- Dating techniques
- Dating Techniques
- ABOUT THE MAGAZINE
- Relative Vs. Absolute Dating: The Ultimate Face-off
- Accuracy of Fossils and Dating Methods
Consequently, the layers in this famous archaeological site represent many different cultures. An early excavator of Hisarlik, Heinrich Schleimann, inadvertently dug through the Troy layer into an earlier occupation and mistakenly assigned the gold artifacts he found there to Troy. Other sites have been continuously occupied by the same culture for a long time and the different layers represent gradual changes. In both cases, stratigraphy will apply.
A chronology based on stratigraphy often can be correlated to layers in other nearby sites. For example, a particular type or pattern of pottery may occur in only one layer in an excavation. If the same pottery type is found in another excavation nearby, it is safe to assume that the layers are the same age. Archaeologists rarely make these determinations on the basis of a single example.
Usually, a set of related artifacts is used to determine the age of a layer. Seriation simply means ordering. This technique was developed by the inventor of modern archaeology, Sir William Matthew Flinders Petrie. Seriation is based on the assumption that cultural characteristics change over time. For example, consider how automobiles have changed in the last 50 years a relatively short time in archaeology. Automobile manufacturers frequently introduce new styles about every year, so archaeologists thousands of years from now will have no difficulty identifying the precise date of a layer if the layer contains automobile parts.
Cultural characteristics tend to show a particular pattern over time. The characteristic is introduced into the culture for example, using a certain type of projectile point for hunting or wearing low-riding jeans , becomes progressively more popular, then gradually wanes in popularity. The method of seriation uses this distinctive pattern to arrange archaeological materials into a sequence. However, seriation only works when variations in a cultural characteristic are due to rapid and significant change over time. It also works best when a characteristic is widely shared among many different members of a group.
Even then, it can only be applied to a small geographic area, because there is also geographic variation in cultural characteristics. For example, 50 years ago American automobiles changed every year while the Volkswagen Beetle hardly changed at all from year to year. Cross dating is also based on stratigraphy. It uses the principle that different archaeological sites will show a similar collection of artifacts in layers of the same age. Sir Flinders Petrie used this method to establish the time sequence of artifacts in Egyptian cemeteries by identifying which burials contained Greek pottery vessels.
These same Greek pottery styles could be associated with monuments in Greece whose construction dates were fairly well known. Since absolute dating techniques have become common, the use of cross dating has decreased significantly. Pollen grains also appear in archaeological layers. They are abundant and they survive very well in archaeological contexts. As climates change over time, the plants that grow in a region change as well.
People who examine pollen grains the study of which is known as pollen analysis can usually determine the genus , and often the exact species producing a certain pollen type. Archaeologists can then use this information to determine the relative ages of some sites and layers within sites. However, climates do not change rapidly, so this type of analysis is best for archaeological sites dating back to the last ice age.
Absolute dating methods produce an actual date, usually accurate to within a few years.
This date is established independent of stratigraphy and chronology. If a date for a certain layer in an excavation can be established using an absolute dating method, other artifacts in the same layer can safely be assigned the same age. Dendrochronology, also known as tree-ring dating, is the earliest form of absolute dating. This method was first developed by the American astronomer Andrew Ellicott Douglas at the University of Arizona in the early s. Douglas was trying to develop a correlation between climate variations and sunspot activity , but archaeologists quickly recognized its usefulness as a dating tool.
The technique was first applied in the American Southwest and later extended to other parts of the world. Tree-ring dating is relatively simple. Trees add a new layer of cambium the layer right under the bark every year. The thickness of the layer depends on local weather and climate.
In years with plenty of rain, the layer will be thick and healthy. Over the lifetime of the tree, these rings accumulate, and the rings form a record of regional variation in climate that may extend back hundreds of years. Since all of the trees in a region experience the same climate variations, they will have similar growth patterns and similar tree ring patterns. One tree usually does not cover a period sufficiently long to be archaeologically useful. However, patterns of tree ring growth have been built up by "overlapping" ring sequences from different trees so that the tree ring record extends back several thousand years in many parts of the world.
The process starts with examination of the growth ring patterns of samples from living trees. Then older trees are added to the sequence by overlapping the inner rings of a younger sample with the outer rings of an older sample. Older trees are recovered from old buildings, archaeological sites, peat bogs, and swamps. Eventually, a regional master chronology is constructed. When dendrochronology can be used, it provides the most accurate dates of any technique. In the American Southwest, the accuracy and precision of dendrochronology has enabled the development of one of the most.
