geochronology

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.

For instance, with reference to the 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^[1]

• eon - e.g. Phanerozoic
• era - e.g. Paleozoic
• period - e.g. Ordovician
• epoch - e.g. Late Ordovician
• age - e.g. Ashgill

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.

• 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 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.

Other radiogenic dating techniques include:

• Fission track dating
• Cosmogenic isotope dating
• Rubidium-strontium dating
• Samarium-neodymium dating
• Rhenium-osmium dating
• Lutetium-hafnium dating
• Paleomagnetic dating
• Thermo-luminescence dating (quartz exposure to heat)

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

Source of error

The geochronologic and chronostratigraphic units can be mixed up^[2].

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

References

1. ^ 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.
2. ^ David Weishampel:The Evolution and Extinction of the Dinosaurs, 1996, Cambridge Press, ISBN 0-521-44496-9

• 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

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