Dating in Archaeology | The Canadian Encyclopedia
Archaeological scientists have two primary ways of telling the age of artefacts and the sites from which they came: relative dating and absolute. Article Index Relative dating includes methods that rely on the analysis of Absolute dating methods mainly include radiocarbon dating. Chronology: Relative and Absolute Dating methods According to the Dictionary of Anthropology, the word chronology means the science of computing dates.
Generally speaking, the more complex a poem or piece of pottery is, the more advanced it is and the later it falls in the chronology. Egyptologists, for example, created a relative chronology of pre-pharaonic Egypt based on increasing complexity in ceramics found at burial sites. Unlike observation-based relative dating, most absolute methods require some of the find to be destroyed by heat or other means. Certain unstable isotopes of trace radioactive elements in both organic and inorganic materials decay into stable isotopes.
This happens at known rates. By measuring the proportion of different isotopes present, researchers can figure out how old the material is. Here are some of the most common radiometric methods: Sometimes called carbon dating, this method works on organic material.
Both plants and animals exchange carbon with their environment until they die. Afterward, the amount of the radioactive isotope carbon in their remains decreases. Measuring carbon in bones or a piece of wood provides an accurate date, but only within a limited range. It would be like having a watch that told you day and night. Also called single crystal argon or argon-argon Ar-Ar dating, this method is a refinement of an older approach known as potassium-argon K-Ar dating, which is still sometimes used.
Both methods date rock instead of organic material. As potassium decays, it turns into argon. But unlike radiocarbon dating, the older the sample, the more accurate the dating — researchers typically use these methods on finds at leastyears old. While K-Ar dating requires destroying large samples to measure potassium and argon levels separately, Ar-Ar dating can analyze both at once with a single, smaller sample.
The uranium-thorium method is often helpful for dating finds in the 40, to ,year-old range, too old for radiocarbon but too young for K-Ar or Ar-Ar. Trapped Charge Dating Brosko Over time, certain kinds of rocks and organic material, such as coral and teeth, are very good at trapping electrons from sunlight and cosmic rays pummeling Earth.
Researchers can measure the amount of these trapped electrons to establish an age. But to use any trapped charge method, experts first need to calculate the rate at which the electrons were trapped.
This includes factoring in many variables, such as the amount of radiation the object was exposed to each year. These techniques are accurate only for material ranging from a few thousand toyears old — some researchers argue the accuracy diminishes significantly afteryears. 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. Description of different dating methods: Till the early part of 19th century quite a good number of relative dating methods have been used in archaeological studies. A short description of each of these methods is given below: Stratigraphy - Stratigraphy is the analysis of a series of layers that exist in the horizontal dimension, studied in the vertical time dimension.
The method is based on the basic geological 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.
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.
Everything Worth Knowing About ... Scientific Dating Methods
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'. 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.
Dating Methods in Prehistory
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. 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.
Principles of Prehistoric Archaeology. Chronology: Relative and Absolute Dating methods
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. 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.
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 B. C However, a margin of 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 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.
Dating Techniques In Archaeology
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.
Goodwin who worked extensively on the patination in 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. Carbon Dating - Radiocarbon dating is a chemical analysis used to determine the age of organic materials based on their content of the radioisotope of carbon The method was developed by Willard F.
Libby and a team of scientists at the University of Chicago.
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In Libby received the Nobel Prize for his method to use Carbon for age determinations in archaeology, geology, geophysics, and other branches of science. It subsequently evolved into the most powerful method of dating and Holocene artefacts and geologic events up to about 50, years.
By radiocarbon method one can date different types of organic or inorganic materials as long as they consist of carbon. The method is actually devised to measure the amount of low level radioactivity of carbon remaining in ancient and dead material of organic origin. Radiocarbon 14C dating is the most widely accepted technique for studying the chronological relationships of archaeological complexes.
Using the radiocarbon method as a source of objective information, we are able to build Stone Age chronologies as well as establish the primary chrono-cultural boundaries. The earth's crust contains potassium of which isotope K40 decays to A40 at a known rate.