by Michael G. Lamoureux, March/April 2009
Today's archaeologist has a wide variety of natural, electro-magnetic, chemical, and radio-metric dating methodologies available to her that can be used to accurately date objects that are just a few hundred years old as well as objects that are a few million years old with high accuracy in the right circumstances. Furthermore, when you consider that many archaeological sites will contain numerous types of artifacts that permit the use of multiple dating methodologies, a modern archaeologist can often employ cross-dating methodologies which can allow for extremely accurate dating as far back as 10,000 years in some regions.
Natural Dating Techniques
A modern archaeologist has almost half a dozen natural dating techniques that she can apply in the field that she can use to quickly determine an approximate date range, which, in the cases of varve analysis and dendrochronology, can often be used to decrease the date range estimate to a matter of just a few years.
One of the oldest natural dating techniques is geochronology, which is based on the principle of superposition -- an object, or layer, on top must have been placed there at a later point in time. Once a geologist has determined the absolute age of a geological formation, the archaeologist can assign an indirect date to objects found in the formation. In archaeology, geochronology lays the foundations for the dating technique better known as stratigraphy that assesses the age of archaeological materials by their association with geological deposits or formations. For example, the successive formation of post-Pleistocene shorelines at Cape Krusenstern Alaska provided J Louis Giddings with a means of ordering sites chronologically.
A prime example of stratigraphy is varve analysis. A varve is a sedimentary bed, or a sequence of such beds, that are deposited in a body of still water in a year. By dividing the rate of sedimentation in terms of units per year by the number of units deposited following a geologic event, an archaeologist or geologist can roughly establish the age of an event in years. The counting and correlation of varves has been used to measure the age of Pleistocene glacial deposits by way of the strata annually deposited in lakes by retreating glaciers. The upper limit of varve dating is dependent upon the region. A sequence of 17,000 years has been established in Scandinavia and a sequence of 20,000 years has been established in the United States in the state of Alaska.
Another example of stratigraphy is biostratigraphy. Chronological information may be conveyed by the presence, absence and form of the bones from one or more animal groups, which were known to have fixed periods of existence, found in a strata at an archaeological site. This technique is central to palaeoanthropology and the development of voles was crucial to the dating of the English Lower Paleolithic site of Boxgrove.
Stratigraphy is not an absolute dating technique as the best it can do is allow for the generation of terminus post quem (TPQ) dates, that provide the earliest possible date of a deposit, and termins ante quem (TAQ) dates, that provide the latest possible dates for a deposit, but still a very useful one as it provides a good reference check against other dating techniques.
A dating technique closely related to stratigraphy is palynology, the science of pollen analysis. If the history of plant life and the relative distribution is known in a region, palynology can be used to provide a reasonably accurate date range based on the plant life, and the average relative distribution thereof, represented in a set of samples
A more exact dating technique using natural formations is that of dendrochronology, which was first used in the 1930s , and which is based on the number, width, and density of the annual growth rings of certain types of long-lived trees. Dendrochronology has been used with great success in Alaska, the South-West US, Northern Mexico, Germany, Greece, Great Britain, Ireland, Norway, and Switzerland. Dendrochronology has produced master tree-ring indices off of the Douglas Fir and Bristlecone Pine in the south-west US that allows for the accurate dating of events and climatic conditions of the past 4000 years. In Germany, a master tree-ring index has been constructed that dates back 6000 years, and in Ireland an index has been constructed that dates back over 7200 years.
The final "natural" dating technique we will discuss is that of sequence dating which makes use of seriation techniques. Based on the observation that patterns of human behavior continually change, sequence dating is based on the principle that as human behavior changes, so does the material products it produces. This allows an archaeologist, who is able to identify the attributes of a class of artifacts that are the most sensitive to change, to construct a sequence of those artifacts that accurately reflects the passage of time. The technique was first applied successfully by Flinders Petrie who used it on pottery to date tombs at the huge prehistoric cemetery at Diospolis Parva, Egypt in 1902.
