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. The latter have generally been available only since 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. Others, such as amino acid racimization and cation-ratio dating, are based on chemical changes in the organic or inorganic composition of a sample. In recent years, a few of these methods have come under close scrutiny as scientists strive to develop the most accurate dating techniques possible. Relative dating methods determine whether one sample is older or younger than another.
In Europe during this era, inquiry into the prehistoric past was discouraged, because the Bible was regarded as the supreme authority on human history and the early history of the earth. For example, creationism dominated scholarly writings on the origin of the universe and humanity, and during this period, fossils of marine organisms that were sometimes found in mountains were described as being washed up by the Great Flood.
Ancient arrow points and other prehistoric stone tools were thought to have been produced by thunderbolts and other natural phenomena. Prehistoric stone arrow points and axes were believed to have fallen from the sky at the moment when thunder stuck. These implements were called thunder-stones, ceraunia, or pierre de foudre. It generally was believed that all living plant and animal species were survivors of the Great Flood and that with careful biblical research, especially on the book of Genesis, it was possible to calculate the age of the earth.
For example, inDr. John Lightfoot, the vice-chancellor of Cambridge University, calculated that the universe was created in BC, on October 23, at am, coincidental with the beginning of the Fall Term. Later inArchbishop James Ussher refined this estimate and suggested that the earth was actually created on the evening preceding October 23, BC.
This is the kind of pedantic to us debate that took place, so that although historical sites were being studied, prehistoric archaeology was being interpreted in light of the Bible. But, in situ materials also required theory for relative sorting, and this was provided by the Danish atomist and geologist Bishop Nicholas Steno They are: the Principle of Superposition: In a sedimentary sequence, the older beds are on the bottom, and the younger beds are on the top; the Principle of Original Horizontality: Sediments tend to be deposited in flat, horizontal layers; and the Principle of Original Lateral Continuity: a flat layer will tend to extend for a considerable distance in all directions.
Through human ingenuity, the last years have been witness to great number of techniques for sorting time applicable to the scientific study of the past. These various dating techniques fall into one or more of three categories: absolute, relative, and radio-metric.
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 genusand 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. 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 activitybut 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.
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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.
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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 inits age will be given as B.
Calibrated dates are given using the actual date, such as c. Potassium-Argon Dating. 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, fromyears 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. Other Methods.
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 datingthis 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.
How Does Radiocarbon Dating Work? - Instant Egghead #28
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. Baillie, M. London U.
Taylor, R. Radiocarbon Dating : An Archaeological Perspective. Orlando, FL: Academic Press, Long, and R. Wood, Michael. In Search of the Trojan War.
New York : New American Library, Richmond, Elliot " Dating Techniques. Richmond, Elliot "Dating Techniques. 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-feewhich 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 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. Cosmic rays: Invisible, high-energy particles that constantly bombard Earth from all directions in space.
Chronology: Tools and Methods for Dating Historical and Ancient Deposits, Inclusions, and Remains
Dendrochronology: Also known as tree-ring dating, the science concerned with determining the age of trees by examining their growth rings. Half-life: Measurement of the time it takes for one-half of a radioactive substance to decay.
Radioactive 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.
Dating methods Dating techniques are procedures used by scientists to determine the age of a specimen. Relative dating methods tell only if one sample is older or younger than another sample; absolute dating methods provide a date in years. Luminescence Dating Laboratory This allows researchers to date materials that cannot be dated using other techniques (e.g., lack of suitable organic remains for radiocarbon dating). Additionally, since it is capable of directly dating cultural materials such as ceramics, the bridging arguments between dating events and target events are minimized. Relative dating techniques permit chronological relationships to be ascertained through physical and/or chemical seriation (cation exchange ratio, fluorine dating, patination, pollen analysis) based on spatial relationships (stratigraphy and cross-dating), differential abundances, technological variations, or combinations thereof.
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. Using thermoluminescence, pottery pieces as old asyears can be dated with precision.
Known as dendrochronology pronounced den-dro-crow-NOL-o-geetree-ring dating is based on the fact that trees produce one growth ring each year. 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. 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 fossilslithologiesand cross-cutting relationships do enable the geologist to give an approximate relative age in field studies. 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 datingradioactive or radiometric dating see carbon dating ; fission-track dating ; potassium-argon dating ; rubidium-strontium dating ; uranium-lead datingand thermoluminescence.
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. They can tell you how old something is to a near-precise date or within a set range, usually with a slight margin of error.
Each has a failsafe built in through the academic method and repeated testing. Multiple tests are carried out on a subject material, choosing a range of samples to ensure that such problems are eliminated. Researchers will also send samples to different labs, ensuring that each is unaware of which other labs are carrying out tests.
