geochronology

geochronology

[jee-oh-kruh-nol-uh-jee]

Dating and interpretation of geologic events in the history of the Earth. The classical technique of geochronology was stratigraphy, including faunal succession. Since the mid 20th century, radiometric dating has provided absolute age data to supplement the relative dates obtained from the fossil record. Radiometric dating is based on the principle that radioactive isotopes in geologic material decay at constant, known rates to daughter isotopes. Seealso carbon-14 dating.

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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.

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 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 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. 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.

Other radiogenic dating techniques include:

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.

Source of error

The geochronologic and chronostratigraphic units can be mixed up.

  • Correct: Tyrannosaurus rex lived in Late Cretaceous.
  • False: Tyrannosaurus rex was found in Late Cretaceous, meaning that a timetraveller found it 67 Ma.

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

External links

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