Seriation dating can also be frequency-based. Based on the assumption that the frequency of an artifact type typically follows a predictable measure in the form of a "battleship curve" from the time of its origin to the time of its disuse, it allows a sequence of archaeological sites with a number of examples of a given object type to be accurately ordered based on the frequency of an artifact type. The most famous example of frequency-based seriation dating is that of James Deetz and Edwin N. Dethlefsen who applied the methodology to tombstones from 18th and 19th century New England and demonstrated that the popularity of the decorative motifs on the headstones did follow a battleship-shaped distribution over time.
Electromagnetic Dating Techniques
Probably the most well-known electromagnetic dating technique is that of archaeomagnetism. Archaeomagnetism, which uses the fact that the earth's magnetic field varies through time and shifts in the horizontal plane (declination angle) as well as the vertical plane (dip angle), allows materials that contain a sufficient amount of iron content to be dated wherever accurate compass readings are available far enough back in time as iron particles trapped in a matrix, which align to magnetic north, will have their orientation fixed when the matrix is heated above its curie point. In some areas, archaeomagnetic alignments have been calibrated to 5,000 years in the past.
Another electromagnetic dating technique is based on electron spin resonance. In some crystal structures, electrical charges build up at a known rate and can be used to date enamel, shells, and calcite deposits between 50,000 and 1,000,000 years old in dry environments.
The final electromagnetic dating technique in common use is that of thermoluminescence dating. Thermoluminescence dating makes use of the fact that free electrons trapped in a mineral's crystal lattice can escape when the mineral is heated to a temperature below incandescence. If one assumes a relatively constant radiation level, a measure of the thermoluminescent output can be used to provide a date when the object was last heated to the point where its free electrons escaped. The method can be quite accurate and is routinely used to date objects several hundred to several thousand years old.
Chemical Dating Techniques
Although not that widely used, archaeologists do have a number of chemically based dating methods to choose from. Perhaps the most common is that of obsidian hydration (rind) dating, developed in 1960 by Irving Friedman and Robert Smith. If an obsidian (recently deposited volcanic) object is trapped for a long period of time in an area where water is present, water vapor will slowly diffuse into a freshly chipped surface. The cumulative hydration, or absorption, of water will form a hydration layer, measurable in microns, on the exposed surfaces that can be detected microscopically. Since the hydration rate with respect to a specific obsidian composition and water temperature is fairly constant, if the obsidian composition is known and the historical temperature of the area was fairly constant year after year, or if regional correction factors are known, fairly accurate dates can be produced. In the right circumstances, the technique can be used to date objects as recent as 200 years or as ancient as 200,000 years old.
Another chemical dating technique available to archaeologists is that of aspartic amino acid racemization which can be used to date bones, teeth, and shells that are between 1,000 years and 1,000,000 years old (if calibrations to local climates are available). It's based on the fact that the chemical structures of amino acids found in all living things changes over time at a known rate given a known set of environmental conditions. More specifically, it uses the fact that the amino acids of the vast majority of living organisms come in what biologists call the levorotary (left) form, even though a dextrorotary (right) form exists for all amino acids except glycine. These amino acids start to spontaneously convert from their levorotary form to their dextrorotary form as soon as a creature dies in a process called "racemization". When the rate of conversion is known, racemization provides a clock that can be used to determine the time of death.
Another chemical dating technique available to archaeologists for dating bone is the bone-nitrogen dating technique. Bones buried in soil lose organic components, and nitrogen in particular, and gain inorganic components, such as fluorine and uranium, in their place. Since bones buried at the same time in the same deposit will lose nitrogen and gain fluorine and uranium at the same rate, an archaeologist can used this as a relative dating technique to determine if bones found in the same matrix were indeed deposited together. Although this technique can not produce an exact age as the rate of nitrogen loss and fluorine gain differs with local environmental conditions, when used in conjunction with other bone dating techniques, such as amino acid racemization, bone-nitrogen dating allows an archaeologist to accurately date a collection of bones by accurately dating just one bone from the set.
Radiometric Dating Techniques
Radiometric dating techniques are based on the fact that unstable radioactive elements have regular rates of decay, or half-lives, that can be used as virtual clocks. The most common forms of radiometric dating are carbon-14, potassium-argon, and thorium-230, although some archaeologists will also make use of radium-strontium, lead-alpha age, and (spontaneous) fission-track radiometric dating. Radiometric dating techniques can be used on any object if the original amount of radioactive isotope, the current amount of radioactive isotope, and the rate of radioactive decay of the radioactive isotope is known.
