of the 19th cent., investigators were able to correctly determine the relative age of many archaeological and geological materials.
Stratigraphic dating is accomplished by interpreting the significance of geological or archaeological strata, or layers. The method begins with the careful drawing and description of strata (the geological or archaeological profile). The profile from one location is then compared with profiles from surrounding sites. Stratigraphic dating assumes that the lower layers in any particular profile are older than the upper layers in that profile ("the law of superposition") and that an object cannot be older than the materials of which it is composed. Igneous masses are dated according to whether they caused metamorphism in the surrounding rock (proof of emplacement in preexisting rock) or whether sediments were deposited on them after they were formed. In geology, a master stratigraphic sequence for a particular region is built up by correlating the strata from different locations with one another. As new locations are investigated, the geologist attempts to fit the new profiles into the master sequence of geological strata for that region. The depth of the strata within the master sequence provides the investigator with the relative date of any particular profile.
Seriation is an archaeological technique involving the description of stylistic changes in artifacts and of changes in the popularity of distinct styles in order to accurately describe the sequence of variation over time. The seriation of stratified deposits permits archaeologists to assess the relative age of particular styles. This information may then be used to surmise the relative age of unstratified deposits (e.g., surface sites).
Technological changes can be used for relative dating of archaeological material. The three-age system devised by the Danish archaeologist Christian Thomsen in the 1830s made use of technological criteria. According to this system, humans passed through three distinct stages of technological development, based on the primary material used to manufacture tools and weapons: the Stone Age, the Bronze Age, and the Iron Age.
Biological criteria can also serve as a means for relative dating. Fossils are useful because certain assemblages of species are characteristic of specific geological eras. Pollen analysis, or palynology, involves the microscopic examination of fossil pollen grains in stratified peat and lake deposits. From this, scientists can establish pollen diagrams (describing the relative abundance of different pollen-producing plants at a given point in time) and floral time charts (showing how climate and flora changed over time). The principle of stratigraphic dating is used to establish the relative age of these floral and fossil assemblages. Through the investigation of many different stratigraphic contexts, a master sequence of fossil and floral assemblages may be devised for a region.
Absolute dating can be achieved through the use of historical records and through the analysis of biological and geological patterns resulting from annual climatic variations, such as tree rings (dendrochronology) and varve analysis. After 1950, the physical sciences contributed a number of absolute dating techniques that had a revolutionary effect on archaeology and geology. These techniques are based upon the measurement of radioactive processes (radiocarbon; potassium-argon, uranium-lead, thorium-lead, etc.; fission track; thermoluminescence; optically stimulated luminescence; and electron-spin resonance), chemical processes (amino-acid racemization and obsidian hydration), and the magnetic properties of igneous material, baked clay, and sedimentary deposits (paleomagnetism). Other techniques are occasionally useful, for example, historical or iconographic references to datable astronomical events such as solar eclipses (archaeoastronomy).
When archaeologists have access to the historical records of civilizations that had calendars and counted and recorded the passage of years, the actual age of the archaeological material may be ascertained—provided there is some basis for correlating our modern calendar with the ancient calendar. With the decipherment of the Egyptian hieroglyphics, Egyptologists had access to such an absolute timescale, and the age, in calender years, of the Egyptian dynasties could be established. Furthermore, Egyptian trade wares were used as a basis for establishing the age of the relative chronologies developed for adjoining regions, such as Palestine and Greece. Thus, Sir Arthur Evans was able to establish an accurate absolute chronology for the ancient civilizations of Crete and Greece through the use of Egyptian trade objects that appeared in his excavations—a technique known as cross-dating.
In dendrochronology, the age of wood can be determined through the counting of the number of annual rings in its cross section. Tree ring growth reflects the rainfall conditions that prevailed during the years of the tree's life. Because rainfall patterns vary annually, any given set of tree ring patterns in a region will form a relatively distinct pattern, identifiable with a particular set of years. By comparing the pattern of tree rings in trees whose lifespans partially overlap, these patterns can be extended back in time. By matching the tree rings on an archaeological sample to the master sequence of tree ring patterns, the absolute age of a sample is established. The best known dendrochronological sequences are those of the American Southwest, where wood is preserved by aridity, and Central Europe, where wood is often preserved by waterlogging.
The varved-clay method is applied with fair accuracy on deposits up to 12,000 years old. Streams flowing into still bodies commonly deposit layers (varves) of summer silt and winter clay through the year. Those laid down during the fall and winter have a dark color because of the presence of dead vegetation; those deposited during the rest of the year have a light color. The stratigraphy may also reflect seasonal variation in the velocity of stream flow. By counting each pair of varves the age of the deposit can be determined.
