5 Absolute dating methods

K. Polley

1. Introduction

Absolute dating determines the age of an object in calendrical years. There are many such dating methods such as C14 or Carbon 14 method of dating, U238 method of dating, K-Ar or Potassium-Argon method of dating etc. Any of these methods is not applicable for all archaeological findings. Because the age range of each method is applicable for a particular time span and any archaeological object is not applicable for all methods of dating. Thus each method has its own time limit and object or material specialty e.g. C14 method is applicable only on organic materials, while K-Ar method is applicable on volcanic deposits.

When a number of geological samples are tested in any absolute dating method, such type of absolute dating is known as ‘chronometric dating method’. The dates that result from such multiple testing, are combined and expressed statistically. On the basis of basic principles or methods, used in a particular dating the absolute dating methods can be divided into four basic categories: a) radio-isotopic methods, which are based on the rate of atomic disintegration in a sample or its surrounding environment; (b) paleomagnetic (correlation) methods, which rely on past reversals of the Earth’s magnetic field and their effects on a sample; (c) organic and inorganic chemical methods, which are based on time-dependent chemical changes in the sample, or chemical characteristics of a sample; and (d) biological methods, which are based on the growth of an organism to date the substrate on which it is found (Bradley, 1999:47-48).

While there are many advantages to absolute-dating methods, all were developed outside the discipline of archaeology and thus require rigorous bridging arguments that link the dated event obtained to the archaeological event of interest. Furthermore, the technical sophistication of many absolute-dating methods is costly in terms of specialized expertise and equipment. Even so, these methods offer archaeologists the possibility to assess the age of archaeological events with greater precision and accuracy than ever before (Truncer, 2008, cited in Pearsall, 2008).

2.   Absolute Dating Methods: A Short Survey

2.1 C¹⁴ Dating:

The best-known absolute dating method is ‘radiocarbon dating’ or C¹⁴, 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 C¹⁴ isotope is unstable and eventually decays to N¹⁴, 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 C¹⁴ is relatively short at 5730 years. Through feeding and photosynthesis, a living organism constantly exchanges C¹⁴ with the atmosphere, but this process ceases upon death. The amount of C¹⁴ 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 (Truncer, 2008, cited in Pearsall, 2008).

There are several ways to measure the amount of C¹⁴ in a sample. The most common technique is to measure the emission of beta-rays when the C¹⁴ isotope decays into N¹⁴. 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 C¹⁴ 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 C¹⁴ 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 (Truncer, 2008, cited in Pearsall, 2008).

2.2 K-Ar Method of Dating:

Potassium-argon dating is based on the decay of the radioisotope K⁴⁰ to a daughter isotope Ar⁴⁰. Potassium is a common component of minerals and occurs in the form of three isotopes, K³⁹ and K⁴¹ , both stable, and K⁴⁰ , which is unstable. The K⁴⁰ occurs in small amounts (0.012% of all potassium atoms) and decays to either Ca⁴⁰ or Ar⁴⁰, with a half-Hfe of 1.31 X 10⁹ yr. Although the decay to Ca⁴⁰ is more common, the relative abundance of Ca⁴⁰ in rocks precludes the use of this isotope for dating purposes. Instead, the abundance of argon is measured and sample age is a function of the K⁴⁰ /Ar⁴⁰ ratio. Argon is a gas that can be driven out of a sample by heating. Thus, the method is used for dating volcanic rocks that contain no argon after the molten lava has cooled, thereby setting the isotopic “clock” to zero. With the passage of time, Ar⁴⁰ is produced and retained within the mineral crystals, until driven off by heating in the laboratory during the dating process. Unlike conventional C¹⁴ dating, K-Ar dating relies on measurements of the decay product Ar⁴⁰; the parent isotope content K⁴⁰ is measured in the sample (Bradley, 1999).

As the abundance ratios of the isotopes of potassium are known, the K⁴⁰ content can be derived from a measurement of total potassium content, or by measurement of another isotope K³⁹. Because of the relatively long half-life of K⁴⁰, the production of argon is extremely slow. Hence, it is very difficult to apply the technique to samples younger than -100,000 years and its primary use has been in dating volcanic rocks formed over the last 30 million years (though, theoretically, rocks as old as 10⁹ years could be dated by this method) (Bradley, 1999).

