Dating Techniques In Archaeology
Using relative and radiometric dating methods, geologists are able to answer the question: how old is this fossil?. Determining a series of relative dating is less susceptible to each other hand,. Absolute dating. Elated tags: paleontology, historical archaeology and fossils. Before the advent of absolute dating methods, nearly all dating was relative. The main .. Scientific dating techniques have had a huge impact on archaeology.
Cation-ratio dating is used to date rock surfaces such as stone artifacts and cliff and ground drawings. It can be used to obtain dates that would be unobtainable by more conventional methods such as radiocarbon dating.
Scientists use cation-ratio dating to determine how long rock surfaces have been exposed. They do this by chemically analyzing the varnish that forms on these surfaces.
The varnish contains cations, which are positively charged atoms or molecules. Different cations move throughout the environment at different rates, so the ratio of different cations to each other changes over time. By calibrating these ratios with dates obtained from rocks from a similar microenvironment, a minimum age for the varnish can be determined.
This technique can only be applied to rocks from desert areas, where the varnish is most stable.
Dating in Archaeology | The Canadian Encyclopedia
Although cation-ratio dating has been widely used, recent studies suggest it has potential errors. Many of the dates obtained with this method are inaccurate due to improper chemical analyses. In addition, the varnish may not actually be stable over long periods of time. Thermoluminescence dating is very useful for determining the age of pottery.
Electrons from quartz and other minerals in the pottery clay are bumped out of their normal positions ground state when the clay is exposed to radiation.
This radiation may come from radioactive substances such as uranium, present in the clay or burial medium, or from cosmic radiation. The longer the radiation exposure, the more electrons get bumped into an excited state. With more electrons in an excited state, more light is emitted upon heating. The process of displacing electrons begins again after the object cools.
Scientists can determine how many years have passed since a ceramic was fired by heating it in the laboratory and measuring how much light is given off. Thermoluminescence dating has the advantage of covering the time interval between radiocarbon and potassium-argon datingor 40,—, years. In addition, it can be used to date materials that cannot be dated with these other two methods. Optically stimulated luminescence OSL has only been used since It is very similar to thermoluminescence dating, both of which are considered "clock setting" techniques.
Minerals found in sediments are sensitive to light. Electrons found in the sediment grains leave the ground state when exposed to light, called recombination. To determine the age of sediment, scientists expose grains to a known amount of light and compare these grains with the unknown sediment. This technique can be used to determine the age of unheated sediments less thanyears old. A disadvantage to this technique is that in order to get accurate results, the sediment to be tested cannot be exposed to light which would reset the "clock"making sampling difficult.
The absolute dating method utilizing tree ring growth is known as dendrochronology. It is based on the fact that trees produce one growth ring each year. The rings form a distinctive pattern, which is the same for all members in a given species and geographical area.
The patterns from trees of different ages including ancient wood are overlapped, forming a master pattern that can be used to date timbers thousands of years old with a resolution of one year. Timbers can be used to date buildings and archaeological sites. In addition, tree rings are used to date changes in the climate such as sudden cool or dry periods.
Dendrochronology has a range of one to 10, years or more. As previously mentioned, radioactive decay refers to the process in which a radioactive form of an element is converted into a decay product at a regular rate. Radioactive decay dating is not a single method of absolute dating but instead a group of related methods for absolute dating of samples. Potassium-argon dating relies on the fact that when volcanic rocks are heated to extremely high temperatures, they release any argon gas trapped in them.
As the rocks cool, argon 40Ar begins to accumulate. Argon is formed in the rocks by the radioactive decay of potassium 40K.
Dating in Archaeology
The amount of 40Ar formed is proportional to the decay rate half-life of 40K, which is 1. In other words, it takes 1. This method is generally only applicable to rocks greater than three million years old, although with sensitive instruments, rocks several hundred thousand years old may be dated.
The reason such old material is required is that it takes a very long time to accumulate enough 40Ar to be measured accurately. Potassium-argon dating has been used to date volcanic layers above and below fossils and artifacts in east Africa. Radiocarbon dating is used to date charcoal, wood, and other biological materials. The range of conventional radiocarbon dating is 30,—40, years, but with sensitive instrumentation, this range can be extended to 70, years.
Radiocarbon 14C is a radioactive form of the element carbon. It decays spontaneously into nitrogen 14N. Plants get most of their carbon from the air in the form of carbon dioxideand animals get most of their carbon from plants or from animals that eat plants.
Relative to their atmospheric proportions, atoms of 14C and of a non-radioactive form of carbon, 12C, are equally likely to be incorporated into living organisms.