The absolute dating methods most widely used and accepted are based on the natural radioactivity of certain minerals found in rocks. Since the rate of radioactive decay of any particular isotope is known, the age of a specimen can be computed from the relative proportions of the remaining radioactive material and its decay products. By this method the age of the earth is estimated to be about 4.5 billion years old. Some of the radioactive elements used in dating and their decay products (their stable daughter isotopes) are uranium-238 to lead-206, uranium-235 to lead-207, thorium-232 to lead-208, samarium-147 to neodymium-143, rubidium-87 to strontium-87, and potassium-40 to argon-40. Each radioactive member of these series has a known, constant decay rate, measured by its half-life, that is unaffected by any physical or chemical changes. Each decay element has an effective age range, including uranium-238 (100 million to 4.5 billion years) and potassium-40 (100,000 to 4.5 billion years).
Other methods that depend on the effects of radioactive decay include fission track dating and thermoluminescence. Fission track dating is based on the fact that when uranium-238 atoms fission within a solid medium such as a mineral or a glass, they expel charged particles that leave a trail of damage (known as fission tracks) preserved in the medium. The number of tracks per unit area is a function of time and the uranium concentration. Thus it is possible to measure the time that has elapsed since the material solidified. Thermoluminescence, used in dating archaeological material such as pottery, is based on the luminescence produced when a solid is heated; that is, electrons freed during radioactive decay and trapped in the crystal lattice are released by heating, resulting in luminescence. When light is used rather than heat to free the accumulated electrons, the technique is known as optically stimulated resonance. Yet another technique measures the quantity of trapped electrons by detecting the amount of microwave radiation they absorb (electron-spin resonance); it has the advantage that it can be utilized several times on a given sample. All of these techniques have proven somewhat unreliable. Museums sometimes use them to determine if a ceramic is an antique or a modern forgery.
The radioactive carbon-14 method of dating is used to determine the age of organic matter that is several hundred years to approximately 50,000 years old. Carbon dating is possible because all organic matter, including bones and other hard parts, contains carbon and thus contains a scalable proportion of carbon-14 to its decay product, nitrogen-14. The carbon-14, along with nonradioactive carbon-13 and carbon-12, is converted to carbon dioxide and assimilated by plants and organisms; when the plant or animal dies, it no longer acquires carbon, and the carbon-14 begins to decay. The conventional method of measuring the amount of radioactive carbon-14 in a sample involved the detection of individual carbon-14 decay events. In the 1980s a new procedure became available. This technique involves the direct counting of carbon-14 atoms through the use of the accelerator mass spectrometer and has the advantage of being able to use sample sizes up to 1,000 times smaller than those used by conventional radiocarbon dating. The accelerator mass spectrometer technique reduces the amount of statistical error involved in the process of counting carbon-14 ions and therefore produces dates that have smaller standard errors than the conventional method.
Paleomagnetic dating is based on changes in the orientation and intensity of the earth's magnetic field that have occurred over time. The magnetic characteristics of the object or area (e.g., a section of the seafloor) in question are matched to a date range in which the characteristics of the earth's magnetism were similar. Paleomagnetic dating is also based on the fact that the earth periodically reverses the polarity of its magnetism. Different igneous and sedimentary rocks are rich in magnetic particles and provide a record of the polarity of the earth when they were formed. These patterns will be reflected in various geological contexts, such as stratigraphic sequences. Scientists date these changes in polarity through another technique, such as potassium-argon radioactive dating. This has resulted in the calibration of the pattern of changes in the earth's polarity over many millions of years. Scientists can date a new profile by measuring for changes in polarity within the strata and then matching the sequence to the calibrated master stratigraphic sequence of geomagnetic polarity reversals. In archaeomagnetic dating, oriented specimens are recovered from baked immobile archaeological features, such as the soil surrounding a hearth, in order to determine the direction of geomagnetic field at the time they were formed. This procedure results in the plotting of a polar curve, which documents changes in the direction of the magnetic poles for a given region. The polar curve itself does not provide an absolute date but must be calibrated by an independent technique, such as radiocarbon dating.
Chemical dating methods are based on predictable chemical changes that occur over time. Examples include amino-acid racemization, which is potentially useful in situations where no other technique is available to date an archaeological site, and obsidian hydration. The latter is applicable in areas such as Mesoamerica, where obsidian is abundant. Many investigators, however, consider it unreliable.