Often events can be dated to within a decade. This precision has allowed archaeologists working in the American Southwest to reconstruct patterns of village growth and subsequent abandonment with a fineness of detail unmatched in most of the world. Radiometric dating methods are more recent than dendrochronology. However, dendrochronology provides an important calibration technique for radiocarbon dating techniques. All radiometric-dating techniques are based on the well-established principle from physics that large samples of radioactive isotopes decay at precisely known rates.
The rate of decay of a radioactive isotope is usually given by its half-life. The decay of any individual nucleus is completely random. The half-life is a measure of the probability that a given atom will decay in a certain time. The shorter the half-life, the more likely the atom will decay. This probability does not increase with time. If an atom has not decayed, the probability that it will decay in the future remains exactly the same. This means that no matter how many atoms are in a sample, approximately one-half will decay in one half-life. The remaining atoms have exactly the same decay probability, so in another half-life, one half of the remaining atoms will decay.
The amount of time required for one-half of a radioactive sample to decay can be precisely determined. The particular radioisotope used to determine the age of an object depends on the type of object and its age. Radiocarbon is the most common and best known of radiometric dating techniques, but it is also possibly the most misunderstood.
It was developed at the University of Chicago in by a group of American scientists led by Willard F. Radiocarbon dating has had an enormous impact on archaeology. In the last 50 years, radiocarbon dating has provided the basis for a worldwide cultural chronology. Recognizing the importance of this technique, the Nobel Prize committee awarded the Prize in Chemistry to Libby in The physics behind radiocarbon dating is straightforward.
Earth 's atmosphere is constantly bombarded with cosmic rays from outer space. Cosmic-ray neutrons collide with atoms of nitrogen in the upper atmosphere, converting them to atoms of radioactive carbon The carbon atom quickly combines with an oxygen molecule to form carbon dioxide. This radioactive carbon dioxide spreads throughout Earth's atmosphere, where it is taken up by plants along with normal carbon As long as the plant is alive, the relative amount ratio of carbon to carbon remains constant at about one carbon atom for every one trillion carbon atoms.
Some animals eat plants and other animals eat the plant-eaters. As long as they are alive, all living organisms have the same ratio of carbon to carbon as in the atmosphere because the radioactive carbon is continually replenished, either through photosynthesis or through the food animals eat.
However, when the plant or animal dies, the intake of carbon stops and the ratio of carbon to carbon immediately starts to decrease.
The half-life of carbon is 5, years. After 5, years, about one-half of the carbon atoms will have decayed. After another 5, years, one-half of the remaining atoms will have decayed. So after 11, years, only one-fourth will remain. After 17, years, one-eighth of the original carbon will remain. After 22, years, one-sixteenth will remain.
Radiocarbon dating has become the standard technique for determining the age of organic remains those remains that contain carbon. There are many factors that must be taken into account when determining the age of an object. The best objects are bits of charcoal that have been preserved in completely dry environments. The worst candidates are bits of wood that have been saturated with sea water, since sea water contains dissolved atmospheric carbon dioxide that may throw off the results. Radiocarbon dating can be used for small bits of clothing or other fabric, bits of bone, baskets, or anything that contains organic material.
There are well over labs worldwide that do radiocarbon dating. In the early twenty-first century, the dating of objects up to about 10 half-lives, or up to about 50, years old, is possible. However, objects less than years old cannot be reliably dated because of the widespread burning of fossil fuels, which began in the nineteenth century, and the production of carbon from atmospheric testing of nuclear weapons in the s and s.
Another problem with radiocarbon dating is that the production of carbon in the atmosphere has not been constant, due to variation in solar activity. For example, in the s, solar activity dropped a phenomenon called the "Maunder Minimum" , so carbon production also decreased during this period. To achieve the highest level of accuracy, carbon dates must be calibrated by comparison to dates obtained from dendrochronology. Calibration of Radiocarbon Dates. Samples of Bristlecone pine, a tree with a very long life span, have been dated using both dendrochronology and radiocarbon dating. The results do not agree, but the differences are consistent.
That is, the radiocarbon dates were always wrong by the same number of years. Consequently, tree-ring chronologies have been used to calibrate radiocarbon dates to around 12, years ago. When radiocarbon dating was first put into use, it was decided that dates would always be reported as B. That way, dates reported in magazine articles and books do not have to be adjusted as the years pass. So if a lab determines that an object has a radiocarbon age of 1, years in , its age will be given as B.
