Absolute dating methods (ANT)
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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 datingas use of the word "absolute" implies an unwarranted certainty of accuracy. In archaeology, absolute dating is usually based on the physical, chemical, and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates coins and written history.
Techniques include tree rings in timbers, radiocarbon dating of wood or bones, and trapped-charge dating methods such as thermoluminescence dating of glazed ceramics. In historical geologythe primary methods of absolute dating involve using the radioactive decay of elements trapped in rocks or minerals, including isotope systems from very young radiocarbon dating with 14 C to systems such as uranium-lead dating that allow acquisition of absolute ages for some of the oldest rocks on Earth.
Radiometric dating is based on the known and constant rate of decay of radioactive isotopes into their radiogenic daughter isotopes.
Particular isotopes are suitable for different applications due to the types of atoms present in the mineral or other material and its approximate age. For example, techniques based on isotopes with half lives in the thousands of years, such as carbon, cannot be used to date materials that have ages on the order of billions of years, as the detectable amounts of the radioactive atoms and their decayed daughter isotopes will be too small to measure within the uncertainty of the instruments.
One of the most widely used and well-known absolute dating techniques is carbon or radiocarbon dating, which is used to date organic remains. This is a radiometric technique since it is based on radioactive decay. Carbon moves up the food chain as animals eat plants and as predators eat other animals.
Relative and absolute dating archaeology
With death, the uptake of carbon stops. It takes 5, years for half the carbon to change to nitrogen; this is the half-life of carbon After another 5, years only one-quarter of the original carbon will remain. After yet another 5, years only one-eighth will be left. By measuring the carbon in organic materialscientists can determine the date of death of the organic matter in an artifact or ecofact. The relatively short half-life of carbon, 5, years, makes dating reliable only up to about 50, years.
The technique often cannot pinpoint the date of an archeological site better than historic records, but is highly effective for precise dates when calibrated with other dating techniques such as tree-ring dating.
An additional problem with carbon dates from archeological sites is known as the "old wood" problem. It is possible, particularly in dry, desert climates, for organic materials such as from dead trees to remain in their natural state for hundreds of years before people use them as firewood or building materials, after which they become part of the archaeological record.
Thus dating that particular tree does not necessarily indicate when the fire burned or the structure was built. For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating. The development of accelerator mass spectrometry AMS dating, which allows a date to be obtained from a very small sample, has been very useful in this regard. Other radiometric dating techniques are available for earlier periods.
One of the most widely used is potassium-argon dating K-Ar dating. Potassium is a radioactive isotope of potassium that decays into argon The half-life of potassium is 1. Potassium is common in rocks and minerals, allowing many samples of geochronological or archeological interest to be dated. Argona noble gas, is not commonly incorporated into such samples except when produced in situ through radioactive decay.
The date measured reveals the last time that the object was heated past the closure temperature at which the trapped argon can escape the lattice. K-Ar dating was used to calibrate the geomagnetic polarity time scale. Thermoluminescence testing also dates items to the last time they were heated. For example, since each Roman emperor had his own face stamped on coins during his realm, and dates for emperor's realms are known from historical records, the date a coin was minted may be discerned by identifying the emperor depicted.
Many of the first efforts of archaeology grew out of historical documents-for example, Schliemann looked for Homer's Troyand Layard went after the Biblical Ninevah-and within the context of a particular site, an object clearly associated with the site and stamped with a date or other identifying clue was perfectly useful. But there are certainly drawbacks.
Outside of the context of a single site or society, a coin's date is useless. And, outside of certain periods in our past, there simply were no chronologically dated objects, or the necessary depth and detail of history that would assist in chronologically dating civilizations.
Without those, the archaeologists were in the dark as to the age of various societies. Until the invention of dendrochronology.
The use of tree ring data to determine chronological dates, dendrochronology, was first developed in the American southwest by astronomer Andrew Ellicott Douglass. InDouglass began investigating tree ring growth as an indicator of solar cycles.
Douglass believed that solar flares affected climate, and hence the amount of growth a tree might gain in a given year.
His research culminated in proving that tree ring width varies with annual rainfall. Not only that, it varies regionally, such that all trees within a specific species and region will show the same relative growth during wet years and dry years.
Timing is Everything - A Short Course in Archaeological Dating
Each tree then, contains a record of rainfall for the length of its life, expressed in density, trace element content, stable isotope composition, and intra-annual growth ring width. Using local pine trees, Douglass built a year record of the tree ring variability.
