Geochronology
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{{Geology to Paleobiology}}
In the [[natural sciences]] under the umbrella of [[natural history]], '''Geochronology''' is the [[science]] of determining the absolute age of [[rock (geology)|rocks]], [[fossil]]s, and [[sediment]]s, within a certain degree of uncertainty inherent within the method used. A variety of dating methods are used by geologists to achieve this.
Geochronology is different in application from [[biostratigraphy]], which is the science of assigning sedimentary rocks to a known geological period via describing, cataloguing and comparing fossil floral and faunal assemblages. Biostratigraphy does not ''directly'' provide an absolute age determination of a rock, merely places it within an ''interval'' of time at which that fossil assemblage is known to have coexisted. Both disciplines work together hand in hand however, to the point they share the same system of naming [[strata|rock layers]] and the time spans utilized to classify layers within a strata. (see table at right for units, left hand columns are rock record, the right units of time each sharing the same names.)
For instance, with reference to the [[Geologic Time Scale|Geologic time scale]], the Upper [[Permian]] ([[Lopingian]]) lasted from 270.6 +/- 0.7 Ma until somewhere between 250.1 +/- 0.4 Ma (oldest known [[Triassic]]) and 260.4 +/- 0.7 Ma (youngest known Lopingian) - a gap in known, dated fossil assemblages of nearly 10 Ma. While the biostratigraphic age of an Upper Permian bed may be shown to be Lopingian, the true date of the bed could be anywhere from 270 to 251 Ma.
On the other hand, a granite which is dated at 259.5 +/- 0.5 Ma can reasonably safely be called "Permian", or most properly, to have intruded in the Permian.
The science of geochronology is the prime tool used in the discipline of [[chronostratigraphy]], which attempts to derive absolute age dates for all fossil assemblages and determine the geologic history of the Earth and extraterrestrial bodies.
== Geochronologic units<ref>According to "'''Glossary of Geology''', 4th. edition, ''American Geological Institute publications'' 1997, ISBN 0-922152-34-9": Geochronological units are divisions of time based on chronostratigraphic units. It is time spans corresponding to the chronostratigraphic material units. Geochronological units in order of decreasing rank are '''eon''', '''era''', '''period''', '''epoch''' and '''age'''. Names of periods and units of lower rank are the same as those of the corresponding chronostratigraphic units; the names of some eras and eons are independently formed.</ref> ==
*[[eon (geology)|eon]] - e.g. [[Phanerozoic]]
*[[era (geology)|era]] - e.g. [[Paleozoic]]
*[[period (geology)|period]] - e.g. [[Ordovician]]
*[[epoch (geology)|epoch]] - e.g. Late Ordovician
*age - e.g. Ashgill
== Dating methods ==
* [[radiometric dating|Radiometric]] techniques measure the decay of [[radioactive isotopes]], and other radiogenic activity.
* Incremental techniques measure the regular addition of material to sediments or organisms.
* Correlation of marker horizons allow age-equivalence to be established between different sites.
== Radiometric dating ==
By measuring the amount of [[Radioactive decay|radiocative decay]] of a [[radioactive isotope]] with a known [[half-life]], geologists can establish the absolute age of the parent material. A number of radioactive isotopes are used for this purpose, and depending on the rate of decay, are used for dating different geological periods.
* [[Radiocarbon dating]]. This technique measures the decay of [[Carbon-14]] in organic material (''e.g.'' plant macrofossils), and can be applied to samples younger than about 50,000 years.
* [[Uranium-lead dating]]. This technique measures the ratio of two lead isotopes (Pb-206 and Pb-207) to the amount of uranium in a mineral or rock. Often applied to the trace mineral zircon in igneous rocks, this method is one of the two most commonly used (along with [[argon-argon dating]]) for geologic dating. Uranium-lead dating is applied to samples older than about 1 million years.
* [[Uranium-thorium dating]]. This technique is used to date [[speleothem]]s, [[coral]]s, [[carbonate]]s, and fossil [[bone]]s. Its range is from a few years to about 700,000 years.
