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23-03-2015, 09:35

Kinds of Archaeological Dating Methods

Within the class of absolute-dating methods, there are five major kinds: (1) counting methods, (2) radioactive isotope decay methods, (3) radioactivity dosimetry methods, (4) magnetic methods, and (5) chemical methods. Not all absolute-dating methods entail the use of high-technology techniques, as evidenced by the development of dendrochronology in the 1910s and 1920s, as discussed above. The extensive use of dendrochronology is limited, however, to contexts in which tree remains are well preserved over appreciable periods of time. These contexts include architectural structures that have sheltered wood elements from decomposition, extremely dry desert or arid environments, or oxygen-deprived wet environments such as swamps or bogs. A similar counting method that makes use of historically unique climate patterns is ‘varve analysis’ - a method devised in the late nineteenth century that measures seasonal variation in sedimentation rates in lakes that receive glacial runoff. These sedimentation rates can vary substantially from year to year based on the duration of the season (higher rates of sedimentation occur during summer months) and temperature. An obvious drawback to this method is the limited number of environmental settings in which it can be applied.

Perhaps the best-known absolute dating method is ‘radiocarbon dating’ or 14C, a method that relies on measuring the degree of 14C decay in a sample and comparing this to the known decay rate of the isotope. The 14C isotope is unstable and eventually decays to 14N, and the decay rate is measured using a convention known as the half-life (the amount of time a sample loses one half of its radioactivity). The half-life of 14C is relatively short at 5730 years. Through feeding and photosynthesis, a living organism constantly exchanges 14C with the atmosphere, but this process ceases upon death. The amount of 14C remaining in an organic sample, then, is dependent on the time of death of the organism’s cells and constitutes the event that is dated.

There are several ways to measure the amount of 14C in a sample. The most common technique is to measure the emission of beta-rays when the 14C isotope decays into 14N. However, since the decay transition is a rare occurrence during the actual analysis of the sample, relatively large samples are needed to obtain sufficient counting statistics, and limits analysis to samples less than 30 000 years old. Another technique uses atomic mass spectrometry (AMS) to separate out 14C atoms by weight and counts their occurrence. This more recent technique has reduced the sample size of charcoal to 5 mg, and extended the age limit of radiocarbon dating back to about 55 000 years. A major refinement in the precision of radiocarbon dating has been the development of calibration curves that take into account the fact that production of radiocarbon in the atmosphere has not been constant over time. For the last several thousand years, calibration curves are obtained by 14C dating samples from the annular rings of trees, the true age of which is known through dendrochronology. The radiocarbon age estimates in different years are then calibrated to fit the dendrochronological age, and the resulting calibration curves can then be used to improve the precision of radiocarbon age estimates for samples of unknown age. Beyond the limits of dendrochronology, radiocarbon calibration curves have been produced through U/Th and TL dating.

Another frequently used decay method is known as ‘potassium-argon’ or K/Ar dating, and is similar to radiocarbon dating in that it dates the separation of the sample pool of the unstable 40K isotope from the atmospheric reservoir. Since 40K decays into 40Ar at a known rate (half-life of 1.28 x 109 years), the deficiency of 40K and the buildup of the decay product (40Ar) in the sample can be used to estimate the age of the separation event. Since the half-life of 40K is significantly longer than 14C, and has undergone much less change over the last few tens of thousands of years, its detection limits are especially well suited to estimating ages beyond where detection of 14C begins to be problematic, at around 50 000 years ago. Uranium series, most commonly ‘U/Th dating’, provides another decay method based on the differential solubility of uranium and thorium compounds. The most common technique measures the deficiency of 230Th compared to 234U, but other uranium decay products, as well as the parent isotopes of 238U, 235U, and 232Th, can also be measured to obtain chronological information.

