DATING METHODS IN ARCHAEOLOGY
Archaeological investigations have no meaning unless the chronological sequence of the events are reconstructed faithfully. The real meaning of history is to trace the developments in various fields of the human past. Towards this end, while investigating the past cultures, archaeology depends on various dating methods. These dating methods can broadly be divided into two categories, i.e. 1 Relative dating methods and 2) Absolute dating methods.
RELATIVE DATING METHODS
This dating method is also known as “Archaeological Dating” or “Historical Chronology”. These are mainly non-scientific dating methods. These methods were relied on especially prior to the introduction of scientific methods of dating. But, even when the scientific methods of absolute dating are available, this method of dating has not lost its importance, as many a time we have to depend solely on relative dating. Even when the absolute dates are available, we have to supplement the information with relative dating. The various methods of relative dating are;
This method depends on the common observation that the height of the habitational area increases as the people continue to live at the same place. The deposit thus occurring forms layers depending on the nature of the material brought in by the people inhabiting the area. According to this method, the upper deposits are younger and the lower deposits are older. Basing on this principle, the cultural assemblages found in different layers can be assigned a chronological personality. For example, if the cultural contents of the lower deposit are Mauryan in character, appropriately this deposit may be assigned a date between 400-200 B.C. Similarly, if the cultural equipment of the upper deposit are of the Sunga period, this deposit has to placed between 200-73 B.C.
This method, however, has some disadvantages. In practice it is found that the mounds are disturbed9+ by all sorts of pits and dumps. Quite often, the archaeologist decided the change of stratum on the basis of the “feed” of the deposit. In such cases subjective element cannot be ruled out. But, for a single culture site the method is quite reliable.
Quite convincing dates are sometimes arrived at by importing parallels from other contemporaneous cultures. This parallelism is formed due to trade relations, particularly wehen trade followed in both directions. For example, beads closely resembling those from the temple repositories at Cnoss and dating from C.1600 B.C were found in a late context (Period V) at Harappa. This closely agrees with the fact that the seals from Indus Valley style from Ur, Kish and Tell Asmar and other sites fall within the range of 2500-1500 B.C. in terms of Mesopotamian dates. By noting the association of these beads it has been possible to trace a archaeological datum line across Indian sub-continent and Mesopotamia.
When a group or type of objects are found together under circumstances suggesting contemporanity they are said to be associated. It is nearly always association with other phenomena that gives a first clue as to the use, the age and chronological attribution of a potential datum, but age at least may sometimes be inferred from position in a geological deposit or a layer of peat. Once a type has been classified by the aid of its context further specimen of the same type, even when found in isolation, can be assigned their place in terms of dates. Association in simplicity can be illustrated by an example, at the port of Arikamedu near Pondicherry. Antiquities and potteries of Roman origin were found in association with the finds of Indian origin. With the aid of these dated imports it was possible to date the associated Indian objects to the first two centuries A.D. Similarly NBP and PGW has helped in fixing the chronology of numerous sites.
Sometimes dates are also obtained with the assistance of astronomy. Clay tablets found in West Asia contain inscriptional evidence with regard to the occurrence of a solar eclipse. Specialists in the field of astronomical research placed this solar eclipse as on 15th June 763 B.C. The Almanac belonging to Maya culture of Central America helped in dating several important sites accurately on the basis of astronomy. Astronomical data have been applied in the study of geological ice ages by calculating the curves for major fluctuation of solar radiation. Consequently, the chronology worked out for the geological deposits helped in dating the prehistoric tools found in these deposits.
DIAGNOSTIC TRAILS AND
There are certain antiquities and potteries which by themselves have acquired a dating value. Whenever and where ever such antiquities are found, associated finds are automatically dated. For example Mauryan terracottas, Roman amphorae, Harappan pottery, NBP , PGW,etc. have obtained such dating value because of their distinct characteristics.