2.3 Uranium Series Dating:

Uranium Series dating is based on the fact that when calcite (or other forms of calcium carbonate) are formed they normally uranium but not thorium, because of the different chemical properties of these two elements. However, as time passes the uranium undergoes radioactive decay producing thorium so old calcite does contain thorium (Parkes, 1986).

Thus, if one knows the concentration of uranium in the newly formed calcite, one can work out how old the calcite is by measuring the amount of thorium present today. The technique is thus, in some ways, the reverse of C14 dating, since in it one measures the amount of the daughter nuclide that has been produced rather than the amount of the parent that remains (Parkes, 1986). Uranium series dating is the only method that can provide absolute dates in the period 30,000 to 100,000 years B.P when neither conventional C14 nor K-Ar dating are possible.

2.4 Dosemetric Dating Methods (Thermoluminescence Dating or TL, Optically Stimulated Luminescence Dating or OSL, Infrared Stimulated Luminescence Dating or IRSL, Electron Spin Resonance Dating or ESR):

‘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 (Truncer, 2008, cited in Pearsall, 2008).

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 2mm 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 K⁴⁰ decay chains with minor sediment contributions from Rb87. One factor that can negatively affect the precision of trapped charge dating is dis-equilibriumin 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 (Truncer, 2008, cited in Pearsall, 2008).

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 500⁰ 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 (Truncer, 2008, cited in Pearsall, 2008).

2.5 Fission Track Dating:

‘Fission-track dating’ is a dosimetric method that works differently than trapped charge dating methods. During normal radioactive decay the nuclei of uranium 238 occasionally undergo a much more violent process: spontaneous fission. In this process the nuclei splits into two roughly equal halves, both with mass numbers in the range 70 to 160. These two halves fly apart with tremendous force and so do great damage to the nearly crystal lattice of the rocks and minerals containing uranium 238.    This damage is not visible in the naked eye, but if the glass or crystal is placed in a suitable etching agent (hydrofluoric acid for glass) the damaged areas are attacked more readily than the rest of the surface, so that the fission tracks are enlarged sufficiently to be visible under an optical microscope.

In many glasses and minerals, the tracks can survive at room temperature for millions of years and are only destroyed by heating to temperatures above 5000C. Thus the melting associated with the production of glass and the firing of pottery destroys any tracks which were present before these events. After the heating the number of tracks then starts to increase steadily with time, and so, by counting the number of tracks per unit area, it is possible to determine the age of the sample (Parkes, 1986). The main use of this dating method has been for dating volcanic glasses like obsidian, but can also be used on some manmade glasses and minerals extracted from pottery. Dating range of this method goes from 20 years to 1 billion years.

2.6 Archaeomagnetic or Paleomagnetic Dating:

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. 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. This dating method is sometimes known as ‘Paleomagnetism or Paleomagnetic Dating Method’. 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 (Truncer, 2008, cited in Pearsall, 2008).

Now a day a paleomagnetic time scale is established on the basis of the paleomagnetic dating of volcanic rocks and sediments. In this scale dating has concentrated on three main boundaries, the Brunhes/Matuyama (0.7 mya.), the Matuyama/Gauss (2.47 mya.) and the Gauss/Gilbert (3.4 mya.). In each case the age of the boundary has been established on the basis of average of a number of K-Ar dates (Walker and Lowe, 1984:264; Walker, 2005:216-217).

Fig 3: The Paleomagnetic timescale of last 3.5 million years. Black areas in the scale indicate the periods of normal polarity; white areas show episodes of reversed polarity. K-Ar ages are shown on the left hand side; astronomically turned ages from deep sea cores are on the right (after, Walker and Lowe, 1984; Walker, 2005:217)

2.7 Dendrochronology:

Dendrochronology or ‘Tree Ring Method of Dating’ uses the unique patterns of varying tree ring widths in some tree species that are sensitive to climate and precipitation fluctuation. By linking multiple tree samples of different ages, master sequences of this patterning have been built that extend back thousands of years. Samples of unknown age can then be fitted against this master sequence to derive an age estimate. One of the principal advantages of dendrochronology is its high precision, with the potential to identify the exact year in which an event took place. The development of dendrochronology was remarkable achievements that occurred well before the advent of radiometric dating techniques such as radiocarbon dating, and both methods continue to be used today (Truncer, 2008, cited in Pearsall, 2008).