‘Dosemetric methods’ analyze materials that contain information on the amount of radiation to which they have been exposed. The total amount of radiation received by a sample can be used to estimate its age if the sensitivity of the sample to radiation is understood and is combined with an estimate of the annual dose rate. ‘Thermoluminescence dating’ (TL), ‘optically stimulated luminescence’ (OSL), ‘infrared stimulated luminescence’ (IRSL), and ‘electron spin resonance’ (ESR) are all examples of dosimetric, or ‘trapped charge dating’ methods. All electrons in a mineral are in a ground state when it is originally formed or reset due to subsequent light or heat energy. In dosimetrically sensitive minerals (e. g., feldspars, quartz, and calcite), exposure to naturally occurring radiation will reposition some electrons away from atoms in the ground state to a higher energy state known as a conduction band. Over time, most electrons will return to their ground states, but some will become trapped at defect sites in the lattice structures of the mineral. Electrons have the potential to accumulate in these lattice defects with the passage of time until all defects are filled and saturation is reached. In the laboratory, energy in the form of light (OSL, IRSL) or heat (TL) can be imparted on the sample to activate the trapped electrons, which then either return to the ground state or recombine with luminescence centers and emit light or luminescence. Luminescence is then measured fairly precisely using a photomultiplier resulting in accurate estimates of total dose. ESR measures the number of trapped electrons differently by bombarding the sample with microwaves in a magnetic field. An ESR spectrometer then records the amount of microwave absorption which is proportional to the number of trapped electrons and holes, which in turn produces the age estimate. The precision of ESR is far less than luminescence methods, but an advantage lies in its nondestructive measurement process that allows multiple measurements to be taken on the same sample.

Estimating the annual dose in using luminescence methods is complex and can be prone to substantial error factors. Annual dose is received both from concentrations of radioactive elements in the sample itself, as well as external environmental sources. Only the ionizing effects of gamma and cosmic rays need to be considered in calculating the external dose rate since short range beta and alpha ray contributions can be eliminated with the removal of the outer 2 mm of the sample. Internal dose rate, however, is primarily due to alpha and beta rays emitted from radioactive elements in the sample and need to be measured. Dose rate is dependent on radioactivity originating from the U, Th, and 40K decay chains with minor sediment contributions from 87Rb. One factor that can negatively affect the precision of trapped charge dating is disequilibrium in the U-series decay chains, which complicates dose rate calculations and may increase the uncertainty of age estimates. Other attenuating factors on radiation such as moisture content and latitude also need to be accounted for. The overall precision of trapped charge dating methods, however, can be as low as 6-7%, a rate that compares favorably with other radiometric dating methods.

A central advantage of trapped charge dating methods is one of accuracy. The events dated are the growth of a crystalline structure such as bone or tooth enamel (ESR), exposure to temperatures in excess of 500 °C during manufacture or subsequent firing events, typically ceramics or heated lithics (TL), or the last exposure of archaeologically relevant sediments to sunlight (OSL, IRSL). These events are often of direct interest to archaeologists, and thus trapped charge dating methods possess the potential to yield ‘archaeological chronologies’ of overall higher precision than other dating methods with intrinsically high methodological precision such as radiocarbon dating. This advantage has gone largely unappreciated by North American archaeologists, although trapped charge dating is used extensively in Europe and other regions to supplement other dating methods. Regardless, the use of trapped charge dating methods will continue to play an increasing role in the development of archaeological chronologies, and OSL dating in particular is poised to make major breakthroughs in constructing reliable archaeological chronologies.

‘Fission-track dating’ is a dosimetric method that works differently than trapped charge dating methods. Fission tracks are created in crystalline minerals when atoms of 238U fission and break apart. Since 238U fissions at a known rate, the number of fission tracks across a quantified crystalline area in a material in which the concentration of uranium is known can be used to estimate age. Fission-track dating is limited, however, in its application to archaeological contexts older than 100 000 because the low frequency at which 238U fissions creates substantial error terms that makes its applicability to younger samples impractical.