THICKNESS OF DEPOSIT AND DATING:
Sir Flinders Petrie had worked out a formula for dating the finds on the basis of the thickness of the deposit. According to him a period of hundred years may be granted for the accumulation of a deposit of one and a half feet. However this formula has not been accepted by many archaeologists. Further this formula has been disproved on many occasions.
Pottery is probably the most abundantly available antique material in any
archaeological site. The types and forms or shape of these pots and also the
antiquities under go evolutionary changes in cource of time,
and thus contribute towards our knowledge of the chronological
asssignments. It is based on the
simple fact that industrial and art forms and for that matter all objects are
subject to evolutionary process. Typological
evolution may take either the form
of improvement or the form of devolution or degeneration.
Working out these changes brought in during the evolutionary process
helps us in fixing chronological slots for different shapes.
Examples may be cited from Greek archaeology where even the shapes of the
pots have been appropriately and approximately dated.
Similarly the forms of the pots of the Maya culture and the shapes and
decorations on the Chinese potteries have been so adequately dated that their
relative dating value is immense. But
in India though the variety of wares are satisfactorily dated the typological
evolution is yet to be worked out.
8. ARCHITECTURE AND DATING
Houses, publica buildings and places of worship are also quite useful in
providing relative dating. It is a
common knowledge that the building and the building material of Harappan
architecture are quite characteristic. Again
the architectural feature of the building of th4e Sultanate period, Mughal
period, Maratha period are quite distinct and have, therefore, accordingly been
assigned different dates. Size of
the bricks, shapes of the pillars, features of the capitals, pattersns of
decorations and styles of paintings -
all contribute towards relative datingin archaeological research.
ABSOLUTE OR SCIENTIFIC DATING METHODS
1. RADIOCARBON OR C-14 DATING:
This is one of the most important methods of dating the ancient objects which contain some carbon in them. This method was discovered by Prof. Willard F. Libby in 1946, which won him Noble Prize in Chemistry. This method has achieved fame within a short time largely because it provides chronology for the prehistoric cultures, when we do not have written records.
Explanation of this Therory:
This method is based on the presence of radio-active carbon of atomic weight 14 in organic matter. Cosmic radiation produces in the upper atmosphere of the earth Neutron particles, some of which hit the atoms of ordinary Nitrogen. This is captured by the nucleus of the nitrogen atom, which gives off a proton and thus changing to Carbon-14. This Carbon-14 in turn is radio-active and by losing an electron reverts to nitrogen. This creation of new carbon atoms and then reverting to nitrogen has achieved a state of equilibrium in the long duration of the earths existence. C-14 along with the carbondioxide enter the living organisms in the process of photosynthesis, and all the radiocarbon atoms that disintegrate in living things are replaced by the C-14 entering the food chain. Thus the process of radio carbon present in the living organism is same as in the atmosphere.
It is further assumed that all living animals derive body material from
the plant kingdom, and also exhibit the same proportion of C-14 material.
Therefore as soon as the organism dies no further radiocarbon is added. At that
time the radioactive disintegration takes over in an uncompensated manner. The
C-14 has a half-life of about 5730 years, i.e. only half the C-14 will remain
after the half-life period. In the disintegration process the Carbon-14 returns
to nitrogen emitting a beta particle in the process. The quantity of the C-14
remaining is measured by counting the beta radiation emitted per minute per gram
of material. Modern C-14 emits about 15 counts per minute per gram, whereas
Carbon-14 which is 5700 years old, emits about 7.5 counts per minute per gram.
for C-14 Dating:
Specimens of organic material which can yield good amount of carbon can be collected for C-14 dating. For example charcoal, wood, shell, paper, leaves, cloth, animal hair, bone, pollen, tooth, iron, prehistoric soot from the ceiling of the caves – practically any material containing some carbon – can be subjected to C-14 dating.