Calibrated dates are given using the actual date, such as c. If an object is too old to be dated by radiocarbon dating, or if it contains no organic material, other methods must be used. One of these is potassium-argon dating. All naturally occurring rocks contain potassium. Some of the potassium in rocks is the radioactive isotope potassium Potassium gradually decays to the stable isotope argon, which is a gas.
When the rock is melted, as in a volcano, any argon gas trapped in the rock escapes. When the rock cools, the argon will begin to build up. So this method can be used to measure the age of any volcanic rock, from , years up to around 5 billion years old. This method is not widely used in archaeology, since most archaeological deposits are not associated with volcanic activity. However, Louis and Mary Leakey successfully used the method to determine the ages of fossils in Olduvai Gorge in Tanzania by examining rocks from lava flows above and below the fossils.
They were able to establish an absolute chronology for humans and human ancestors extending back two million years. At Laetolli, in Tanzania, volcanic ash containing early hominid footprints was dated by this method at 3. Uranium is present in most rocks. This isotope of uranium spontaneously undergoes fission.
The fission fragments have a lot of energy, and they plow through the rock, leaving a track that can be made visible by treating the rock. So by counting fission tracks, the age of the rock can be determined. Like potassium-argon dating , this can only be used to determine the age of the rock, not the age of the artifact itself.
Thermoluminescence is a recently developed technique that uses the property of some crystals to "store" light. Sometimes an electron will be knocked out of its position in a crystal and will "stick" somewhere else in the crystal. These displaced electrons will accumulate over time. If the sample is heated, the electrons will fall back to their normal positions, emitting a small flash of light. By measuring the light emitted, the time that has passed since the artifact was heated can be determined. This method should prove to be especially useful in determining the age of ceramics, rocks that have been used to build fire rings, and samples of chert and flint that have been deliberately heated to make them easier to flake into a projectile point.
Science continues to develop new methods to determine the age of objects. As our knowledge of past chronologies improves, archaeologists will be better able to understand how cultures change over time, and how different cultures interact with each other. As a result, this knowledge will enable us to achieve a progressively better understanding of our own culture. A Slice through Time: Dendrochronology and Precision Dating. Radiocarbon after Four Decades: In Search of the Trojan War. New American Library, Dating techniques are procedures used by scientists to determine the age of an object or a series of events.
The two main types of dating methods are relative and absolute. Relative dating methods are used to determine only if one sample is older or younger than another. Absolute dating methods are used to determine an actual date in years for the age of an object. Before the advent of absolute dating methods in the twentieth century, nearly all dating was relative. The main relative dating method is stratigraphy pronounced stra-TI-gra-fee , which is the study of layers of rocks or the objects embedded within those layers. This method is based on the assumption which nearly always holds true that deeper layers of rock were deposited earlier in Earth 's history, and thus are older than more shallow layers.
The successive layers of rock represent successive intervals of time. Since certain species of animals existed on Earth at specific times in history, the fossils or remains of such animals embedded within those successive layers of rock also help scientists determine the age of the layers. Similarly, pollen grains released by seed-bearing plants became fossilized in rock layers. If a certain kind of pollen is found in an archaeological site, scientists can check when the plant that produced that pollen lived to determine the relative age of the site.
Absolute dating methods are carried out in a laboratory. The most widely used and accepted form of absolute dating is radioactive decay dating. Radioactive decay refers to the process in which a radioactive form of an element is converted into a nonradioactive product at a regular rate. The nucleus of every radioactive element such as radium and uranium spontaneously disintegrates over time, transforming itself into the nucleus of an atom of a different element.
In the process of disintegration, the atom gives off radiation energy emitted in the form of waves. Hence the term radioactive decay. Each element decays at its own rate, unaffected by external physical conditions. By measuring the amount of original and transformed atoms in an object, scientists can determine the age of that object. Invisible, high-energy particles that constantly bombard Earth from all directions in space. Also known as tree-ring dating, the science concerned with determining the age of trees by examining their growth rings.
Measurement of the time it takes for one-half of a radioactive substance to decay. The predictable manner in which a population of atoms of a radioactive element spontaneously disintegrate over time. The age of the remains of plants, animals, and other organic material can be determined by measuring the amount of carbon contained in that material.