Archaeomagnetism, or ‘archaeomagnetic dating’, is a method that pursues correlating an event of archaeological interest with the position of Earth’s magnetic north (which is continually changing). These magnetic positions can provide absolute dates during the time interval of the last few thousand years once they are correlated with dates derived from other methods, such as radiocarbon. Magnetic materials such as iron will tend to align themselves according to the direction of the Earth’s magnetic field at any particular time and place. If these magnetic materials are in fired contexts, such as in archaeological hearth or kiln features, the direction of magnetic north can be preserved to a degree that dating the firing event is possible. Like dendrochronology, archaeomagnetic dating requires constructing reference curves, or a master sequence of direction change in magnetic north, using the orientation of samples of known age based on historical records or, more commonly, radiocarbon dating. These master sequences need to be constructed for each region of study because secular variation in magnetic north is regionally specific. Archaeomagnetic dating is often used in concert with other available dating methods, although it is extensively used in some areas, such as the American Southwest. Another form of magnetic dating documents reversal events of the Earth’s north and south magnetic poles by analyzing the magnetic properties of minerals in preserved sediments. Documenting reversed polarity events has proved to be a robust relative-dating method, but can only provide general age estimates and is restricted to relatively old archaeological contexts. For example, the most recent reversal, the Blake subchron, has been dated to 104 000-117000 BP.

A final set of absolute-dating methods can be referred to as chemical dating methods that rely on isolating and documenting the temporal dimension of a reaction or set of reactions. Amino acid racemi-zation, obsidian hydration, fluorine dating, and in situ cosmogenic isotope dating fall into this category. Chemical dating methods are complicated by the fact that reaction rates are not just product of time, but are influenced by a number of other environmental variables as well (e. g., moisture content, temperature, pressure, etc.). Due to the relatively large number of variables that need to be independently measured, serious problems with amino acid racemi-zation remain to be resolved and applications of cos-mogenic isotope dating in archaeology are in an early state of development. Indeed, all three methods are more powerful when used to assess the relative age of archaeological events rather than absolute age.

‘Amino acid racemization’ relies on the rate of decomposition of protein in bone and shell in order to assess age. A major drawback to the method is that temperature, and thus the rate of protein decomposition, or racemization, must be assumed to have remained constant over time. These assumptions then allow age to be assessed using linear projections, but both assumptions are highly problematic for time periods that clearly demonstrate significant climatic change. Some attempts have been made to control for this shortcoming by creating calibration curves using radiocarbon dating, producing racemization rates to estimate ages beyond the limits of radiocarbon range. However, this calibration still carries the assumption that variation in temperature during earlier periods was similar to that of the later radiocarbon calibrated time span. A more robust application of amino acid racemization is known as aminostratigraphy, a relative-dating method that differentiates stratigraphic units based on protein ratios, and has led to the construction of broad regional chronologies.

The parameters of ‘obsidian hydration’ are better known; however, a number of attributes need to be documented for its successful application. Obsidian hydration measures the hydration layer of an exposed surface of glass or silica-enriched rock such as obsidian. Hydration of a new surface begins the moment it is exposed, so the event dated using this method is one consistent with archaeological interests (e. g., the manufacture or recycling of obsidian artifacts). The thickness of the hydration layer is dependent on the hydration rate, which if known, can yield an age of exposure of the surface. Calculating the hydration rate can be difficult, however, since the rate is dependent on composition, moisture, temperature, and pressure. Even so, obtaining sufficient estimates for these attributes is far from impossible, and many successful applications of obsidian hydration have been made. In a relative-dating role, obsidian hydration offers the possibility of identifying and ordering assemblages of mixed age. In addition, because the chemical composition of obsidian differs significantly from outcrop to outcrop, provenance studies have been very successful in accurately characterizing different sources and allocating artifacts to them. Such provenance information, when combined with the chronological information of obsidian hydration, provides a powerful set of tools to document change in the archaeological record.

Fluorine, or ‘fluorine-uranium-nitrogen dating’ of bone can be achieved by measuring the amount of fluorine in the sample through an irreversible reaction that results in fluoroapatite replacing hydroxyapatite. Because the amount of fluorine in the environment and the reaction rate is extremely variable, fluorine dating is best suited to a relative-dating role. Fluorine dating was used to uncover the Piltdown hoax, in which a skull was fabricated from bones of different species and age and purported to represent a human ancestor.