Carbon-14 dating, which is based on the decay of carbon 14, with a half-life of 5730 years, to nitrogen-14 through beta-decay, measures the age of an organic object using the ratio of carbon-14 to carbon-12 that remains in the object. Since carbon-14 rapidly disintegrates compared to other radioactive elements, the method is generally limited to 50,000 years, although it can sometimes be used to date objects as old as 70,000 years when correction factors are available.
Although carbon-14 dating is fairly accurate, since the concentration of carbon-14 in the atmosphere to carbon-12 has varied over time (due to changes in the earth's magnetic field, alterations in solar activity, and the industrial activities of humans), dates may only be off by a few decades for more recent objects and dates for objects tens of thousands of years old can be off by as much as 5,000 years, especially if the sample was contaminated (by percolating ground water, for instance). More precisely, without calibrations, radiocarbon age determinations for items older than 3500 years old become increasingly inaccurate as you go back in time. Objects deposited before 1500 BC are generally found to be at least 150 years too recent, while objects deposited before 4000 BC are generally found to be at least 700 years too recent.
One advantage of the carbon-14 method, which was one of the first radiometric dating methods developed, is that only a handful of charcoal, burned bone, shell, hair, wood, or other organic substance is required for laboratory analysis. When it was invented, it allowed the direct dating of small and valuable items such as bone tools, wooden artifacts, papyri, and human fossils for the first time.
Perhaps the most common radiometric dating technique is potassium-argon dating. Based on the presence of potassium-40, which is abundant in micas, feldspars, and hornblendes and has a half-life of 1.3 billion years, potassium-argon dating makes use of the fact that 11 of every 100 potassium atoms that decay become argon 40. The method can be used to accurately date rocks that were formed as early as 20,000 years and as far back as 5,000,000,000 years, as long as the rocks were not heated to 125 Celsius in the interim, as this is the temperature where argon will begin to leak. (In these circumstances, it can tell you the last time the rock was heated.) The method, which is particularly helpful in dating formations associated with the remains of fossil hominids and Lower Paleolithic tools, has been successfully used to date stone flakes and chopping tools from Koobi Fora in Northern Kenya to approximately 2,000,000 years ago and the remains of Zinjanthropus to approximately 1,750,000 years ago.
Another common radiometric dating technique, which is primarily used to date older oceanic settlements, is thorium 230 which has a half-life of 80,000 years. The ionium-thorium dating method, which is based on the assumption that the initial ionium content of accumulating sediments has remained constant for the total section under study, is generally applied to deep-sea sediments formed during the last 300,000 years.
A less common radiometric dating technique, which is primarily used to date ancient igneous and metamorphic terrestrial rocks as well as lunar samples, is that of radium-strontium dating. It's often used to cross-check potassium-argon dates as the strontium element is not diffused by mild heating.
Another less common radiometric dating technique, known as lead-alpha age dating, uses the total lead content and uranium-thorium alpha-particle activity of zircon, monazite, and xenotime concentrates to determine the age of the rock. It is based on the fact that Uranium-235 and Uranium-238 both decay to lead, lead-207 in the first case and lead-206 in the second. Since uranium 238 has a half-life of 4,500,000,000 years, it can be used to date rocks as old as the earth.
The final radiometric dating technique we will cover is that of spontaneous fission-track dating. Primarily used on glass greater than 100,000 years old where a 10%+ error margin is acceptable, it attempts to calculate the age of a mineral or glass by the spontaneous fission of uranium-238 through a calculation of the spontaneous density to the induced density. The method works best on micas, tektites, and meteorites and can date rocks as old as 1,000,000 years. However, rocks that have been subjected to high temperatures or exposed to cosmic-ray bombardment on the earth's surface are prone to yield erroneous ages.
Today's archaeologist has a wide variety of natural, electro-magnetic, chemical, and radio-metric dating methodologies available to her that can be used to accurately date objects that are just a few hundred years old as well as objects that are a few million years old with high accuracy in the right circumstances.