Background (Encyclopedia of Global Warming)
Because the geological and climatological history of Earth began long before recorded history, scientific dating methods are necessary to determine when many climatic events occurred. For example, such methods could be used to determine when glacial deposits formed or when a boulder was dropped on top of those deposits by a melting glacier.
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Primordial Isotopes (Encyclopedia of Global Warming)
When the Earth formed, it inherited an inventory of radioactive elements that have been decaying ever since. The decay constant for a particular isotope can be determined by measuring the rate at which disintegrations occur in a sample of known mass. Half-lives are calculated from decay constants. Known half-lives of radioactive isotopes enable scientists to determine the age of some objects that contain those isotopes.
For example, water moving through the ground will often dissolve small amounts of uranium. A stalagmite may form from this water in a cave as the water evaporates, incorporating any uranium 238 (U238) present. The uranium will decay to produce thorium 234 (Th234). Thorium is insoluble in water, so it can be assumed that the stalagmite initially contained no thorium. Thorium, too, is radioactive, and may decay into U234, which decays to Th230, which is also radioactive. Using the decay constants and the amounts of U238, U234, and Th230 present in a specimen, scientists can calculate how long it has been since the uranium came out of solution. This technique is limited to ages less than 500,000 years.
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Cosmogenic Isotopes (Encyclopedia of Global Warming)
Cosmic rays are subatomic particles traveling at very high velocities. When they strike the nucleus of an atom, they can eliminate nucleons, altering the identity of the atom. An atom of nitrogen 14 (N14), for instance, might become carbon 14 (C14), or atoms of silicon or oxygen might become beryllium 10 (Be10) or aluminum 26 (Al26). C14, Be10, and Al26 are all radioactive, and their decay constants are known, so they provide a means of dating organic material and the surfaces of boulders.
On Earth, C14 is generally created only as a result of cosmic ray bombardment in the atmosphere, so only atmospheric carbon replenishes its C14 level. Nonatmospheric C14 decays over time without replenishment. An organism will interchange carbon with the atmosphere while it is alive, maintaining a relatively constant ratio of C14 to carbon 12 (C12), but once it dies that interchange will cease and the ratio will decrease. By assuming a historically constant ratio of C14 to C12 in the atmosphere (and thus in living organisms) and by comparing that ratio to the ratio in a sample of tissue from a deceased organism, it is possible to determine how many half-lives of C14 have passed since the organism died. The assumption of a constant ratio is known to be invalid, but it will produce the same errors in all samples, giving the same results for samples of the same age. If the goal of analysis is to compare different samples with one another...
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Nonradiometric Methods (Encyclopedia of Global Warming)
Dendrochronology. Dendrochronology is a method for determining the age of wood by counting and examining annual tree rings. The thickness of a given ring in a tree is determined by environmental factors obtaining during the year in which the ring was formed. Such factors as temperature and rainfall affect the rate of growth and overall health of trees. As a result, patterns of ring thickness in trees that were alive at the same time in the same area tend to resemble one another. Matching patterns of ring thickness between trees of known and unknown age can thus provide evidence that the trees were alive at the same time. The reliability of this method has been extended back to about ten thousand years.
Varves. Just as trees have annual growth cycles, so do sediments deposited in lakes in regions near glaciers. In the summer, rains bring coarse sediments into the lake. In the winter, fine clays have time to settle out. The banded sediments that result from this seasonal alternation are called varves. Just as with tree rings, patterns of thick and thin layers can be correlated in different varved sequences. Some sequences cover more than thirteen thousand years.
Lichenometry. Lichens grow at fairly constant rates in a given area. In a given area, rocks covered by larger lichens have surfaces that have been exposed longer than have the surfaces of rocks covered by...
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Context (Encyclopedia of Global Warming)
Understanding climate change requires knowledge of Earth’s climatological history, which in turn requires methods capable of dating events of climatic significance over the last few million years. As technology has improved, the precision and accuracy of these methods has increased dramatically, and the size of the samples required for accurate dating has decreased by orders of magnitude.