"In situ cosmogenic isotope dating’ is a relatively new radiometric method that, like obsidian hydration, measures how long a surface has been exposed to the atmosphere. Rather than using hydration rates, cos-mogenic isotope dating measures the abundance of in situ cosmogenic isotopes in surface or near-surface materials that have been exposed to the bombardment of high-energy neutrons contained in cosmic rays. In situ cosmogenic isotopes are created when these high-energy neutrons interact with atoms in minerals located in the topmost 1 m or so of deposits. Below 1.5 m, cosmogenic isotope production completely ceases, representing a serious application limit for many archaeological deposits. Partial burial or intermittent burial of artifacts and structures can slow the isotope clock significantly, requiring an accurate understanding of depositional history. Conversely, an artifact or structure buried within the top 1.5 m can accumulate a significant amount of inherited cosmogenic isotopes before surface exposure. In addition, the low production rates of commonly measured cosmogenic isotopes result in timescales that are often more compatible with geological investigations than archaeological ones. However, continuing improvements in the technical ability to measure ever smaller atomic quantities has led to some optimism that common archaeological timescales will soon be encompassed.

Cosmogenic isotope dating primarily makes use of six isotopes, two of which are stable (3He and 21Ne) and four of which are radioactive (10Be, 14C, 26Al, 36Cl). The relatively short timescales of archaeology restrict exposure dating studies to isotopes that accumulate relatively quickly in minerals, namely 3He, 14C, and 36Cl. Cosmogenic isotope production is dependent on elevation (higher rates at higher elevations), magnetic field strength (higher rates with distance from the equatorial magnetic field), and solar activity (high solar activity decreases cosmic ray flux). At present, all of these factors will require additional future research to better assess in situ cosmogenic isotope production rates. Best case scenarios for nearterm future archaeological applications of cosmogenic isotope dating will probably include stone structures, monuments, or petroglyphs whose surfaces have been continually exposed to cosmic rays and have experienced minimal erosion.

Two relative-dating methods that have been briefly mentioned, seriation and stratigraphic superposition, deserve further comment because they still constitute the most common forms of dating used in archaeology today. The principle of stratigraphic superposition was developed in early geological inquiries of the seventeenth and eighteenth centuries. The central proposition is that older deposits lie below younger ones. It is important to stress that the geological law of superposition refers to the age of the deposition event, not necessarily the age of the artifacts within the deposit. However, barring the subsequent disturbance or reworking of the deposits (e. g., reversed stratigraphy), the age of artifact manufacture frequently does roughly coincide with the age of the deposition event, making superposition useful to archaeologists in building chronology.

Unlike other dating methods, seriation was developed by archaeologists. Consequently, the dated attributes are archaeological and in accord with archaeological target events. Seriation uses a distinctive patterning in the distribution of historical classes (often classes of pottery style) through time to generate the chronological order of archaeological events. The distinctive pattern is one of a continuous, nonrepetitive distribution if one documents the presence or absence of attributes (occurrence seriation), or monotonic (unimodal) if the frequencies of historical classes are recorded (frequency seriation). Stratigraphic superposition initially supplied information on which end of the order was oldest (and which was youngest), but it was soon realized that even undated surface assemblages, if described as stylistic classes, could be ordered chronologically if they were arranged in such a way that all classes exhibited unimodal frequencies. Only recently has it come to be understood why stylistic classes behave in this manner.

Seriation is successful because it provides the ability to empirically assess initial chronological constructs against outside data. Initial chronologies and historical class descriptions could be revised or discarded. For almost 70 years, archaeologists knew seriation worked, but were unable to explain why, merely stating that styles wax and wane in popularity. This response did not address ‘why’ styles act this way. We now know that stylistic traits are akin to neutral traits in biology which also display unimodal distributions due to the lack of selection pressure. This theoretical realization has led to new research on the underlying principles of seriation, and has generated new interest in documenting and explaining temporal and spatial variation in the archaeological record.