Absolute Dating Introduction Type of Absolute Dating:

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1 Absolute Dating Introduction Prehistory deals with a long span of time. Therefore, one of the most important aims of prehistorian is to establish the chronological sequence of the past events. In the early stages of prehistoric studies the time sequence were framed in a relative way. That is, by comparing one event with that of another one. It was only during nineteen forties onwards, there evolved dating techniques which can establish dates absolutely. At present, there are a number of dating methods in prehistory. These methods can be discussed under two categories, absolute and relative dating methods. Absolute dating is also known as chronometric dating. It is primarily sought to facilitate time sequence in terms of years. It provides the actual time spanned by a site sequence with close approximation. These methods can provide chronological sequence of even geographically isolated events or culture. Thus the rate of change, differential development in separate areas, and the identification of the geographic sources of widespread cultural influences can be established with the help of different absolute dating. Absolute dating or chronometric dating usually demands high technology laboratory and hence costly. It also demands the help of sciences like geophysics, geochemistry, astronomy, nuclear physics etc. Type of Absolute Dating: i) Radio Carbon Dating, ii) Potassium- Argon Dating, iii) Thermoluminescence, iv) Dendrochronology, v) Electron Spin Resonance, vi) Fission Track Dating, vii) Palaemagnetic dating, viii) Obsidian hydration, ix) Varve analysis, x) Amino Acid Racemaization etc. 1

2 Here our main concern is on Carbon-14 and Potassium-Argon dating methods only. Radio Carbon Dating: It is the single most useful method of dating for the archaeologists, but it has its limitations both in terms of accuracy, and for the time range where it is useful. It has transformed our understanding of the past, helping archaeologist to establish for the first time a reliable chronology of the world cultures. Radiocarbon dating or C14 method has revolutionized the study of archaeology during the 40 s. It is one of the most important and popular method of absolute or chronometric dating. The method is a chemical analysis used to determine the age of organic materials based on their content of the radioisotope of carbon 14. The method was developed in the 1940s by Willard F. Libby and a team of scientists at the University of Chicago. In 1949, the American Chemist W.F. Libby published the first radiocarbon dates. During World War II, he had been one of the several scientists studying cosmic radiation, the sub-atomic particles that constantly bombard the earth, producing high energy neutrons. These neutrons react with nitrogen atoms in the atmosphere to produce atoms of carbon-14 called C14 or radiocarbon which are unstable because they have eight neutrons in the nucleus instead of the usual six as for ordinary carbon (C12). This instability leads to radioactive decay of C14 at a regular rate. In 1960 Libby received the Nobel Prize in chemistry for his method to use Carbon-14 for age determinations in archaeology, geology, geophysics, and other branches of science. It subsequently evolved into the most powerful method of dating late Pleistocene and Holocene artefacts and geologic events up to about 50,000 years. This method is very useful in determining the chronology of a site consisting of charcoal and charred bones. Here we may recall the words of Agrawal and Yadava(1995) The origin of carbon- 14 is in the atmosphere about 15km above the surface of the earth where cosmic ray produces neutrons combine with nitrogen nuclei and knock off a proton from it thus producing carbon-14. The basic principle of the method is that Cosmic rays produce neutrons in the 2

3 atmosphere. These neutrons hit the atoms of nitrogen present in the air and produce a proton and an atom of carbon-14 instead of 12. Carbon-14 is a radioactive isotope of ordinary carbon-12. The reaction that produces a heavy isotope of carbon c 14 is as follows - N 14 + n C 14 + P N14+ Chemically, C14 seems to behave exactly as ordinary non radioactive carbon C12 does. Therefore, like C12, the c 14 atoms readily mix with the oxygen in the earth and produce radioactive carbon dioxide, C14O2. This carbon dioxide is absorbed by plant life in the normal way. Herbivores eat the plants and carnivores eat the herbivores while man, who are omnivores eats the plants, herbivores and carnivores. Thus, the C14 originally produce in the environment equally distributed among all living things and maintained at a constant level till their death. As long as a matter is living and hence in exchange with the atmosphere, it continues to receive C12 and C14 atoms in a constant proportion. After death the organism is no longer in exchange with the atmosphere and no longer absorbs atoms of contemporary carbon. On the other hand, C14 that remains in the organism starts to decay in a constant rate. In something over 7200 years only a half of the C14 remains and in similar period only half of that residue. If therefore, the amount of C14 left with the charcoal or bones found in a given site can be determined then the age of the organic material can also be determined. The half life (t½) is measured by counting the number of beta radiations emitted per minute per gram of the given material. By radiocarbon method one can date many a different type of organic and 3