Quantity of Samples:
Quantity of samples sent for radicarbon dating should be sufficient enough to give proper results. It has been found that a sample yielding five grams of pure carbon is generally sufficient. As quite a bit of sample is lost in the pre-pigmentation process one should try to collect as big sample as possible.
Charcoal 8-12 gm.(Desired) 1 gm.(Minimum)
Wood 10-30 3
Shell 50-100 5
Bone 100-500 200
Iron (Cast iron) 100-150 30
Iron (Steel) 300-500 150
Bones are generally affected by ground water carbonates and are therefore
least reliable for dating. Charred bones are better preserved and are therefore
relatively more reliable. Charcoal is best material specially if derived from
short live plants.
D. How to
While collecting samples for radio carbon dating we should take utmost care, and should observe the following principles and methods.
1. Sample should be collected from and undisturbed layer. Deposits bearing, pit activities and overlap of layers are not good for sampling.
2. The excavator himself should collect the sample from an undisturbed area of the site which has a fair soil cover and is free of lay water associated structures like ring wells and soakage pits.
3. Samples which are in contact or near the roots of any plants or trees should not be collected because these roots may implant fresh carbon into the specimens.
4. Handling with bare hands may add oil, grease, etc to the sample. Therefore, it is better to collect samples with clean and dry stainless steel sclapels or squeezers. It may also be collected with the help of glass.
5. Stainless steel, glass, polythene and aluminium are free from carbonatious organic material. Therefore sampling should be done with such material only.’
6. Samples should be sundried before pacing in aluminium thin foils and placed in a glass jar or secured safely in thick polythene covers. Before pacing the soil should be removed while it is wet at the site./ As far as possible pure sample material alone should be collected and sent to the laboratory.
7. A small card should be attached to the pacing showing the details regarding the name of the sample and date of its pacing.
E. Method of Sample
Before removing the sample from the site we should note down the data or
the environment of the sample. We have to fill the data sheets, which should be
done at the time of sampling and should be submitted along with the sample to
the dating laboratory. These sheets require data on environment and stratigraphy
of the sample, and archaeological estimates of its dating. This data help in
obtaining and objective interpretation of dates.
F. Limitation and Errors of C-14 Dating:
There are a number of technical difficulties inherent in this method of dating. Some of the main difficulties in C-14 dating are;
1. The first difficulty is that the quantity required for a single determination is comparatively large. It will be difficult to obtain sufficient quantities of samples, especially in the case of valuable museum specimens.
2. The second difficulty is that the radio active decay does not take place at a uniform rate but is a random process, and is therefore, governed by the laws of statistical probability.
3. The third and most important difficulty is that, the initial ratio of C-14 to C-12 is very small and difficult to measure with precision.
4. Another difficulty that has to be taken into serious consideration is the possibility of uneven distribution of radio carbon in organic matter.
5. If the specimen is analyzed after having been exposed to contamination by carbon compounds of an age younger than its own, radio carbon age is liable to be reduced. The best results can be obtained from specimens, which were preserved under very dry conditions, or even enclosed in rock tombs of the like.
6. Very dangerous contamination is done, very often, by the growth of fungus and bacteria on the surface of the specimen which even when removed from the specimen may falsify its actual age.
Though there are some drawbacks and technical difficulties, the radiocarbon method is a reliable, efficient and most useful method of dating the archaeological specimens. We are helpless in the case of contamination done by the natural agencies in the past, but we can overcome most of the difficulties by paying sufficient care and attention while collecting the samples. It is the duty of an archaeologist to study with care the condition of preservation of specimens submitted for analysis and, in fact, to submit only specimens that can be regarded as fool-proof as is possible in the circumstances.
Dendrochronology is a method that uses tree-ring analysis to establish chronology. A major application of dendrochronology in archaeology, as a tool for establishing dates from the samples of wood and articles made out of wood is not only in working out primary chronologies but also in cross checking the already known dates by other methods. This method makes it possible to date individual ruins to within a year , or even a season in which they were built. Often, the tree-ring analysis from a site can give strong clues about the length of occupation, certain periods of building or repair activities at the site. Another application of tree-ring analysis is the inference of past environmental conditions, which is extremely useful to the archaeologists.
