Further Reading

Holland, H. D., and K. K. Turekian. Treatise on Geochemistry. 9 Vols. Amsterdam: Elsevier, 2004. marshall, C., and R. Fairbridge. Encyclopedia of Geochemistry, Berlin: springer, 2006.

geochronology Geochronology is the study of time with respect to Earth history, and includes both absolute and relative dating systems as well as correlation methods. Absolute dating systems include a variety of geochronometers such as radioactive decay series in specific isotopic systems that yield a numerical value for the age of a sample. Relative dating schemes include cross-cutting features, and discontinuities such as igneous dikes and unconformities, with the younger units being the cross-cutting features or those overlying the unconformity.

During the 19th and early 20th centuries geo-chronologic techniques were crude. Many ages were estimated by the supposed rate of deposition of rocks, and correlation of units with unconformities with other, more complete sequences. With the development of radioactive dating it became possible to refine precise or absolute ages for specific rock units. Radiometric dating operates on the principle that certain atoms and isotopes are unstable. These unstable atoms tend to decay into stable ones by emitting a particle or several particles. Alpha particles have a positive charge and consist of two protons and two neutrons. Beta particles are physically equivalent to electrons or positrons. These emissions are known as radioactivity. Half-life is the time it takes for half of a given amount of a radioactive element to decay to a stable element. By matching the proportion of original unstable isotope to stable decay product, and knowing the half-life of that element, one can thus deduce the age of the rock. The precise ratios of parent-to-daughter isotopes are measured by a mass spectrometer.

William F. Libby (1908-80) developed radiocarbon, or carbon 14, dating techniques at the University of Chicago in 1946. This major breakthrough in dating organic materials is now widely used by archaeologists, Quaternary geologists, oceanographers, hydrologists, atmospheric scientists, and paleoclima-tologists. Cosmic rays entering Earth's atmosphere transform regular carbon (carbon 12) to radioactive carbon (carbon 14). Within about 12 minutes of being struck by cosmic rays in the upper atmosphere, the carbon 14 combines with oxygen to become carbon dioxide that has carbon 14. The radioactive carbon dioxide diffuses through the atmosphere and is absorbed by vegetation (plants need carbon dioxide to make sugar by photosynthesis). Every living thing contains carbon. While it is alive, each plant or animal exchanges carbon dioxide with the air. Animals also feed on vegetation and absorb its carbon dioxide. At death the animal no longer exchanges carbon 14 with the atmosphere, but the radioactive element continues to decay within the organic material. Theoretically analysis of this carbon 14 can reveal the date when the object once lived by the per cent of carbon 14 atoms still remaining in the object. The radiocarbon method has subsequently evolved into one of the most powerful techniques to date late Pleistocene and Holocene artifacts and geologic events up to about 50,000 years old.

Dendrochronology is the study of annual growth rings in trees for dating the recent geological past. The ages of trees may be determined most simply by counting the number of annual growth rings that form in the trunk of the tree each year. Sometimes this practice is done in conjunction with carbon 14 or other dating techniques to verify the ages of specific rings. This field is closely related to dendroclimatol-ogy, the study of the sizes and relative patterns of tree growth rings to yield information about past climates. Tree rings are most clearly developed in species from temperate forests but not well formed in tropical regions, where seasonal fluctuations are not as great. Most annual tree rings consist of two parts, and early wood consisting of widely spaced thin-walled cells, followed by late wood, consisting of thinly spaced, thick-walled cells. The changes in relative width and density of the rings for an individual species correlates to changes in climate such as soil moisture, sunlight, precipitation, and temperature and also reflects unusual events such as fires or severe drought stress.

The longest dendrochronology record goes back 9,000 years, using species such as the Bristle Cone Pine, found in the southwestern united states, and Oak and Spruce species from Europe. To extend the record from a particular tree, one can correlate rings between individuals that lived at different times in the same microenvironment close to the same location.

uranium, thorium, and lead isotopes form a variety of geochronometers using different parent/ daughter pairs. Uranium 238 decays to lead 206 with a half-life of 4.5 billion years. Uranium 235 decays to lead 207 with a half-life of 0.7 billion years, and thorium 232 decays to lead 208 with a half-life of 14.1 billion years. Uranium, thorium, and lead are generally found together in mixtures, and each one decays into several daughter products (including radium) before turning into lead. The thorium 230/uranium 234 disequilibrium method is one of the most commonly used uranium-series techniques and can be used to date features as old as Precambrian. The fact that uranium is much more soluble than thorium forms the basis for this method, so materials such as corals, mollusks, calcic soils, bones, carbonates, cave deposits, and fault zones are enriched in uranium with respect to thorium.