Carbon, a radioactive form of the element carbon, is created in the atmosphere by cosmic rays invisible, high-energy particles that constantly bombard Earth from all directions in space. When carbon falls to Earth, it is absorbed by plants. These plants are eaten by animals who, in turn, are eaten by even larger animals. Eventually, the entire ecosystem community of plants and animals of the planet, including humans, is filled with a concentration of carbon As long as an organism is alive, the supply of carbon is replenished. When the organism dies, the supply stops, and the carbon contained in the organism begins to spontaneously decay into nitrogen The time it takes for one-half of the carbon to decay a period called a half-life is 5, years.
By measuring the amount of carbon remaining, scientists can pinpoint the exact date of the organism's death.
The range of conventional radiocarbon dating is 30, to 40, years. With sensitive instrumentation, this range can be extended to 70, years. In addition to the radiocarbon dating technique, scientists have developed other dating methods based on the transformation of one element into another. These include the uranium-thorium method, the potassium-argon method, and the rubidium-strontium method. Thermoluminescence pronounced ther-moeloo-mi-NES-ence dating is very useful for determining the age of pottery. The older the pottery, the brighter the light that will be emitted.
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Using thermoluminescence, pottery pieces as old as , years can be dated with precision. Known as dendrochronology pronounced den-dro-crow-NOL-o-gee , tree-ring dating is based on the fact that trees produce one growth ring each year.
- what are the strengths and limitations of relative age dating.
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Narrow rings grow in cold or dry years, and wide rings grow in warm or wet years. Thus, the growth pattern of a tree of a known age can be used as a standard to determine the age of similar trees. The ages of buildings and archaeological sites can also be determined by examining the ring patterns of the trees used in their construction. Dendrochronology has a range of 1 to 10, years or more. Relative dating techniques date specimens in relation to one another; for example, stratigraphy is used to establish the succession of fossils. Absolute or chronometric techniques give an absolute estimate of the age and fall into two main groups.
The first depends on the existence of something that develops at a seasonally varying rate, as in dendrochronology and varve dating. The other uses some measurable change that occurs at a known rate, as in chemical dating , radioactive or radiometric dating see carbon dating ; fission-track dating ; potassium—argon dating ; rubidium—strontium dating ; uranium—lead dating , and thermoluminescence.
Depositional rates of sediments have also been employed as a dating method, but only recently has absolute dating been made possible through the use of radioactive isotopes. Of the various methods the last is obviously the most precise, but fossils , lithologies , and cross-cutting relationships do enable the geologist to give an approximate relative age in field studies.
A relative time scale, constructed in the last century, is based on correlations between palaeontological and stratigraphic data. The rate at which sediments accumulate can also be used for dating see varve. Absolute dating relies on the decay of radioactive isotopes of elements present in the material to be dated see decay constant ; decay curve ; decay series ; isotopic dating; radiocarbon dating ; and radiometric dating.
Dating techniques are procedures used by scientists to determine the age of rocks, fossils, or artifacts. Relative dating methods tell only if one sample is older or younger than another; absolute dating methods provide an approximate date in years. Many absolute dating techniques take advantage of radioactive decay , whereby a radioactive form of an element decays into a non-radioactive product at a regular rate.
In recent years, a few of these methods have come under close scrutiny as scientists strive to develop the most accurate dating techniques possible. It is based on the assumption which nearly always holds true that deeper layers were deposited earlier, and thus are older, than more shallow layers. Although these units may be sequential, they are not necessarily continuous due to erosional removal of some intervening. Stratigraphy is the principle method of relative dating, and in the early years of dating studies was virtually the only method available to scientists.
The technique works best if the animals belonged to species, which evolved quickly, expanded rapidly over a large area, or suffered a mass extinction. The unit of the calendar is the pollen zone. In most cases, this tells us about the climate of the period, because most plants only thrive in specific climatic conditions. This dating technique was first conducted by Hare and Mitterer in , and was popular in the s. Amino acid racimization is based on the principle that amino acids except glycine, which is a very simple amino acid exist in two mirror image forms called stereoisomers.
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, temperature, and pH of the postmortem conditions.
It can be used to obtain dates that would be unobtainable by more conventional methods such as radio-carbon dating. Although cation-ratio dating has been widely used, recent studies suggest it has many problems. Finally, some scientists have recently suggested that the cation ratios may not even be directly related to the age of the sample. Thermoluminescence dating is useful for determining the age of pottery. This radiation may come from radioactive substances such as uranium, present in the clay or burial medium, or from cosmic radiation.