2.8 Varve Chronology:

A characteristic feature of many lake sediment sequences is the presence of regularly laminated sediments consisting of thin, horizontally bedded layers of different structure and texture. Often, these laminae are arranged in couplets with relatively coarse-grained layers alternating with finer-grained bands. In other situations, the couplets may consist of alternations between organic and inorganic laminae, or between sedimentary units rich in diatoms or iron oxides and thin sedimentary bands that are relatively deficient in these components. Such regular, or rhythmical, variations in sediment accumulation reflect seasonal variations in sediment supply to, or in chemical or biogenic processes within, the lake ecosystem. In many cases, the couplets will reflect the annual cycle of deposit ion, in which case they are termed varves. Because they are deposited annually, varves form a basis for dating, for by counting varve sequences, time intervals can be established and a ‘floating’ chronology developed. If varve series can be dated by radiometric or other means (by radiocarbon dating of included organic materials, for example), then the floating varve chronology can be linked to the calendar timescale.

The potential of varves as a basis for dating was initially recognized by the Swedish geologist Gerard de Geer who, in 1884, made the first attempt to count and correlate varve sequences in the Stockholm area of Sweden. His aim was to use the varved sequences that had accumulated in front of the decaying Fennoscandian ice sheet to establish a timescale for deglaciation. After this work varve chronology has subsequently been applied in a range of other lacustrine contexts of Europe (Walker, 2005:132-133).

2.9 Lichenometry:

Lichens are composite plants consisting of algae and fungi. Lichens could be used as basis for dating was first suggested by the Austrian scientist Roland Beschel in a paper published in 1950. Lichens are rapid colonizers of bare surfaces and, once established, there is a progressive increase in size of the thallus by slow marginal growth. Hence, the larger the thallus (in terms of its diameter), the older the lichen, and the greater the time that has elapsed since colonization. Where a surface has been exposed to lichen colonization (following glacier retreat, for example), provided that (a) the growth rates of the lichens are known, and (b) no significant time interval has elapsed between surface exposure (in this case deglaciation) and lichen colonization, an estimate can be made for the timing of substrate exposure by measuring the size of the lichen thalli on that surface. It was on the basis of these two key principles that Beschel developed the technique of lichenometry (Walker, 2005).

By measuring lichen thallus diameters (usually the largest lichen) on surfaces of known age, lichen growth rates can be established and used to construct a growth curve showing the relationship between lichen size and age. Surfaces of known age (referred to as fixed points) might include humanly constructed features such as building walls or gravestones, or natural features such as rock surfaces whose ages might be inferred from old photographs or historical records. Once the growth curve has been developed, surfaces of unknown age can be dated by relating lichen diameters on those surfaces to the growth-rate curve, thereby deriving a calendar age (Walker, 2005).

4. Summery

• Absolute dating determines the age of an object in calendrical years.
• On the basis of methodological differences absolute dating can be divided into four categories, such as- radio-isotopic methods, paleomagnetic (correlation) methods, organic and inorganic chemical methods and biological methods.
• In C14 dating method determination of age of an object can be measured on the basis of the measurement of radioactive carbon particle present in that object.
• Unlike C14 dating, K-Ar dating relies on measurements of the decay product Ar40 in the sample.
• In Uranium series method of dating age of an object can be measured on the basis of the proportion of Uranium and Thorium in that object.
• ‘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.
• Fission track method estimates age of an object by counting number of U238 fission tracks in the quantified crystalline area of that object.
• 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).
• Dendrochronology or ‘Tree Ring Method of Dating’ uses a master sequence of the unique patterns of varying tree ring widths in some tree species to date ancient tree trunks used by prehistoric people to make different objects.
• In Varve chronology annually deposited horizontal bedded layers of lake sediments are used to establish chronology of human activity and past climatic fluctuations in a region.
• Lichenometry measures age of an object on the basis of the growth pattern of lichen on a particular object.