Relative and Absolute Dating methods

Definition of Chronology

Types of Dating

• Relative dating fixes a time frame in relation to other strata or material and not in absolute dates in numbers.
• But it is difficult to know the total time span involved in the intervals between the things.
• absolute dating technique exhibit chronology in terms of years. precise and accurate dating.
• Relative dating, however, is still applied in those areas where the deposits containing organic materials cannot be dated by any absolute dating techniques.

1. Relative Dating:

1. Stratigraphy
2. Typology
3. Cross dating
4. Sequence dating
5. Fluorine, uranium and nitrogen analysis
6. Palaeontology
7. Palynology
8. Patination.

2. Absolute Dating

1. Carbon Dating
2. Potassium- argon Dating
3. Thermoluminescence
4. Dendrochronology
5. Electron Spin Resonance
6. Fission Track Dating
7. Palaemagnetic dating
8. Obsidian hydration
9. Varve analysis
10. Amino Acid Racemaization.

Distinction between Relative and Absolute Dating:

• Relative, inaccurate, disturbed by factors like tectonic shifts, chance, accidents.
• 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.

Stratigraphy -

• Stratigraphy is the analysis of a series of layers that exist in the horizontal dimension, studied in the vertical time dimension.
• principle that among the layers, the earlier deposit lies under later deposit.
• Therefore, whenever a stratigraphic sequence is observed during the excavation of a site, relative ages of the cultural levels can be worked out. The stratigraphic associations of artefact types within and between archaeological sites are regarded as one of the very important method of relative dating.
• Soan civilizarion dating Td 012345.
• Disturbed by tectonic shifts.

Typology -

• An archaeological site mainly consists of artefacts and tools. The tools found in the site are classified on the basis of their form or shape and accordingly different categories or types are made.
• These are then arranged from simple to elaborate or from poorly preserved to well preserved or from crude to refine etc. Then a relative antiquity is derived based on the presumption that simple, poorly preserved and crude tools are earlier than the elaborate, well preserved and refined ones.
• The typological method although was used for obtaining relative dating earlier, it has come in for much criticism now-a-days.
• According to Wheeler 'the values of typological classification are liable to be local rather than universal' but when the method is used in corroboration with stratigraphic method, it may become very helpful'.
• Doesn’t tell us about which species made the tools and there can be various cultures which used the same tools.
• Aurignacian. Solutrean typology.

Sequence Dating -

• In 1881 a relative dating method was developed by the great Egyptologist Sir Flinders Petrie in Egypt.
• The method is based on the fact that artefacts change in predictable ways through time. Among all other artefacts it is found that pottery changes with time frequently and a sequence of pottery design can thus be worked out.
• By detecting this trend of change, an archaeologist can trace associated cultural changes and make short term time distinction.
• While the excavation of grave was carried out in Egypt, Petrie found that the graves were associated with varied pottery. He analyse the features of pottery such as handles of pot and worked out a sequence showing their change, progressing from functional entities to mere decorations.
• The changes on pots were than correlated with other artefacts from graves and he finally ended with a series of numbered pottery stages that he labelled 'sequence dates'.
• This method was later on applied by the great anthropologist, Kroeber in determining the relative age of some sites in south western United States, Mexico and Peru.

Cross Dating -

• Cross dating involves the comparison of artefacts found in different stratigraphic levels. The method is based on the fact that the similar artefacts are approximately contemporary.
• The greater the similarity, the closer the ages. Certain artefacts like coins, pottery, arrowheads etc., which have limited occurrence in a given culture provide best sample for cross dating.
• When such type of objects are found in two different sites, according to cross dating sites are considered roughly the same age. Different pottery types which changes very frequently with time and hence occur for a very shorter period of time are better 'index fossil' or 'horizon marker' than others.

Fluorine, Uranium, and Nitrogen analysis:

• Prehistoric sites often consist of many bone remains. The basic principle of the method is that the longer a bone will be placed in soil, the more fluorine will be caught in and hence can suggest a relative date.
• All bones whether of animal or of human lying in the same level exhibit similar fluorine percentage in them. Therefore, if the quantity of fluorine remains same in both kinds of bone, it is sure that they belong to the same age.
• The bones acquired from a lower level show more fluorine in them whereas the bone remains coming from the upper level contains less fluorine. Relative ages of different bones at the same site thus can be established by measuring their fluorine contents.
• The method cannot provide an absolute age because the amount of fluorine differs from soil to soil, which gives a differential rate of absorption.
• Like fluorine, uranium or nitrogen content of the bones also can be measured. Since fluorine and uranium levels in the bone increase with time, while nitrogen decreases, such measurements helps to place in sequence the cultural phases with which the different bones may be associated.
• In fact the analysis of fluorine, uranium or nitrogen is regarded as one of the very important technique for relative dating.
• Disproved piltdown forgery.

Palaeontology -

• Palaeontology, the study of fossilised remains of bones in archaeological sites also provide relative dates. The method is based on the fact that some animals migrate or become extinct with the change of climate.
• That is, climate has a direct relationship with the presence or absence of certain animals. For example, if evidence for Elephas antiques (a forest elephant) is found, one can assume a temperate climate, while the presence of E. primigenous (a steppe elephant) indicates a steppes or tundra environment of almost glacial conditions.
• Likewise, it is found that in North America the arrival of man caused the extinction of mammals such as the mammoth, horse, camel and several species of bison.
• These can be dated approximately about 6000 B.C However, a margin of 1000 years error might be there as all of them have not become extinct at once and some have lived in isolated areas in which case the dating of fauna associated with other evidence is inexact and misleading.
• Smaller species of animals like rodents, birds, some molluscs and snails are found very sensitive to changes in climate than the larger mammals. In Northern Ireland it has been possible to show changes in coastal environment since the time of human occupation by studying changes in tidal - zone molluscs found in archaeological sites.

Palynology -

• Lennart Von Post, a Swedish Scientist, was the first to develop this palaeobotanical method in 1916.
• By this method a microscopic analysis of pollens extracted from trees are used to identify various trees and a pollen diagram is prepared.
• The pollen diagram in which relative frequencies of various species are plotted helps in tracing out the changing vegetation of an area.
• Acid peat or bog deposit is ideal sources of animal pollen, but dry sites, and clays contain enough pollen to provide a sequence. Pollens in soil underlying or overlying archaeological sites may be correlated with the already known regional pollen sequence and the age of the site thus can be dated.
• A very good example of application of pollen method is the archaeological site at Choukoutien in China.

Patination -

• There is no precise definition for the term patination though it generally means chemical alteration of rock surfaces exposed to atmospheric conditions.
• The amount of patina on the stone is an index of its age valuable for relative placement of the stone artefact in the technological development.
• The chemical alterations of the stone are usually brought about by the action of iron oxides through time.
• The observation of the amount of patina on a stone may be used at sites where there is a long sequence and demonstrates that those tools which lie in the bottom level may have more patina than those in the upper levels.
• The different types of tools from the river gravels, terraces of rivers or lakes can be differentiated in the relative amounts of patina on the basis of which of the relative ages can be assigned on the artefacts.
• A.J.H. Goodwin who worked extensively on the patination in 1960 lists many variables involved in patina formation as well as different type of patination. That can be used fruitfully for the tools from stratified deposits.