Dating Techniques | curtains-and-drapes.info
Understand the difference between relative dating and absolute dating ships, fossils, and cultural materials to tell “relative ages” from one geologic unit. Contrast difference between absolute dating techniques, relative dating methods Research in archaeology terms chronometric dating methods of past cultures. They use absolute dating methods, sometimes called numerical dating This is different to relative dating, which only puts geological events in.
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. 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. 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 magnetism present in the clay is nullified once the pottery, bricks or klins are heated above degree centigrade. This implanted magnetism can be measured and the date of its firing estimated. The dating of ancient pottery by Thermoluminiscence measurements was suggested by Farrington Daniels of the University of Wisconsin in America 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. The glow emitted is directly proportional to the radiation it received multiplied by the years. It is present in nearly every mineral.
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. 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.
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. 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 timethe 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 ,, years ago.
Obsidian is a natural glass substance that is often formed as a result of volcanic activity.
Dating Methods in Prehistory
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. 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. Fluorine is found naturally in ground water. This water comes in contact with skeletal remains under ground.
When this occurs, the fluorine in the water saturates the bone, changing the mineral composition. Over time, more and more fluorine incorporates itself into the bone. By comparing the relative amounts of fluorine composition of skeletal remains, one can determine whether the remains were buried at the same time.
A bone with a higher fluorine composition has been buried for a longer period of time. Absolute dating is the term used to describe any dating technique that tells how old a specimen is in years.
These are generally analytical methods, and are carried out in a laboratory. Absolute dates are also relative dates, in that they tell which specimens are older or younger than others. Absolute dates must agree with dates from other relative methods in order to be valid. This dating technique of amino acid racimization was first conducted by Hare and Mitterer inand was popular in the s. It requires a much smaller sample than radiocarbon dating, and has a longer range, extending up to a few hundred thousand years.
It has been used to date coprolites fossilized feces as well as fossil bones and shells. These types of specimens contain proteins embedded in a network of minerals such as calcium. Amino acid racimization is based on the principle that amino acids except glycine, a very simple amino acid exist in two mirror image forms called stereoisomers. Living organisms with the exception of some microbes synthesize and incorporate only the L-form into proteins.
When these organisms die, the L-amino acids are slowly converted into D-amino acids in a process called racimization. The protons are quickly replaced, but will return to either side of the amino acid, not necessarily to the side from which they came. This may form a D-amino acid instead of an L—amino acid. The rate at which the reaction occurs is different for each amino acid; in addition, it depends upon the moisture, temperatureand pH of the postmortem conditions.
The higher the temperature, the faster the reaction occurs, so the cooler the burial environment, the greater the dating range. The burial conditions are not always known, however, and can be difficult to estimate.
For this reason, and because some of the amino acid racimization dates have disagreed with dates achieved by other methods, the technique is no longer widely used.
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.
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.
Principles of Prehistoric Archaeology. Chronology: Relative and Absolute Dating methods
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.
Dating Techniques In Archaeology
As the rocks cool, argon 40Ar begins to accumulate. Argon is formed in the rocks by the radioactive decay of potassium 40K. 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. The rate at which this process occurs is called the half-life.
This is the time required for half of the 14C to decay into 14N. The half-life of 14C is 5, years. This allows them to determine how much 14C has formed since the death of the organism.