The modern science of dendrochronology was pioneered by A.E.Douglass in 1904. Tree ring analysis is based on the phenomenon of formation of annual growth rings in many trees, such as conifers. These rings are shown by the trees growing in regions with regular seasonal changes of climate. As a rule trees produce one ring every year. The annual ring is formed by the `cumbium’ which lies between the old wood and bark. When growing season (rainy season) begins, sets of large, thinly-walled cells are added to the wood. As the season advances towards the end of the season, the cells added to the wood become increasingly smaller and more thickly walled. This process repeats in the following years also.
The formation of rings is affected by drought and prosperous seasons. In the years with unfavourable weather the growth rings will be unusually narrow. On the other hand, during years with exceptionally large amounts of rain the tree will form much wider growth rings. Most of the trees in a give area show the same variability in the width of the growth rings because of the conditions they all endured. Thus there is co-relation between the rings of one tree to that of another.
Further, one can correlate with one another growth rings of different trees of same region, and by counting backwards co-relating the inner rings of younger trees with the outer rings of older trees we can reconstruct a sequence of dates.
Scientists have prepared a sort of calendar for the last three thousand yeas. By comparing a sample with these calendars or charts we can estimate the age of that sample. Thus it is possible to know the age of the wood used for making furniture or in the construction work.
The main disadvantage with the system is that, we require a sample showing at least 20 growth rings to make an objective estimation of its age. Hence smaller samples cannot be dated. This method can date the sample upto the time of cutting the tree, but not the date when it was actually brought into use. Still more serious defect is that, the system is liable to give earlier dates, when the wood from the inner core of the trunk is used.
3. ARCHAEOMAGNETIC DATING: This method is based on the fact that the magnetic field of the earth is changing constantly in direction and proporationate intensity, and that these changes lead to measurable records. The magnetic waves present in the earth implant magnetism to the buried objects in the form of thermo-remnant magnetism. The magnetism present in the clay is nullified once the pottery, bricks or klins are heated above 500 degree centigrade. Fresh magnetism corresponding to the magnetic field of the earth at the time of object’s firing is imparted, which remains permanently. This implanted magnetism can be measured and the date of its firing estimated.
THERMOLUMINISCENCE DATING OF
The dating of ancient pottery by Thermoluminiscence measurements was suggested by Farrington Daniels of the University of Wisconsin in America (1953). This method can date ancient pottery with an accuracy of plus or minus 10%.
Thermoluminescence is the release in the form of light of stored energy from a substance when it is heated. All ceramic material contain certain amounts of radioactive impurities (uranium, thorium, potassium). When the ceramic is heated the radioactive energy present in the clay till then is lost, and fresh energy acquired gradually depending on the time of its existence. The thermoluminescence observed is a measure of the total dose of radiation to which the ceramic has been exposed since the last previous heating, i.e. in the klin. For calculating dates the sample is heated upto 5000C and thermoluminiscence observed as a glow is measured with very sensitive instruments. The glow emitted is directly proportional to the radiation it received multiplied by the years.
4. POTASSIUM – ARGON DATING: Potassium (K) is one of the elements that occur in great abundance in the earth’s crust. It is present in nearly every mineral. In its natural form potassium contains a small fraction of radio-active material. For each 100 radio-active potassium atoms that decay 89% form calcium and 11% become Argon, one of the rare gases. During rock formation, especially lava, tuffs, pumice, etc. Virtually all argon that had accumulated in the parent material will escape. The process of radio-active decay of potassium continues and the argon accumulated again which when measured will give a clue as to the age of the rock. This method has dated samples which are 4.5 billion years to 2500 years old.