Scientists looking at isotope-ratio records in marine sediment cores have sometimes found that different radiometric techniques indicate different dates for the same climatic excursion. As the climatic excursions were found to be global and strongly correlated with known astronomical cycles, it became possible to determine their age with greater accuracy, validating some results over others. This correlation with astronomical cycles could also be used to calibrate radiometric dating methods, just as counting tree rings was used to calibrate C14 dating methods. As methods developed, it became possible to date specific geologic and climatic events, such as the encroachment or retreat of ice from a given area, the speed of uplift of a surface, or the rate of development of a valley.
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Further Reading (Encyclopedia of Global Warming)
Cremeens, David L., and John P. Hart, eds. Geoarchaeology of Landscapes in the Glaciated Northeast: Proceedings of a Symposium Held at the New York Natural History Conference 6. Albany: State University of New York: State Education Deptartment, 2003. Contains an outstanding example of dating, using the New England Varve sequence, uranium-thorium calibrations, C14 dates, and calendar C14 dates; shows how all these methods can be used in concert to reinforce one another. Figures, maps, bibliography.
Ruddiman, William F. Earth’s Climate Past and Future. 2d ed. New York: W. H. Freeman, 2008. This elementary college textbook has several sections concerning dating methods, their limitations, errors, and resolution. Illustrations, figures, tables, maps, bibliography, index.
Thurber, David L., et al. “Uranium-Series Ages of Pacific Atoll Coral.” Science 149, no. 3679 (1965): 55-58. An early paper describing the uranium-thorium method; uses graphs to illustrate the viability of the technique. Figures, tables, bibliography.
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Dating methods (American Indians Ready Reference)
Article abstract: A variety of methods, both absolute and relative, are used by scholars to establish dates for archaeological sites and artifacts
Archaeological sites in the Americas are assigned ages based either on absolute dating methods, in which a date in calendar years is provided, or on relative dating methods, in which an age either relatively older or younger is provided. The main absolute dating methods used in the Americas, occupied since about 9500 b.c.e., include radiocarbon dating, obsidian hy- dration dating, and dendrochronology. The major relative dating techniques are stratigraphic analysis, seriation, and cross-dating.
The invention of radiocarbon dating by Willard Libby in 1949 revolutionized archaeology by providing an absolute means for dating sites. The radiocarbon method is based on the knowledge that cosmic radiation entering the earth's atmosphere produces neutrons which react with nitrogen to produce carbon 14. Carbon 14 is unstable and subject to gradual radioactive decay. Living organisms maintain the same level of carbon 14 as that found in the atmosphere because of their intake of carbon dioxide. At death, however, the ratio of radioactive carbon 14 to stable carbon 12 decays at a constant and known rate. Half of the original carbon 14 disappears in 5,568 years, known as the “half-life” of carbon 14. Organic material, notably charcoal, wood, shell, bone,...
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Dating Methods (World of Earth Science)
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. The latter have generally been available only since 1947. Many absolute dating techniques take advantage of radioactive decay, whereby a radioactive form of an element is converted into another radioactive isotope or 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 undergone continual refinement as scientists strive to develop the most accurate dating techniques possible.
Relative dating methods determine whether one sample is older or younger than another. They do not provide an age in years. Before the advent of absolute dating methods, nearly all dating was relative. The main relative dating method is stratigraphy.
Stratigraphy is the study of layers of rocks or the objects embedded within those layers. It is based on the assumption (which, except at unconformities, nearly always holds true) that deeper layers were deposited earlier, and thus are older than more shallow layers. The sequential layers of rock represent sequential intervals of time. Although these units may be sequential, they are not necessarily continuous due to erosional removal of some intervening units. The smallest of these rock units that can be matched to a specific time interval is called a bed. Beds that are related are grouped together into members, and members are grouped into formations.
Seriation is the ordering of objects according to their age. It is a relative dating method. In a landmark study, archaeologist James Ford used seriation to determine the chronological order of American Indian pottery styles in the Mississippi Valley. Artifact styles such as pottery types are seriated by analyzing their abundances through time. This is done by counting the number of pieces of each style of the artifact in each stratigraphic layer and then graphing the data. A layer with many pieces of a particular style will be represented by a wide band on the graph, and a layer with only a few pieces will be represented by a narrow band. The bands are arranged into battleship-shaped curves, with each style getting its own curve. The curves are then compared with one another, and from this the relative ages of the styles are determined. A limitation to this method is that it assumes all differences in artifact styles are the result of different periods of time, and are not due to the immigration of new cultures into the area of study.