When the organism dies, however, its body stops incorporating new carbon. The ratio will then begin to change as the 14C in the dead organism decays into 14N. 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. 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. 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.HIST 1111 - Prehistory: Dating Methods
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. The ratio of potassium to Argon may be measured to ascertain date of minerals and rocks in a deposit. This method is able to cover a wide range of time even far greater than C method because, the half life of the radioactive potassium is million years.
The method has 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.
The advantage of the method is that it works well in case of the sites which areyears 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. Therefore the method is restricted to the areas where volcanic rocks rich in potassium are available.
Aitken and co- workers. Initially designed to date archaeological ceramics, it was subsequently extended to other mineral materials, such as burnt flint. This is based on the fact that objects such as pottery that have been heated in the past can be dated by the measurement of their Thermoluminescence TL glow. Thermoluminiscence TL is the emitted light in the pottery which can be measured. If the ground up pottery is reheated, it emits light. The phenomenon results from radio-active influence of the metallic elements like uranium and potassium present in the clay and surrounding soils.
By the use of Thermoluminescence TL dating methods and the results obtained could make it possible to provide a new chronological framework for archaeological and anthropological knowledge. For example, the new chronology based on Thermoluminescence TL dating enabled in revising some prior assumptions about the evolution of lithic industries and the nature of hominids present in the Near East at various stages of the Middle Palaeolithic. Dendrochronology - The age of wooden objects can be determined by means of Dendrochronology or tree ring analysis.
It determines the calendar years of tree-ring formation and the felling dates of trees, which helps to determine the age of wooden objects with a great precision.
Dendrochronology has therefore become well established in the field of archaeology, art history and cultural heritage. The method depends on the fact that trees growing in temperate zones have clearly defined annual rings of growth.
As these tree rings represent annual growth, merely by counting rings one can count the age of the tree and hence its association. This dating method with latest methodological advances helps us in defining the calendar year in which the tree-rings were formed and in interpreting such dating in terms of the age of a wooden object. Despite many difficulties found for ESR dating of bones and carbonates, tooth enamel dated by Electron Spin Resonance ESR has been proven as a reliable method in its application to fossil teeth and quartz.
Radiocarbon dating, however, can only be used for dating objects that are less than 50 years. Dendrochronology Dendrochronology is a method that studies the rings of tree trunks to define characteristic sequences by analyzing the morphology of growth rings for a given species.
This method is based on the principle that the variation in tree growth from one year to another is influenced by the degree of precipitation, sunshine, temperature, soil type and all ambient conditions and that, consequently, reference patterns can be distinguished. Several sets of rings from different trees are matched to build an average sequence. Subsequently, overlapping series of average sequences from trees that died at different times and come from various sources ie, the wood of historic buildings, archaeological and fossil woods are used to build a chronological sequence covering several hundred years which becomes a reference.
Finally, absolute dating is obtained by synchronizing the average sequences with series of live and thus datable trees and thus anchors the tree-ring chronology in time. Dendrochronology mainly uses softwood species that are sensitive to changes in growth conditions, while hardwoods show rather little variation in ring width. This method provides very accurate dating, sometimes to the nearest year.
It is especially used to develop calibration curves used to correct data obtained from radiocarbon dating, a technique that remains imprecise due to fluctuations in the concentration of carbon 14 in the atmosphere over the centuries. Thermoluminescence Thermoluminescence uses the phenomenon of ionizing radiations that naturally occur in the atmosphere. This technique relies on a unique physicochemical property of certain minerals especially quartz and feldspar that have an imperfect structure and therefore retain radioactive elements in the natural environment.
When these minerals are heated while a pot is being baked during the occupation of an archaeological site, for instance, the traps formed by their crystal structure are emptied and the clock is reset to zero. Subsequently, the total flow rate of irradiation paleodose since the reset is calculated by heating the specimen once more, and this result is then compared to the annual input recorded by a dosimeter installed on the archaeological site where the object being dated was found.
Thermoluminescence is a technique that requires complex manipulation. To obtain a date for a single pottery sample, it is necessary to perform a laboratory fractionation of the clay mineral used in the manufacture of the pottery and prepare nearly 75 sub-samples; some of these are heated to release the level of thermoluminescence, while others receive a radiation dose to measure their sensitivity to radiation.
Thermoluminescence can replace radiocarbon dating to date events that occurred more than 50 years ago; it is used mainly for dating stone fireplaces, ceramics and fire remains. Aitken editorsChronometric Dating in Archaeology ; W. Adams, Archaeological Typology and Practical Reality: Harris, Principles of Archaeological Stratigraphy