Age determination ( Dr. AJEET JAISWAL- DEPT OF ANTHROPOLOGY)
The determination of an absolute age normally involves either a radioactive decay process or some natural, annual, rhythmic process. In the context of dating works of art, the relevant techniques are radiocarbon dating, thermoluminescence dating and dendrochronology.
1. Radiocarbon dating.
Radiocarbon dating is possible because of the continuous formation of the radioactive isotope of carbon (carbon-14) that occurs in the upper atmosphere when neutrons produced by cosmic rays interact with atmospheric nitrogen atoms. Carbon-14 combines with oxygen to form radioactive carbon dioxide, which then mixes throughout the atmosphere, dissolves in the oceans and, via the photosynthesis process and the food chain, enters all plant and animal life. Since carbon-14 is produced by cosmic ray neutrons at a more or less constant rate and, at the same time, is lost by radioactive decay, an equilibrium concentration of carbon-14 is established: the ratio of carbon-14 to carbon-12 atoms being approximately 1 to a million million. All living plants and animals therefore contain this equilibrium concentration of carbon-14. However, once dead, the plant or animal no longer takes in carbon dioxide from the atmosphere, so that the carbon-14 lost by radioactive decay is not replaced, and therefore its concentration slowly decreases—by half in 5730 years. Consequently, by measuring the carbon-14 concentration in, for example, dead wood and comparing it with the concentration in living wood, the age of the dead wood can be determined.
Radiocarbon dating is applied to materials that once formed part of the biosphere. In the context of works of art, this includes wood, bone, ivory, all types of textile, leather, parchment and paper. Until recently the carbon-14 content of such material could be determined only by counting the number of beta particles emitted by those carbon-14 atoms undergoing radioactive decay. This measurement technique requires comparatively large samples (at least 1 g of carbon, which is equivalent to some 10 g of wood or 50 g of bone), and the resultant damage to works of art was generally unacceptable. However, in the late 1970s the technique of accelerator mass spectrometry was developed. With this technique the total number of carbon-14 atoms present in a sample are counted, and as a result only 1 mg of carbon is required. Consequently, radiocarbon dating now has a significant role to play in the dating of works of art, in particular textiles and ivories. A much publicized application in the 1980s was the dating of the Shroud of Turin (Turin Cathedral) to the 13th–14th century ad rather than to the period of Christ.
The basic assumption on which radiocarbon dating depends is that the carbon-14 concentration in living plants and animals has always been the same as it is now. Unfortunately, for a number of reasons, this assumption is not strictly valid. Therefore, in order to obtain true calendar dates the radiocarbon dates must first be calibrated. A calibration curve extending back some 8000 years (see fig. 3) has now been established using long sequences of tree-rings dated by dendrochronology (see §3 below). The correction necessary to convert radiocarbon dates into calendar dates varies from less than one hundred to several hundred years, depending on the period under consideration. Typically, the corrected radiocarbon dates are accurate to within ±50–100 years.
2. Thermoluminescence dating.
Thermoluminescence (TL) dating depends on the fact that when crystalline materials, for example quartz and feldspars, are subjected to ionizing radiation (e.g. alpha particles, beta particles and gamma rays) a proportion of the free electrons thus produced are trapped at defects in the crystalline lattice. When the material is subsequently heated the electrons escape from the traps, and if they then combine with another type of defect, referred to as a luminescence centre, the energy associated with recombination is released in the form of light or thermoluminescence. The intensity of this light provides a measure of the number of trapped electrons and hence a measure of the ionizing radiation dose that the material received.
3. High-precision radiocarbon calibration curve for the past 8000 years based on the dendrochronological dating of Irish oak; the straight line represents the ideal 1:1 correspondence between radiocarbon age and dendrochronological (calendar) age
In the production of pottery, clay is fired at temperatures in the range of 500°–1000°C, and the TL acquired due to the action of ionizing radiation during geological times is removed. The TL clock is thus reset to zero. Subsequently, the stored TL or number of trapped electrons builds up again, due to the action of ionizing radiation from radioactive impurities (i.e. a few parts per million of uranium and thorium and a few per cent of potassium), both in the pottery itself and associated with its burial or storage environment.