The term faunal dating refers to the use of animal bones to determine the age of sedimentary layers or objects such as cultural artifacts embedded within those layers. Scientists can determine an approximate age for a layer by examining which species or genera of animals are buried in it. The technique works best if the animals belonged to species that evolved quickly, expanded rapidly over a large area, or suffered a mass extinction. In addition to providing rough absolute dates for specimens buried in the same stratigraphic unit as the bones, faunal analysis can also provide relative ages for objects buried above or below the fauna-encasing layers.
Each year seed-bearing plants release large numbers of pollen grains. This process results in a "rain" of pollen that falls over many types of environments. Pollen that ends up in lakebeds or peat bogs is the most likely to be preserved, but pollen may also become fossilized in arid conditions if the soil is acidic or cool. Scientists can develop a pollen chronology, or calendar, by noting which species of pollen were deposited earlier in time, that is, residue in deeper sediment or rock layers, than others. A pollen zone is a period of time in which a particular species is much more abundant than any other species of the time. In most cases, this also reveals much about the climate of the period, because most plants only thrive in specific climatic conditions. Changes in pollen zones can also indicate changes in human activities such as massive deforestation or new types of farming. Pastures for grazing livestock are distinguishable from fields of grain, so changes in the use of the land over time are recorded in the pollen history. The dates when areas of North America were first settled by immigrants can be determined to within a few years by looking for the introduction of ragweed pollen.
Pollen zones are translated into absolute dates by the use of radiocarbon dating. In addition, pollen dating provides relative dates beyond the limits of radiocarbon (40,000 years), and can be used in some places where radiocarbon dates are unobtainable.
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 in 1967, and was popular in the 1970s. 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. This means that the ratio of the D-form to the L-form is zero (D/L=0). When these organisms die, the L-amino acids are slowly converted into D-amino acids in a process called racimization. This occurs because protons (H+) are removed from the amino acids by acids or bases present in the burial environment. 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 Lmino acid. The reversible reaction eventually creates equal amounts of Lnd D-forms (D/L=1.0).
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. 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. Cation ratio dating relies on the principle that the cation ratio (K++Ca2+)/Ti4+ decreases with increasing age of a sample. 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. When the ceramic is heated to a very high temperature (over 932°F [500°C]), these electrons fall back to the ground state, emitting light in the process and resetting the "clock" to zero. 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 dating, or 40,00000,000 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 1984. 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 than 500,000 years 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. Narrow rings grow in cold and/or dry years, and wide rings grow in warm years with plenty of moisture. 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. Dendrochronology has a range of one to 10,000 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. As the rocks cool, argon-40 (40Ar) begins to accumulate. Argon-40 is formed in the rocks by the radioactive decay of potassium-40 (40K). The amount of 40Ar formed is proportional to the decay rate (half-life) of 40K, which is 1.3 billion years. In other words, it takes 1.3 billions years for half of the 40K originally present to be converted into 40Ar. 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 30,0000,000 years, but with sensitive instrumentation, this range can be extended to 70,000 years. Radiocarbon (14C) is a radioactive form of the element carbon. It decays spontaneously into nitrogen-14 (14N). Plants get most of their carbon from the air in the form of carbon dioxide, and 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. While a plant or animal is alive, the ratio of 14C/12C in its body will be nearly the same as the 14C/12C ratio in the atmosphere. 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 5,730 years. Scientists can estimate how many years have elapsed since an organism died by comparing the 14C/12C ratio in the remains with the ratio in the atmosphere. This allows them to determine how much 14C has formed since the death of the organism.
One of the most familiar applications of radioactive dating is determining the age of fossilized remains, such as dinosaur bones. Radioactive dating is also used to authenticate the age of rare archaeological artifacts. Because items such as paper documents and cotton garments are produced from plants, they can be dated using radiocarbon dating. Without radioactive dating, a clever forgery might be indistinguishable from a real artifact. There are some limitations, however, to the use of this technique. Samples that were heated or irradiated at some time may yield by radioactive dating an age less than the true age of the object. Because of this limitation, other dating techniques are often used along with radioactive dating to ensure accuracy.
