dating: Absolute Dating

Absolute Dating

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. Since 1950 the physical sciences contributed a number of absolute dating techniques that have had a revolutionary effect on archaeology and geology. These techniques are based upon the measurement of radioactive processes (radiocarbon; potassium-argon, uranium-lead, uranium-thorium, 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, uranium-234 to thorium-230, 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 proved 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 hominin 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.

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