4 inorganic materials as long as they consist of carbon. The datable organic materials are charcoal, pollen, woods, twigs, seeds, bones, antler, shells, fish remains, insect s remains etc. The inorganic materials are sediments and soils, ice core, metal casting ore, underground water pigment from wall painting and work arts etc.the ideal material for radiocarbon dating is the wood charcoal that is found in an archaeological site. Unaltered wood from dry sites, soot, grasses, dung (animal and human), well-preserved antler or tusk, paper, calcareous tufa formed by algae, lake muds (gyttja), parchment, peat, and chemically unaltered mollusc shells all contain enough C14 and can provide sample for dating. Unburned bone contains a substance called collagen, which is rich in carbon, and this can be extracted and dated. The amount of sample requirement for radiocarbon dating method is different in different laboratories. For example, in a laboratory at Cambridge the recommended sample requirement are as follows: charcoal grams; wood grams; shell grams; peat grams; bones grams, etc. While selecting a sample for dating, the important thing to be noted is that it has to be taken with great care in order not to contaminate it with more recent material. When Libby invented this method, he used a modified form of Geiger counter where he could measure carbon -14 per unit weight of carbon. Later on, with the use of developed and improved machinery the selected carbon sample is turned into gas and liquid form. The sample is not handled after it has been made ready for conversion to a gas. Thus the danger of radioactive contamination from the atmosphere has been eliminated. With the gasproportional counters, a much smaller sample is needed to make an age determination. The calculation of the age sample is done on the basis of the half-life value of carbon-14. Recently, C-14 counting is done with the help of mass spectrometry. With the original methods, the effective range of radiocarbon dating was about 30,000 years. With the more modern systems, 50,000 years is the 4

5 practical limit, but for samples of great importance an elaborate, expensive, and time-consuming process of isotopic enrichment can be carried out that will enable the scientist to ascertain the age of material as old as 70,000 years (Haring, de-vries, and de-vries 1958). The dates derived from such samples are, however, not absolute in terms of years. They simply indicate an interval of time within which the actual age most probably lies. This is expressed in terms of a date which is midway in this interval, with the limits indicated by a plus or minus probable error. Thus a typical C14 date might read ± years B.P. This means that the date of the given specimen lies between 8600 to 8000 years B.P. Using the radiocarbon method as source of objective information, archaeologists are able to build Stone Age chronologies as well as established the primary chrono-cultural boundaries. Advantages and disadvantages: The main advantages of the C14 method are as follows i) It gives us the absolute chronology. ii) The material for dating is generally available in archaeological sites. iii) This method is low cost. iv) It has readily-accessible facilities. There are, however disadvantages also as given below i) The radio carbon method can adequately date organic materials up to 50,000 years old only. ii) The rate of which C14 is produced in the atmosphere has fluctuated considerably because of changes in the strength of the earth s magnetic fields and alteration in solar activity. iii) The materials dated by C12 typically are quite porous (easily contaminated by old/young carbon), iv) One must be very careful with pre-treatment and sample selection. v) The radiocarbon dates are expressed in BP and are accompanied by a statistical margin of error which corresponds to the measurement of a random phenomenon. The calibration is necessary to transform these results into calendar dates. The radiocarbon method for dating has proved itself useful when the laboratory uses all possible care and the archaeologist supplies material that has been collected with proper methods. Today, this method is the most widely accepted technique for studying the chronological relationship of archaeological complexes. 5