Assuming that the firing associated with its production was the last heating to which the pottery was subjected, then in principle the age of the pottery can be determined from three measurements: (1) the natural TL stored at the present day; (2) the TL sensitivity (i.e. the amount of TL produced per unit dose of radiation); and (3) the radiation dose received per year. In practice the situation is not quite so simple, and many measurements and checks involving several grams of sample are required in order to obtain an accurate date for a piece of pottery. Further, for an accurate date a knowledge of the burial or storage environment of the pottery is essential, since this contributes significantly to the overall annual radiation dose. Therefore, as a method of accurate dating, thermoluminescence can normally be applied only to pottery sherds obtained from archaeological sites during controlled excavations. Even so, the resulting age determination is still typically accurate only to ±5–10%.
In the case of fine art ceramics, the limitation on sample size and the lack of information on storage history means that the date obtained by TL is even less accurate. However, the method is normally adequate to distinguish between recently made objects—fakes—and those manufactured in antiquity. Typically for such authenticity testing, some 30 mg of sample is drilled from an inconspicuous area of the ceramic, usually the base. In estimating the age a range of values are assumed for the environmental contribution to the annual radiation dose. In addition to pottery and porcelain, the core material from bronzes that have been cast using a clay core can also be tested by TL, since this material has again normally been fired to a sufficiently high temperature to reset its TL clock to zero.
3. Dendrochronology.
The basis of dendrochronology lies in the fact that in temperate climates, where there is a contrast between the seasons, trees grow by the addition of a well-defined annual ring formed between the bark and the sapwood. The width of each ring depends on the prevailing climatic conditions, principally rainfall and temperature. Therefore, trees of a single species growing in similar localities should have a similar pattern of ring widths, uniquely defined by their common climatic history. This similarity in ring pattern provides the basis for cross-dating by which a tree-ring sequence of unknown age is matched with one of known age.
Long chronologies, sometimes referred to as master curves, are established, starting with living trees. The timescale is then extended by using large timbers from felled trees or from buildings, with ring patterns that sufficiently overlap the existing chronology to be certain of an exact match. In this way master chronologies have been built up that extend over several thousands of years and are accurate to within one year.
In order to be able to use dendrochronology for dating works of art a number of conditions must be fulfilled. First, the wood needs to be of a species for which there is a master chronology, and there must normally be at least 100 rings in the sample to be dated to ensure an exact match with the master pattern. Then, if the date is to be precise to the year of felling, either the bark must be present or an estimate must be made of the number of rings lost during the preparation of the timber. Finally, to determine the date of actual use, allowance must be made for seasoning the timber. If these conditions are adequately fulfilled, then dendrochronology provides a valuable technique for dating panel paintings and building timbers.
Read more: VII. Age determination - 1. Radiocarbon dating., 2. Thermoluminescence dating., 3. Dendrochronology - Dating, Carbon, Radiocarbon, Radiation, Pottery, and Ring
Finding the age of rocks based on the presence of naturally occurring long-lived radioactive isotopes of several elements in certain minerals and rocks. Measurements of rock ages have enabled geologists to reconstruct the geologic history of the Earth from the time of its formation 4.6 × 109 years ago to the present. Age determinations of rocks from the Moon have also contributed to knowledge of the history of the Moon, and may someday be used to study the history of Mars and of other bodies within the solar system. See also Radioactivity.
Many rocks and minerals contain radioactive atoms that decay spontaneously to form stable atoms of other elements. Under certain conditions these radiogenic daughter atoms accumulate within the mineral crystals so that the ratio of the daughter atoms divided by the parent atoms increases with time. This ratio can be measured very accurately with a mass spectrometer, and is then used to calculate the age of the rock by means of an equation based on the law of radioactivity. The radioactive atoms used for dating rocks and minerals have very long half-lives, measured in billions of years. They occur in nature only because they decay very slowly. The pairs of parents and daughters used for dating are listed in the table. See also Dating methods.