* [[Potassium-argon dating]] and [[argon-argon dating]]. These techniques date metamorphic, [[igneous]] and [[volcanic]] rocks. They are also used to date [[volcanic ash]] layers within or overlying [[paleoanthropology|paleoanthropologic]] sites. The younger limit of the argon-argon method is a few thousand years.
Other radiogenic dating techniques include:
* [[Fission track dating]]
* Cosmogenic isotope dating
* [[Rubidium-strontium dating]]
* [[Samarium-neodymium dating]]
* [[Rhenium-osmium dating]]
* [[Lutetium-hafnium dating]]
* [[Paleomagnetism|Paleomagnetic dating]]
* [[Thermo-luminescence dating]] (quartz exposure to heat)
==Luminescence Dating==
Luminescence dating techniques observe 'light' emitted from materials such as quartz, diamond, feldspar, and calcite. Many types of luminescence techniques are utilized in geology, including [[optically stimulated luminescence]] (OSL), [[cathodoluminescence]] (CL), and [[thermoluminescence]] (TL). [[Thermoluminescence]] and [[optically stimulated luminescence]] are used in archaeology to date 'fired' objects such as pottery or cooking stones, and can be used to observe sand migration.
== Incremental dating ==
[[Incremental dating]] techniques allow the construction of year-by-year annual chronologies, which can be fixed (''i.e.'' linked to the present day and thus [[calendar]] or [[sidereal]] time) or floating.
* [[Dendrochronology]]
* [[Ice core]]s
* [[Lichenometry]]
* [[Varve]]s
==Source of error==
The geochronologic and chronostratigraphic units can be mixed up<ref>David Weishampel:''The Evolution and Extinction of the Dinosaurs'', 1996, Cambridge Press, ISBN 0-521-44496-9</ref>.
*Correct: ''[[Tyrannosaurus]] rex'' lived in Late [[Cretaceous]].
*False: ''Tyrannosaurus rex'' was found in Late Cretaceous, meaning that a timetraveller found it 67 [[Ma (unit)|Ma]].
== References ==
{{refimprove|date=September 2007}}
<references/>
*Dalrymple G.B., Grove M., Lovera O.M., Harrison, T.M., Hulen, J.B., and Lanphere, M.A. (1999),"Age and thermal history of the Geysers plutonic complex (felsite unit), Geysers geothermal field, California: a 40Ar/39Ar and U–Pb study", Earth Planet. Sci. Lett. v. 173 p. 285–298.
*Dickin, A. P. (1995). ''Radiogenic Isotope Geology''. Cambridge, Cambridge University Press. ISBN 0-521-59891-5
*Faure, G. (1986). ''Principles of isotope geology''. Cambridge, Cambridge University Press. ISBN 0-471-86412-9
*Faure, G., and Mensing, D., (2005), "Isotopes - Principles and applications". Third Edition. J. Wiley & Sons. ISBN 0-471-38437-2
*Lowe, J.J., and Walker, M.J.C. (1997), ''Reconstructing Quaternary Environments'' (2nd edition). Longman publishing ISBN 0-582-10166-2
*Ludwig, K.R., and Renne, P.R., (2000) "Geochronology on the Paleoanthropological Time Scale", Evolutionary Anthropology 2000,v. 9,Issue 2, p. 101-110.
*Renne, P.R., Ludwig, K.R., and Karner,D.B. (1998), "Progress and challenges in geochronology", Science Progress, v. 83 No. 1, p.107-121.
*Renne, P.R., Sharp, W.D., Deino. A.L., Orsi, G., and Civetta, L. )1997) "40Ar/39 Ar Dating into the Historical Realm: Calibration Against Pliny the Younger". Science, v. 277, p. 1279-1280.
*Smart, P.L., and Frances, P.D. (1991), ''Quaternary dating methods - a user's guide''. Quaternary Research Association Technical Guide No.4 ISBN 0907780083
== See also ==
*[[Age of the Earth]]
*[[Arthur Holmes]]
*[[Fritz Houtermans]]
*[[Alfred O. C. Nier]]
*[[Thomas Edvard Krogh]]
{{Time Topics}}
{{Chronology}}
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