Chronology and dating methods - Wikibooks, open books for an open world
According to the Dictionary of Anthropology, the word chronology means the Unlike relative dating, absolute or chronometric dating is primarily sought to. Absolute dating is the process of determining an age on a specified chronology in archaeology and geology. Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty of accuracy. Absolute dating provides a numerical age or range in contrast with relative. 1. Paleoanthropology. Fossil Man and Fossil Men. Relative dating methods. • Stratigraphy: based on superposition of geologic and cultural deposition.
It cannot be used to accurately date a site on its own. However, it can be used to confirm the antiquity of an item. Optically stimulated luminescence OSL [ edit ] Optically stimulated luminescence OSL dating constrains the time at which sediment was last exposed to light.
During sediment transport, exposure to sunlight 'zeros' the luminescence signal. Upon burial, the sediment accumulates a luminescence signal as natural ambient radiation gradually ionises the mineral grains. Careful sampling under dark conditions allows the sediment to be exposed to artificial light in the laboratory which releases the OSL signal. The amount of luminescence released is used to calculate the equivalent dose De that the sediment has acquired since deposition, which can be used in combination with the dose rate Dr to calculate the age.
Dendrochronology The growth rings of a tree at Bristol ZooEngland. Each ring represents one year; the outside rings, near the bark, are the youngest. Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree rings, also known as growth rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year.
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.Anthropology optional for UPSC - Dating methods like relative and absolute dating for finding age
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.
What Is Chronometric Dating? | Sciencing
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. Both of the latter materials have allowed dating of Early and Middle Pleistocene sites which are not datable using other methods.
In particular, recent discoveries of human remains in Western Europe have been proposed to be sites of the earliest arrival of humans there, and have been dated to the Early Pleistocene by Electron Spin Resonance ESR using quartz and tooth enamel. Electron Spin Resonance ESR method can be applied to different types of samples in various environments; its contribution to the elaboration of a chronostratigraphic frame is of a great importance for the understanding of the Homo erectus dispersals out of Africa and especially for the first settlements in Europe.
Palaeomagnetic Dating - It is an important means of crosschecking the dates based on the constantly shifting nature of the earth magnetic field, both in direction and intensity.
The measurement of the earth's magnetic field in several places of the world for centuries has shown that it varies with time. A number of studies have shown that a record of past magnetic field in the form of angles of declination and dip can be trapped in baked clay. When clay is heated to a certain degree, the magnetic elements of baked clay realign themselves along lines dictated by the intensity and character of the magnetic field of the earth at that time. On cooling the magnetic elements are frozen and can be recorded as long as the clay is preserved.
This is called remnant magnetism. When records of past angles of declination and dip have been kept it is possible to compare the values of historic records and arrive at the date of archaeological specimens of fired clay. Fluxgate, Spinner magnetometer, Super-conduction magnetometer are the instruments used for measuring magnetic remenence. The method was used in Great Britain by Aitken in detail. It has also been used in Japan and Arizona. In India the method is applied in dating Karewa sediments in Kashmir.
The reliability of the application of the method depends on certain conditions such as The availability of good records of change of magnetic paths near the prehistoric sites, Occurrence of series of already dated baked clay in the area against which objects of unknown dates can be dated, Availability of archaeological samples which are found in their place of first occurrence.
Fire places and kiln thus provide best samples for dating. Varve analysis - Varve analysis, one of the oldest dating methods which demonstrate seasonal variation and also reflect the climatic conditions of ancient time.
The word varve in Swedish means annual layers of sediments deposited at the bottom of the lakes by the runoff from melting glacial ice. The method is based on the relative thickness of the varves and their comparison to the new sections as in tree ring analysis. Formation of varves depends on climatic variation. In summer when ice melts coarse sediments deposits at the bottom and in winter when the lake is frozen, the finer sediments deposit at the top. It is possible to measure the relative thickness of the varves and obtain a series to which one can compare and correlate new sections as they are discovered.
The application of varve dating is restricted by several factors. First, it is because varve accumulation occurs only to glacial areas of the world.
Second, many of the Pleistocene glacial areas has receded nowadays and affecting the supply of sediments. Therefore outside Scandinavia it is difficult to find continuous sequence of varves reaching the present. The longest sequence known goes back only 17, years. Third, the varves may form frequently rather than annually depending on the pattern of the melting. However, instead of the limitations, varve analysis can be used indirectly for archaeological dating.
In North America, Ernst Antevs has made several attempts to relate Pleistocene geological formations in the American Southwest to events that produced varves in the northern parts of North America. Fission Track Technique - This technique dates material ranging from 20 years to 1,, years before the present. Fission track dating is based upon the fact that over geological time spontaneous fission of uranium impurities produces minute sub- microscopic damage trails in the most insulating solids, both long period of time.
As the damage sites are permanent, a simple count of their number allows an age to be measured. The density of track depends on the uranium content as well as the age of the sample i. The sample is immersed in the proper chemical reagent, which allows rapid attack of the narrow regions of damage and slower dissolution of the adjacent unchanged material. Such treatment are used to create etch pits of optical size, each one making a single fission site. The method requires sufficient uranium to produce a track density, which can be counted within reasonable time.
Amino Acid Racemisation Technique Amino Acid Racemisation, a method of absolute dating applied to human fossils is developed very recently. This is based on the fact that all living organisms have L-amino acid in their protein and after death, and over a long period of time all the L-amino acids except glycine undergo change called racemisation and became non protein D-amino acid.
The proportion of these D-amino acids increases with time. Thus the age of the skeletal materials that are found in archaeological sites can be estimated by determining the amount of change, racemisation that has occurred. Amino Acid Racemisation AAR is a useful comparative dating method with great potential, but it introduces problems of calibration and the requirement of an intimate knowledge of the palaeoenvironmental conditions of the bone deposition site.
They are called chronometric because they allow one to make a very accurate scientific estimate of the date of an object as expressed in years. They do not, however, give "absolute" dates because they merely provide a statistical probability that a given date falls within a certain range of age expressed in years.
Relative Vs. Chronometric dating in Archaeology ?
Chronometric methods include radiocarbon, potassium-argon, fission-track, and thermoluminescence. The most commonly used chronometic method is radiocarbon analysis.
It measures the decay of radioactive carbon 14C that has been absorbed from the atmosphere by a plant or animal prior to its death. Once the organism dies, the Carbon begins to decay at an extremely predictable rate. Radioactive carbon has a half-life of approximately 5, years which means that every 5, years, half of the carbon will have decayed. This number is usually written as a range, with plus or minus 40 years 1 standard deviation of error and the theoretical absolute limit of this method is 80, years ago, although the practical limit is close to 50, years ago.
Because the pool of radioactive carbon in the atmosphere a result of bombardment of nitrogen by neutrons from cosmic radiation has not been constant through time, calibration curves based on dendrochronology tree ring dating and glacial ice cores, are now used to adjust radiocarbon years to calendrical years.