Uranium-lead dating also uses the known original abundance of isotopes of uranium and the known decay rates of parents to daughter isotopes. This technique is useful for dating rocks up to billions of years old. All naturally occurring uranium contains uranium 238 and uranium 235 in the ratio of 137.7:1. Uranium 238 decays to lead 206 with a half-life of 4,510 Ma through a process of eight alpha-decay steps and six beta-decay steps. Uranium 235 decays to Lead 207 (with a half-life of 713 Ma) by a similar series of stages that involves seven alpha-decay steps and four beta-decay steps. Uranium-lead dating techniques were initially applied to uranium minerals such as uraninite and pitchblende, but these are rare, so geochronologists developed precise methods of measuring isotopic ratios in other minerals with only trace amounts of uranium and lead (zircon, sphene). The amount of radiogenic lead in all these methods must be distinguished from naturally occurring lead; this is calculated using their abundance with lead 204, which is stable. After measuring the ratios of each isotope relative to lead 204, the ratios of uranium 235/lead 207 and uranium 238/lead 206 should give the same age for the sample, and a plot with each system plotted on one axis shows each age.

If the two ages agree, the ages will plot on a curve known as concordia, which tracks the evolution of these ratios in Earth versus time. Ages that plot on concordia are said to be concordant. In many cases, however, the ages determined by the two ratios are different, and they plot off the concordia curve. This occurs when the system has been heated or otherwise disturbed during its history, causing a loss of some of the lead daughter isotopes. Because lead 207 and lead 206 are chemically identical, they are usually lost in the same proportions.

The thorium-lead dating technique is similar to the uranium-lead technique; it uses the decay from thorium 232 to lead 208 (releasing six helium 4), with a half-life of 13,900 years. Minerals used for this method include sphene, zircon, monazite, apatite, and other rare U-Th minerals. The ratio of lead 208/thorium 232 is comparable with lead 207/ura-nium 235. This not totally reliable method is usually employed in conjunction with other methods. In most cases the results are discordant, showing a loss

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Concordia diagram showing the concordia curve that traces the evolution of the 206Pb/238U vs. 207Pb/235U ratio with time, from the present to 3.5 billion years (Ga) ago. The discordia curve shows the path that the ratio would follow if the rock example used crystallized at 3.2 billion years ago and lost lead (for example, through metamorphism) at 1.0 billion years ago.

of lead from the system. The Th-Pb method can also be interpreted by isochron diagrams.

Potassium-argon dating is based on the decay of radioactive potassium into calcium and argon gas at a specific rate and is accomplished by measuring the relative abundances of potassium 40 and argon 40 in a sample. The technique is potentially useful for dating samples as old as 4 billion years. Potassium is one of the most abundant elements in Earth's crust (2.4 percent by mass). One out of every 100 potassium atoms is radioactive potassium 40, with 19 protons and 21 neutrons. If a beta particle hits one of the protons, the latter can convert into a neutron. With 18 protons and 22 neutrons, the atom becomes argon 40, an inert gas. For every 100 potassium 40 atoms that decay, 11 become argon 40.

By comparing the proportion of potassium 40 to argon 40 in a sample and knowing the decay rate of potassium 40, one can estimate the age of the sample. The technique works well in some cases but is unreliable in samples that have been heated or recrystallized after formation. Since it is a gas, argon 40 can easily migrate in and out of potassium-bearing rocks, changing the ratio between parent and daughter.

Fission-track dating determines the thermal age of a sample, the time lapsed since the last significant heating event (typically above 215°F, or 102°C). Fission tracks are paths of radiation damage made by nuclear particles released by the spontaneous fission, or radioactive decay, of uranium 238. Fission tracks are created at a constant rate in uranium-bearing minerals, so by determining the density of tracks present, one can determine the amount of time that has passed since the tracks began to form in the mineral. Fission-track dating is used for determining the thermal ages of samples between about 100,000 and 1,000,000 years old; it is also used for estimating the uplift and erosional history of areas by recording when specific points cooled past 215.6°F (102°C).

Thermoluminescence is a chronometric dating method based on the fact that some minerals give off a flash of light when heated. The intensity of the light is proportional to the amount of radiation to which the sample has been exposed and the length of time since the sample was heated. Luminesence results from heating a substance, and thus liberating electrons trapped in its crystal defects. The phenomenon is used as a dating technique, especially for pottery. The number of trapped electrons is assumed to be related to the quantity of ionizing radiation to which the specimen has been exposed since firing, since the crystal defects are caused by ionizing radiation, and therefore the sample's age. Thus, measuring the amount of light emitted upon heating allows one to estimate of the age of the sample.

A number of other isotopic systems can be used for geochronology, but they are less common or less reliable than the methods described above. Geochronologists also incorporate relative and correlation dating techniques, such as strati-graphic correlation of dated units, to explore the wider implications of ages of dated units. A paleo-magnetic timescale has been constructed for the past 180 million years, and in many situations one can determine the age of a particular part of a stratigraphic column or location on the seafloor by examining the geomagnetic properties of that position on the column and correlating it with a known geomagnetic period. Finally, geochronologists use structural cross-cutting relationships to determine which parts of a succession are older or younger than a dated sample. Eventually the geochronolo-gist is able to put together a temporal history of a rock terrane by dating several samples and combining these ages with cross-cutting observations and correlation with other units.

See also paleomagnetism; radioactive decay; stratigraphy, stratification, cyclothem.

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