6 Potassium Argon Dating: This method is similar to carbon dating. The Potassium-Argon (K-R40) method is used by Geologists to date rocks which are hundreds or even thousands of millions of years old. It is also one of the most appropriate techniques for dating early human habitation sites in Africa, which can be up to 5 million years old. It is restricted to volcanic rock and not more recent than around 100,000 years. The earth s crust contains Potassium of which isotope K40 decays to A40 at a known rate. The ratio of Potassium-Argon may be measured to ascertain date of minerals and rocks in a deposit. Potassium is one of the most abundant chemical elements found in the earth crust. In many rock forming minerals, it is one of the major components. There are three naturally occurring isotopes of potassium namely- 39K, 40K, and 41K. That is the mass number of potassium is 39, 40, 41. It is found that 88.8 percent of 40K decays to 40Ca ( Ca with mass number 40 by emission of electron) beta particle and 11.2 percent of 40K decays to 40Ar by a process called electron capture. Therefore in a mineral rich with Potassium there seems to be a gradual decay of Potassium and subsequent growth of Ca40 and Ar40 as long as time passes. Principle: This method is able to cover a wide range of time even far greater than C14 method. Like the radiocarbon (C14) dating method Potassium- Argon dating method is based on the principle of radioactive decay. In this case, the steady but very slow decay of radioactive isotope Potassium-40 (K40) to the inert gas Argon 40 (Ar40) in volcanic rock. Knowing the rate of K40 its half life is around million of years a measure of the quantity of Ar40 trapped within a 10gram rock sample give an estimate of the date of the rock s formation. As with all radioactive methods, it is important to be clear about what sets the radioactive clock to zero. In this case, it is the formation of the rock through volcanic activity, which drives off any Argon formerly present. 6

7 The dates obtained in the laboratory are in effect geological dates for rock samples. Happily, some of the most important areas for the study of the Lower Palaeolithic, notably the rift valley in Africa are area of high volcanic activity. This means that archaeological remains often lie on geological strata formed by geologic action, and hence suitable for K- Ar40 dating. In addition these are often overlaid by comparable volcanic rock, so that dates for these two geological strata provide a chronological sandwich between the upper and lower slices of which the archaeological deposits are set. This method has also proved quite useful in dating some hominid fossils as employed in the site of Olduvai Gorge in east Africa where the remains were as old as 1.75 million years. K-Ar40 method has enabled the pre-historians to work out the chronology of human beings and their evolution. The earliest hominid forms go beyond 4 million years. Advantage and disadvantage: The advantage of the method is that it works well in case of the sites which are 500,000 years old. But the disadvantage of the method is that it can be applied to only to those rocks and minerals which are rich in potassium. Since those rocks are available only in volcanic areas, the method is restricted to those areas. Conclusion: The method which helps to determine date of a specimen in terms of years and thereby an event associated with the event is known as absolute dates. There are various types of absolute dating technique evolved during 1940 s onwards. Carbon dating and Potassium Argon dating are two important absolute dating techniques. With the radiocarbon dating the remnant radioactive carbon present in any organic or inorganic material found in archaeological site can be measured. Potassium argon dating method is based on the principle that potassium present in rocks or minerals decay in a constant rate to argon which can be measured. Carbon dating can date up to years while Potassium dating can date up to millions of years old specimens. Out of the two methods carbon dating is much more widely applicable method. The hominid fossil record is extremely fragmentary and chronology is essential in unraveling the complex record of human development. In applying the dating methods to a particular site, it is important to consider the purposes and limitations of absolute or chronometric dating as applied to prehistoric sites. With the development of the several absolute dating around the world, archaeologists are now able to construct a single, multiregional time frame for all hominid evolution. 7

8 In recent days, archaeologists have access to powerful tools for establishing chronometric ages of any site. There are considerable efforts which have been expended on constructing an absolute chronology for prehistoric sites that are too old for radiocarbon dating. Newer dating methods now exist and these methods can span the time range, they can also extend much further back in time, and are applicable to a wide variety of archaeological deposits. Most of the methods have been developed over the decades, and further developments and new methods are coming into play. Notably, we see improvements in the several methods from its invention till date. *** 8