| Parent-daughter pairs used for dating rocks and minerals | ||
| Parent | Daughter | Half-life, 109 years |
| Potassium-40 | Argon-40 | 11.8 |
| Potassium-40 | Calcium-40 | 1.47 |
| Rubidium-87 | Strontium-87 | 48.8 |
| Samarium-147 | Neodymium-143 | 107 |
| Rhenium-187 | Osmium-187 | 43 |
| Thorium-232 | Lead-208 | 14.008 |
| Uranium-235 | Lead-207 | 0.7038 |
| Uranium-238 | Lead-206 | 4.468 |
The rubidium-strontium method is based on rubidium-87, which decays to stable strontium-87 (87Sr) by emitting a beta particle from its nucleus. The abundance of the radiogenic strontium-87 therefore increases with time at a rate that is proportional to the Rb/Sr ratio of the rock or mineral. The method is particularly well suited to the dating of very old rocks such as the ancient gneisses near Godthaab in Greenland, which are almost 3.8 × 109 years old. This method has also been used to date rocks from the Moon and to determine the age of the Earth by analyses of stony meteorites.
The potassium-argon method is based on the assumption that all of the atoms of radiogenic argon-40 that form within a potassium-bearing mineral accumulate within it. This assumption is satisfied only by a few kinds of minerals and rocks, because argon is an inert gas that does not readily form bonds with other atoms. The K-Ar method of dating has been used to establish a chronology of mountain building events in North America beginning about 2.8 × 109 years ago and continuing to the present. In addition, the method has been used to date reversals of the polarity of the Earth's magnetic field during the past 1.3 × 107 years. See also Orogeny; Paleomagnetism.
The uranium, thorium-lead method is based on uranium and thorium atoms which are radioactive and decay through a series of radioactive daughters to stable atoms of lead (Pb). Minerals that contain both elements can be dated by three separate methods based on the decay of uranium-238 to lead-206, uranium-235 to lead-207, and thorium-232 to lead-208. The three dates agree with each other only when no atoms of uranium, thorium, lead, and of the intermediate daughters have escaped. Only a few minerals satisfy this condition. The most commonly used mineral is zircon (ZrSiO4), in which atoms of uranium and thorium occur by replacing zirconium. See also Lead isotopes (geochemistry); Radioactive minerals.
The common-lead method is based on the common ore mineral galena (PbS) which consists of primordial lead that dates from the time of formation of the Earth and varying amounts of radiogenic lead that formed by decay of uranium and thorium in the Earth. The theoretical models required for the interpretation of common lead have provided insight into the early history of the solar system and into the relationship between meteorites and the Earth.
The fission-track method is based on uranium-238 which can decay both by emitting an alpha particle from its nucleus and by spontaneous fission. The number of spontaneous fission tracks per square centimeter is proportional to the concentration of uranium and to the age of the sample. When the uranium content is known, the age of the sample can be calculated. This method is suitable for dating a variety of minerals and both natural and manufactured glass. Its range extends from less than 100 years to hundreds of millions of years. See also Fission track dating.
The samarium-neodymium method of dating separated minerals or whole-rock specimens is similar to the Rb-Sr method. The Sm-Nd method is even more reliable than the Rb-Sr method of dating rocks and minerals, because samarium and neodymium are less mobile than rubidium and strontium. The isotopic evolution of neodymium in the Earth is described by comparison with stony meteorites. See also Meteorite.
The rhenium-osmium method is based on the beta decay of naturally occurring rhenium-187 to stable osmium-187. It has been used to date iron meteorites and sulfide ore deposits containing molybdenite.
Relative and quantitative techniques used to arrange events in time and to determine the numerical age of events in history, geology, paleontology, archeology, paleoanthropology, and astronomy. Relative techniques allow the order of events to be determined, whereas quantitative techniques allow numerical estimates of the ages of the events. Most numerical techniques are based on decay of naturally occurring radioactive nuclides, but a few are based on chemical changes through time, and others are based on variations in the Earth's orbit. Once calibrated, some relative techniques also allow numerical estimates of age. See also Archeology; Astronomy; Geology; Radioisotope.
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