When there is concurrence, we can be quite certain of the date or date range that results from the test. The second major advantage is that we can date material without destroying it.
As time has gone by, new developments mean smaller and smaller samples are required for more accurate dates. This is especially true for radiocarbon dating. The range of options available offer a significant advantage.
The sheer number of choices, some of which overlap, means that if an anomalous result comes up with one method, other methods may be applied to ensure that the anomaly is just that or confirm a change in thinking regarding the dating of such material.
Most problems associated with such radiometric, chemical and other absolute dating methods are the result of user error rather than flaws in the method.
The first major issue with any absolute dating method is ensuring that you're selecting the right material from the right places and not including later contaminants; these test results will be skewed, throwing up anomalous results.
It's easy to date inclusions or to accidentally select contaminants from the material. Further limitations exist in dating material that has been reused.
One example of reused wood from ancient tomb showed the wood to be far older than the construction of the tomb It was the case, and the method was not flawed, but the reliance on this method requires other cts to be considered to ensure that we are not solely relying on absolute dating methods in isolation.
One of the greatest problems that archaeologists have had to handle is the overlap and replacement of Neanderthal with anatomically modern humans in Central Europe Contamination by modern carbon sources suggests that the dates often thrown up at the greater end of the range of radiocarbon dating suggest that traditionally understood dates of the appearance of modern humans, disappearance of Neanderthals and the extent to which they overlap on the continent, suggests that dates acquired over the last 50 years may be too young in some instances.
Relative dating methods do not seek to put an exact date on a layer, artefact or activity although it can within a reasonable amount of doubt.
It seeks to explain each item in context of its relationship to everything else, placing it in a sequence. With relative dating, we can see that artefact A came after artefact B by examining its evolution in design or methods of production. We can also see and explain how one geological layer came after another. Here are the most common methods.
It observes sedimentary rock layers for signs of fossilized organic material. This data is used to explain not evolution although it can - that's not its purposebut the sequence of succession for the lifeforms that occupied that particular landscape at a given time, and to examine when a layer was set down. It does not give dates, but it does demonstrate landscape changes through the organic life that occupied it in that time frame. Pieced together, we can build a profile over larger areas Palaeomagnetism : Useful in Earth Sciences such as geology and geography, as well as archaeology and anthropology, there is surprisingly much to learn about the palaeomagnetic record the study of the magnetic field of the past.
It's contributed to the study of continental drift and plate tectonics in the former and dating pottery and brick firing in the latter In archaeology, the study has provided unequivocal and solid dates for the earliest occupation of humans in China and Western Europe, including several relative studies of the archaeological landscape.
Palynology : This is the study of fungal spores and plant pollen during their sexual reproduction stage. Archaeologists and anthropologists can use surviving materials to build a chronology of changes to a landscape over time This can be used to build a landscape history, a profile of land occupation by humans, and tell us much about the local climate at any given time.
Often used in conjunction with absolute methods such as radiocarbon dating. This is a broad area within geology, and in archaeology and anthropology, that examines layers of a landscape. It says nothing about the age of each layer, merely the sequence of deposition. The principles mentioned below make up the theory of the science.
Cross-Cutting Relationships : Used in geology, this is one of the main defining principles of the science. It's the process of examining relationships and interactions between geological layers to determine a sequence - usually to understand which are earlier.
Through it, we come to understand and explain how disrupted layers are older than the actual layers It challenges the principle that a sublayer is always earlier though it is in most cases. Tectonic plates can push rock layers beneath others, creating mountain ranges Harris Matrix : This is a tool of stratigraphy rather than a method used in archaeological contexts, utilizing some of the three Principles listed below. A Harris Matrix is a diagram similar to a flowchart that breaks complex stratigraphic layers into a most likely sequence.
It does not state the age of the layers but sets down the most likely process by which the sequence came to be. Usually, they will use three labels: layers a stratigraphic layercuts a feature showing where a later addition cut through each layer and fills when the cut was filled - naturally, a fill cannot predate the cut of which it is a part Law of Inclusions: Like cross-cutting, the premise for this is that any anomalous clasts in geologic layers or inclusions found within an archaeological stratigraphic layer must be older than the layer itself, even if deposited later.
There are many reasons why we should never attempt to date inclusions as proof of the age of the layer; the anomalies that inclusions throw up is just one of them. It's important not to confuse the age of the item with the date of deposition Principle of Lateral Continuity : Mostly used in geology but with some stratigraphic use in landscape archaeology too, it defines that layers that have become separated or split but otherwise appear to share a relationship must have been deposited at the same time.