geochronology

In the natural sciences under the umbrella of natural history, Geochronology is the science of determining the absolute age of rocks, fossils, and sediments, within a certain degree of uncertainty inherent within the method used. A variety of dating methods are used by geologists to achieve this. The interdisciplinary approach of using several methods can often achieve best results. ^[2]

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 rock layers and the time spans utilized to classify layers within a strata. (See table at right for terminology.)

For instance, with reference to the Geologic time scale, the Upper Permian (Lopingian) lasted from 270.6 +/- 0.7 Ma (Ma = millions of years ago) 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.

Dating methods

Radiometric dating

By measuring the amount of Radioactive 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. More slowly decaying isotopes are useful for longer periods of time, but less accurate in absolute years. With the exception of the radiocarbon method, most of these techniques are actually based on measuring an increase in the abundance of a radiogenic isotope, which is the decay-product of the radioactive parent isotope.^[3][4][5] Two or more radiometric methods can be used in concert to achieve more robust results.^[6] Most radiometric methods are suitable for geological time only, but some such as the radiocarbon method and the ⁴⁰Ar/³⁹Ar dating method can be extended into the time of early human life ^[7] and into recorded history.^[8]

Some of the commonly-used techniques are:

• Radiocarbon dating. This technique measures the decay of carbon-14 in organic material and can be best applied to samples younger than about 60,000 years.^[9]
• Uranium-lead dating. This technique measures the ratio of two lead isotopes (lead-206 and lead-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 speleothems, corals, carbonates, and fossil bones. 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 paleoanthropologic sites. The younger limit of the argon-argon method is a few thousand years.

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 cores
• Lichenometry
• Varves

Correlation of marker horizons

Marker horizons are geological units in different geographic locations but which are of the same age. This allow age-equivalence to be established between different sites. ^[10] For example, tephra is often used in archaeology.

Differences between chronostratigraphy and geochronology

It is important not to confuse geochronologic and chronostratigraphic units. ^[11] Geochronological units are periods of time, thus it is correct to say that Tyrannosaurus rex lived during the the Late Cretaceous Epoch. ^[12] Chronostratigraphic units are geological material, so it is also correct to say that fossils of the genus Tyrannosaurus have been found in the Upper Cretaceous Series. ^[13] In the same way, it is entirely possible to go and visit an Upper Cretaceous Series deposit - such as the Egyptian mangrove deposit where the Tyrannosaurus fossils were found - but it is naturally impossible to visit the Late Cretaceous Epoch as that is a period of time.

References

1. ^ International Commission on Stratigraphy. "International Stratigraphic Chart". http://www.stratigraphy.org/chus.pdf. Retrieved on 2008-06-17. 
2. ^ Renne, P.R., Ludwig, K.R. and Karner, D.B. 1998. Progress and challenges in geochronology. Science Progress, 83, 107-121 [1]
3. ^ Dickin, A. P. 1995. Radiogenic Isotope Geology. Cambridge, Cambridge University Press. ISBN 0-521-59891-5
4. ^ Faure, G. 1986. Principles of isotope geology. Cambridge, Cambridge University Press. ISBN 0-471-86412-9
5. ^ Faure, G., and Mensing, D. 2005. "Isotopes - Principles and applications". 3^rd Edition. J. Wiley & Sons. ISBN 0-471-38437-2
6. ^ 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 and Planetary Science Letters, 173, 285–298 [2]
7. ^ Ludwig, K.R. and Renne, P.R. 2000. Geochronology on the Paleoanthropological Time Scale. Evolutionary Anthropology, 9, 101-110 [3]
8. ^ Renne, P.R., Sharp, W.D., Deino. A.L., Orsi, G., and Civetta, L. 1997. ⁴⁰Ar/³⁹Ar dating into the historical realm: Calibration against Pliny the Younger. Science, 277, 1279-1280 [4]
9. ^ Plastino, W.; Kaihola, L.; Bartolomei, P.; Bella, F. (2001). "Cosmic Background Reduction In The Radiocarbon Measurement By Scintillation Spectrometry At The Underground Laboratory Of Gran Sasso". Radiocarbon 43 (2A): 157–161. https://digitalcommons.library.arizona.edu/objectviewer?o=http%3A%2F%2Fradiocarbon.library.arizona.edu%2Fvolume43%2Fnumber2A%2Fazu_radiocarbon_v43_n2a_157_161_v.pdf. 
10. ^ Demidov, I.N. 2006. Identification of marker horizon in bottom sediments of the Onega Periglacial Lake. Doklady Earth Sciences, 407, 213-216 [5]
11. ^ David Weishampel:The Evolution and Extinction of the Dinosaurs, 1996, Cambridge Press, ISBN 0-521-44496-9
12. ^ Barrick, R.E. and Showers, W.J. 1994. Thermophysiology of Tyrannosaurus rex: Evidence from Oxygen Isotopes. Science, 265, 222-224 [6]
13. ^ Smith, J.B., Lamanna, M.C., Lacovara, K.J., Dodson, P. Jnr., Poole, J.C. and Giegengack, R. 2001. A Giant Sauropod Dinosaur from an Upper Cretaceous Mangrove Deposit in Egypt. Science, 292, 1704-1707 [7]

Further reading

• Lowe, J.J., and Walker, M.J.C. (1997), Reconstructing Quaternary Environments (2nd edition). Longman publishing ISBN 0-582-10166-2
• Smart, P.L., and Frances, P.D. (1991), Quaternary dating methods - a user's guide. Quaternary Research Association Technical Guide No.4 ISBN 0907780083
• Geochronology bibliography Talk:Origins Archive

See also

• Age of the Earth
• List of geochronologic names
• Thermochronology
• Closure temperature
• Arthur Holmes
• Fritz Houtermans
• Alfred O. C. Nier
• Thomas Edvard Krogh

External links

• International Commission on Stratigraphy

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