Clark Wissler, an anthropologist researching Native American groups in the Southwest, recognized the potential for such dating, and brought Douglass subfossil wood from puebloan ruins. Unfortunately, the wood from the pueblos did not fit into Douglass's record, and over the next 12 years, they searched in vain for a connecting ring pattern, building a second prehistoric sequence of years.
Inthey found a charred log near Show Low, Arizona, that connected the two patterns. It was now possible to assign a calendar date to archaeological sites in the American southwest for over years. Determining calendar rates using dendrochronology is a matter of matching known patterns of light and dark rings to those recorded by Douglass and his successors.
Relative dating methods are unable to determine the absolute age of an object or event, but can determine the impossibility of a particular event happening before or after another event of which the absolute date is well known. In this relative dating method, Latin terms ante quem and post quem are usually used to indicate both the most recent and the oldest possible moments when an event. Jun 27, Relative dating is a less advanced technique as compared to absolute dating. In relative dating, mostly the common sense principles are applied, and it is told that which artifact or object is older than the other one. Most commonly, the ancient factors of the rocks or objects are examined using the method called stratigraphy. Our planet inherits a large number of artifacts and monuments bestowed upon us by older historic civilizations. These remains are subjected to dating techniques in order to predict their ages and trace their history. This ScienceStruck post enlists the differences between the Author: Prachi Patkar.
Dendrochronology has been extended in the American southwest to BC, by adding increasingly older archaeological samples to the record. There are dendrochronological records for Europe and the Aegean, and the International Tree Ring Database has contributions from 21 different countries. The main drawback to dendrochronology is its reliance on the existence of relatively long-lived vegetation with annual growth rings.
Secondly, annual rainfall is a regional climatic event, and so tree ring dates for the southwest are of no use in other regions of the world. It is certainly no exaggeration to call the invention of radiocarbon dating a revolution. It finally provided the first common chronometric scale which could be applied across the world.Dating methods in archaeology and paleoanthropology Part I
Invented in the latter years of the s by Willard Libby and his students and colleagues James R. Arnold and Ernest C.
Anderson, radiocarbon dating was an outgrowth of the Manhattan Projectand was developed at the University of Chicago Metallurgical Laboratory.
Essentially, radiocarbon dating uses the amount of carbon 14 available in living creatures as a measuring stick. All living things maintain a content of carbon 14 in equilibrium with that available in the atmosphere, right up to the moment of death. When an organism dies, the amount of C14 available within it begins to decay at a half life rate of years; i.
Start studying Archaeology Dating Methods. Learn vocabulary, terms, and more with flashcards, games, and other study tools. Method for relative dating of bones-Bones in ground absorb fluorine & uranium from groundwater Absolute Dating. Designation of the age of an event, object or fossil in terms of solar years (absolute time). Jan 10, Prior to the discovery of radiometric dating which provided a means of absolute dating in the early 20th century, archaeologists and geologists were largely limited to the use of relative dating techniques to determine the geological events. Absolute dating is the process of determining an approximate computed age in archaeology and geology. Relative dating determines the age of artifacts or site, as older or younger or the same age as others, but does not produce precise dates. Absolute dating, methods that produce specific chronological dates for objects and occupations, was not available to archaeology until well into the 20th century.
Comparing the amount of C14 in a dead organism to available levels in the atmosphere, produces an estimate of when that organism died. So, for example, if a tree was used as a support for a structure, the date that tree stopped living i.
The organisms which can be used in radiocarbon dating include charcoal, wood, marine shell, human or animal bone, antler, peat; in fact, most of what contains carbon during its life cycle can be used, assuming it's preserved in the archaeological record. The farthest back C14 can be used is about 10 half lives, or 57, years; the most recent, relatively reliable dates end at the Industrial Revolutionwhen humankind busied itself messing up the natural quantities of carbon in the atmosphere.
Further limitations, such as the prevalence of modern environmental contamination, require that several dates called a suite be taken on different associated samples to permit a range of estimated dates.
See the main article on Radiocarbon Dating for additional information. Over the decades since Libby and his associates created the radiocarbon dating technique, refinements and calibrations have both improved the technique and revealed its weaknesses. Calibration of the dates may be completed by looking through tree ring data for a ring exhibiting the same amount of C14 as in a particular sample-thus providing a known date for the sample.
Such investigations have identified wiggles in the data curve, such as at the end of the Archaic period in the United States, when atmospheric C14 fluctuated, adding further complexity to calibration.
One of the first modifications to C14 dating came about in the first decade after the Libby-Arnold-Anderson work at Chicago. One limitation of the original C14 dating method is that it measures the current radioactive emissions; Accelerator Mass Spectrometry dating counts the atoms themselves, allowing for sample sizes up to times smaller than conventional C14 samples.