The application of this method to archaeology depends on locating the widespread distribution of localities that have recently (in the last half-million years) experienced volcanic activity forming layers over the culture-bearing deposits. The city of Pompeii in Italy is a good example of the destruction caused by volcanic activity. This method is more useful in dating the prehistoric sites. The starting phase of the Palaeolithic period in India is pushed back by atleast one million years from the earlier dating of about 5 lakh years B.Cto 1.4 million years B.C. This unique example comes from a sit known as Bori in Maharashtra, where it was found that a layer yielding flake tools is overlain by a layer of volcanic ash. When this ash was subjected to Potassium-Argon dating it yielded a date of 1.4 million years, thus suggesting that the tools found below this ash are older than 1.4 million years.
5. FISSION TRACK DATING: This method has been jointly developed by Fleischer, Price and Walker. Initially this method was developed to date the meteorites and other extra-terristrial objects, but it is now being applied to archaeological purposes as well.
It is known that may minerals and natural glasses (obsidian, tektites) contain very small quantities of uranium. Through time , the uranium undergoes a slow spontaneous process of decay. This method of dating depends upon the measurements of detectable damage called tracks in the structure of glasses caused by the fission. These tracks disappear when the glass is heated above a critical temperature and fresh tracks formed in course of time. The fresh tracks are counted to date the sample. This method is suitable for dating objects which have undergone heating process some 100,000-1,000,000 years ago.
6. OBSIDIAN HYDRATION DATING: Obsidian is a natural glass substance that is often formed as a result of volcanic activity. Prehistoric man was impressed by the naturally sharp edges produced when a piece of obsidian was fractured, and hence, preferred the material in tool making. The dating of obsidian artifacts is based on the fact that a freshly made surface of obsidian will absorb water from its surroundings to form a measurable hydration layer. The surface of obsidian has a strong affinity for water as is shown by the fact that the vapour pressure of the absorption continues until the surface is saturated with a layer of water molecules. These water molecules then slowly diffuses into the body of the obsidian. The mechanical strains produced as a result throughout the hydrated layer can be recognized under polarized light.
Each time a freshly fractured surface is prepard on a piece of obsidian, the hydration process begins afresh. The absorption takes place at a steady rate. The water content increases with time. Measurement of the hydrated layer gives a clue as to the age of the object under test.
7. FLOURINE DATING: This method is adopted to date the ancient bones. The elements flourine in the form of fluorides occurs in the ground water. This fluoride is gradually absorbed and fixed in the phosphatic mineral matter of bones and teeth buried in the soil. The fluorine content of fossil bones increases with the passage of time, but at rate which varies from sit to sit, depending on the hydrological conditions, climate, type of matrix and amount of fluorine in circulation.
The fluorine method is most suitable for the relative dating of bones in gravelly or sandy alluvial deposits in temperate regions. This method is useful when the containing deposit is alluvial clay, but it is of no use in cave earth or volcanic soil. Its usefulness is limited to distinguishing modern from prehistoric and prehistoric from Pleistocene like that.
The fluorine content of a
specimen may vary with the texture or type of material that is sampled. Spongy
bones absorb more fluorine than compact or harder bones. The fluorine content of
the bones help in estimating the date by the chemical analysis, which requires
about 1/10 gram of
8. PALAEONTOLOGICAL METHOD:
This method depends on the changes that have occured in the body structure of some animals during the interglacial periods of the Pleistocene epoch. During the interglacial periods the climate changed from wet to dry and vice versa. These changes obviously effect the flora and fauna, which try to adapt to the condtions by subjecting some changes in the body structure. For example, during colder spells animals tend to develop mor fur. Traditionally the elephant, rhinoceros, bear, hyena, pig, bovid, deer and antelope have provided the index fossil. This method is applicable, especially, to Palaeolithic period, which has undergone the Pleistocene changes. Finds of index fossils in the tool bearing geological deposits are sufficient to date these deposits, and consequently the tools themselves.