Accurate radiocarbon dating is that diagenic (after death) demands consideration regarding potential contamination of a specimen and a proper application of changes in the 14C/12C ratio in the atmosphere over time. 14C levels can be measured in tree rings and used to correct for the 14C/12C ratio in the atmosphere at the time the organism died, and can even be used to calibrate some dates directly. Although the magnitude of change of the 14C/12C ratio sometimes stirs controversy, with proper calibration and correction, radiocarbon dating correlates well with other dating techniques and consistently proves to be an accurate dating techniquespecially for Pleistocene and Holocene period analysis.
Uranium series dating techniques rely on the fact that radioactive uranium and thorium isotopes decay into a series of unstable, radioactive "daughter" isotopes; this process continues until a stable (non-radioactive) lead isotope is formed. The daughters have relatively short half-lives ranging from a few hundred thousand years down to only a few years. The "parent" isotopes have half-lives of several billion years. This provides a dating range for the different uranium series of a few thousand years to 500,000 years. Uranium series have been used to date uranium-rich rocks, deep-sea sediments, shells, bones, and teeth, and to calculate the ages of ancient lakebeds. The two types of uranium series dating techniques are daughter deficiency methods and daughter excess methods.
In daughter deficiency situations, the parent radioisotope is initially deposited by itself, without its daughter (the isotope into which it decays) present. Through time, the parent decays to the daughter until the two are in equilibrium (equal amounts of each). The age of the deposit may be determined by measuring how much of the daughter has formed, providing that neither isotope has entered or exited the deposit after its initial formation. Carbonates may be dated this way using, for example, the daughter/parent isotope pair protactinium-231/uranium-235 (231Pa/235U). Living mollusks and corals will only take up dissolved compounds such as isotopes of uranium, so they will contain no protactinium, which is insoluble. Protactinium-231 begins to accumulate via the decay of 235U after the organism dies. Scientists can determine the age of the sample by measuring how much 231Pa is present and calculating how long it would have taken that amount to form.
In the case of daughter excess, a larger amount of the daughter is initially deposited than the parent. Non-uranium daughters such as protactinium and thorium are insoluble, and precipitate out on the bottoms of bodies of water, forming daughter excesses in these sediments. Over time, the excess daughter disappears as it is converted back into the parent, and by measuring the extent to which this has occurred, scientists can date the sample. If the radioactive daughter is an isotope of uranium, it will dissolve in water, but to a different extent than the parent; the two are said to have different solubilities. For example, 234U dissolves more readily in water than its parent, 238U, so lakes and oceans contain an excess of this daughter isotope. This excess is transferred to organisms such as mollusks or corals, and is the basis of 234U/238U dating.
Some volcanic minerals and glasses, such as obsidian, contain uranium-238 (238U). Over time, these substances become "scratched." The marks, called tracks, are the damage caused by the fission (splitting) of the uranium atoms. When an atom of 238U splits, two "daughter" atoms rocket away from each other, leaving in their wake tracks in the material in which they are embedded. The rate at which this process occurs is proportional to the decay rate of 238U. The decay rate is measured in terms of the half-life of the element, or the time it takes for half of the element to split into its daughter atoms. The half-life of 238U is 4.47x109 years.
When the mineral or glass is heated, the tracks are erased in much the same way cut marks fade away from hard candy that is heated. This process sets the fission track clock to zero, and the number of tracks that then form are a measure of the amount of time that has passed since the heating event. Scientists are able to count the tracks in the sample with the aid of a powerful microscope. The sample must contain enough 238U to create enough tracks to be counted, but not contain too much of the isotope, or there will be a jumble of tracks that cannot be distinguished for counting. One of the advantages of fission track dating is that it has an enormous dating range. Objects heated only a few decades ago may be dated if they contain relatively high levels of 238U; conversely, some meteorites have been dated to over a billion years old with this method.
Although certain dating techniques are accurate only within certain age ranges, whenever possible, scientists attempt to use multiple methods to date specimens. Correlation of dates via different dating methods provides a highest degree of confidence in dating.
See also Evolution, evidence of; Fossil record; Fossils and fossilization; Geologic time; Historical geology