The longer the exposure to the radiation, the more electrons that are bumped into an excited state, and the more light that is emitted upon heating. Scientists can determine how many years have passed since a ceramic piece was fired by heating it in the laboratory and measuring how much light is given off. Optically stimulated luminescence has only been used since To determine the age of a sediment, scientists expose grains to a known amount of light and compare these grains with the unknown sediment.
This absolute dating method is also known as dendrochronology. Dendrochronology has a range of , years or more. As previously mentioned, radioactive decay refers to the process in which a radioactive form of an element is converted into a nonradioactive product at a regular rate. When volcanic rocks are heated to extremely high temperatures, they release any argon gas trapped in them. Radiocarbon 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.
Atoms of 14 C and of a non-radioactive form of carbon, 12 C, are equally likely to be incorporated into living organisms — there is no discrimination. This allows us to determine how much 14 C has formed since the death of the organism. A problem with radiocarbon dating is that diagenic after death contamination of a specimen from soil, water, etc. This can lead to inaccurate dates. Another problem lies with the assumptions associated with radiocarbon dating. This is not completely true. Uranium series have been used to date uranium-rich rocks, deep-sea sediments, shells, bones, and teeth, and to calculate the ages of ancient lake beds.
In the case of a daughter excess, a larger amount of the daughter is initially deposited than the parent. Some volcanic minerals and glasses, such as obsidian, contain uranium U. See also Pollen analysis ; Strata. Cambridge University Press, Progress and Pitfalls in Radiocarbon Dating. All these labors have not led to a single unexpected finding such as a human fossil from the time of the dinosaurs, or a Jurassic dinosaur in the same rocks as Silurian trilobites. Paleontologists now apply sophisticated mathematical techniques to assess the relative quality of particular fossil successions, as well as the entire fossil record.
Relative Vs. Absolute Dating: The Ultimate Face-off
These demonstrate that, of course, we do not know everything and clearly never will , but we know enough. Today, innovative techniques provide further confirmation and understanding of the history of life. Biologists actually have at their disposal several independent ways of looking at the history of life - not only from the order of fossils in the rocks, but also through phylogenetic trees.
Phylogenetic trees are the family trees of particular groups of plants or animals, showing how all the species relate to each other. Phylogenetic trees are drawn up mathematically, using lists of morphological external form or molecular gene sequence characters. Modern phylogenetic trees have no input from stratigraphy, so they can be used in a broad way to make comparisons between tree shape and stratigraphy.
The majority of test cases show good agreement, so the fossil record tells the same story as the molecules enclosed in living organisms. Dating in geology may be relative or absolute. Relative dating is done by observing fossils, as described above, and recording which fossil is younger, which is older. The discovery of means for absolute dating in the early s was a huge advance.
The methods are all based on radioactive decay:. The first radiometric dates, generated about , showed that the Earth was hundreds of millions, or billions, of years old.
Accuracy of Fossils and Dating Methods
Since then, geologists have made many tens of thousands of radiometric age determinations, and they have refined the earlier estimates. Age estimates can be cross-tested by using different isotope pairs. Results from different techniques, often measured in rival labs, continually confirm each other. Every few years, new geologic time scales are published, providing the latest dates for major time lines.
Older dates may change by a few million years up and down, but younger dates are stable. For example, it has been known since the s that the famous Cretaceous-Tertiary boundary, the line marking the end of the dinosaurs, was 65 million years old.
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Repeated recalibrations and retests, using ever more sophisticated techniques and equipment, cannot shift that date. It is accurate to within a few thousand years. The fossil record is fundamental to an understanding of evolution. Fossils document the order of appearance of groups and they tell us about some of the amazing plants and animals that died out long ago. Fossils can also show us how major crises, such as mass extinctions, happened, and how life recovered after them.
If the fossils, or the dating of the fossils, could be shown to be inaccurate, all such information would have to be rejected as unsafe. Geologists and paleontologists are highly self-critical, and they have worried for decades about these issues. Repeated, and tough, regimes of testing have confirmed the broad accuracy of the fossils and their dating, so we can read the history of life from the rocks with confidence.
Educators have permission to reprint articles for classroom use; other users, please contact editor actionbioscience. Currently, he is studying certain basal dinosaurs from the Late Triassic and the quality of different segments of the fossil record. He holds the Chair in Vertebrate Paleontology at the University of Bristol, UK, in addition to chairing the Masters program in paleobiology at the university.
Your one-stop source for information on evolution. Michael Benton wrote another article, Evidence of Evolutionary Transitions , for this website which explains how fossils support the stages of evolutionary history. Data bases and software for studying the quality of the fossil record.