Fluorine dating is useful to scientists dating early hominid remains. Buried bones take up fluorine from surrounding soils. The amount of fluorine taken up is proportional to the amount in the surrounding deposit and the length of time the bone has been buried. Varying concentrations of fluorine in different deposits preclude the method from being considered absolute, but it can be used to measure the relative ages of bones found in the same deposit.
See E. F. Zeuner, Dating the Past (4th ed. 1970); R. H. Dott and R. L. Batten, Evolution of the Earth (1988); M. J. Aitken, Science-based Dating in Archaeology (1990); W. B. Harland et al., A Geologic Time Scale 1989 (1990).
Method of dating very old rocks by means of the amount of common lead they contain. Common lead is any lead from a rock or mineral that contains a large amount of lead and a small amount of the radioactive precursors of lead (i.e., the isotopes uranium-235, uranium-238, and thorium-232). By this method, the age of the Earth has been estimated to be circa 4.6 billion years. This figure is in good agreement with the age of meteorites and the age of the Moon as determined independently.
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Method of age determination that makes use of the radioactive decay of uranium-238 to uranium-234; the method can be used for dating sediments from either a marine or a playa lake environment. Because this method is useful for the period circa 100,000–1.2 million years before the present, it helps in bridging the gap between the carbon-14 dating method and the potassium-argon dating method.
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Method of estimating the age of rocks, minerals, and meteorites from measurements of the amount of the stable isotope strontium-87 formed by the decay of the unstable isotope rubidium-87 that was present in the rock at the time of its formation. The method is applicable to very old rocks because the transformation is extremely slow: the half-life, or time required for half the initial quantity of rubidium-87 to disappear, is approximately 48.8 billion years. Seealso dating.
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Method for determining the age of igneous rocks based on the amount of argon-40 in the rock. Radioactive potassium-40 decays to argon-40 with a half-life of about 1.3 billion years, making this method useful for dating rocks that are billions of years old. A more sophisticated method, called argon-argon dating, provides a more accurate estimate of the original potassium-40 content by means of the ratio of argon-40 to argon-39 in the rock, thus yielding a more accurate age determination. Seealso dating.
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Method of dating that depends on the production of helium during the decay of radioactive isotopes of uranium and thorium. Because of this decay, the helium content of any mineral or rock capable of retaining helium will increase during the lifetime of that mineral or rock, and the ratio of helium to its radioactive progenitors then becomes a measure of geologic time. Fossils may also be dated by helium dating. The relatively large amount of helium produced in rocks may make it possible to extend helium dating to rocks and minerals as young as a few tens of thousands of years old.
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Method of determining the age of a mineral that utilizes the damage done by the spontaneous fission of uranium-238, the most abundant isotope of uranium. The fission results in radiation damage, or fission tracks, that can be made visible by preferential leaching (removal of material by solution) of the host substance with a suitable chemical reagent; the leaching process allows the etched fission-track pits to be viewed and counted under a microscope. The amount of uranium present can be determined by irradiation that produces thermal fission of uranium-235, which produces another population of tracks, related to the uranium concentration of the mineral. Thus, the ratio of naturally produced, spontaneous fission tracks to induced fission tracks is a measure of the age of the sample. See also dating.
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In geology and archaeology, the process of determining an object's or event's place within a chronological scheme. Scientists may use either relative dating, in which items are sequenced on the basis of stratigraphic clues (see stratigraphy) or a presumed evolution in form or structure, or absolute dating, in which items are assigned a date independent of context. The latter type includes potassium-argon and carbon-14 dating; both are based on the measurement of radioactive decay. The record of changes in polarity of the Earth's magnetic field has provided a timescale for seafloor spreading and long-term marine sedimentation. Dendrochronology has proved useful in archaeology and climatology. Seealso fission-track dating; helium dating; lead-210 dating; rubidium-strontium dating; uranium-234–uranium-238 dating; uranium-thorium-lead dating.
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Method of determining the age of once-living material, developed by U.S. physicist Willard Libby in 1947. It depends on the decay of the radioactive isotope carbon-14 (radiocarbon) to nitrogen. All living plants and animals continually take in carbon: green plants absorb it in the form of carbon dioxide from the atmosphere, and it is passed to animals through the food chain. Some of this carbon is radioactive carbon-14, which slowly decays to the stable isotope nitrogen-14. When an organism dies it stops taking in carbon, so the amount of carbon-14 in its tissues steadily decreases. Because carbon-14 decays at a constant rate, the time since an organism died can be estimated by measuring the amount of radiocarbon in its remains. The method is a useful technique for dating fossils and archaeological specimens from 500 to 50,000 years old and is widely used by geologists, anthropologists, and archaeologists.
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