Geochronology

Geochronology is the
Geochronology is different in application from biostratigraphy, which is the science of assigning sedimentary rocks to a known geological period via describing, cataloging and comparing fossil floral and faunal assemblages. Biostratigraphy does not directly provide an absolute age determination of a rock, but 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 where they share the same system of naming
The science of geochronology is the prime tool used in the discipline of
Dating methods
Segments of rock (strata) in chronostratigraphy | Time spans in geochronology | Notes to geochronological units |
---|---|---|
Eonothem | Eon | 4 total, half a billion years or more |
Erathem | Era | 10 defined, several hundred million years |
System | Period | 22 defined, tens to ~one hundred million years |
Series | Epoch | 34 defined, tens of millions of years |
Stage | Age | 99 defined, millions of years |
Chronozone | Chron |
subdivision of an age, not used by the ICS timescale |
Radiometric dating
By measuring the amount of
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.[8][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. Monazite geochronology is another example of U–Pb dating, employed for dating metamorphism in particular. 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 paleoanthropologicsites. The younger limit of the argon–argon method is a few thousand years.
- Electron spin resonance(ESR) dating
Fission-track dating
Cosmogenic nuclide geochronology
A series of related techniques for determining the age at which a geomorphic surface was created (
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).[11] 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.
Paleomagnetic dating
A sequence of paleomagnetic poles (usually called virtual geomagnetic poles), which are already well defined in age, constitutes an apparent polar wander path (APWP). Such a path is constructed for a large continental block. APWPs for different continents can be used as a reference for newly obtained poles for the rocks with unknown age. For paleomagnetic dating, it is suggested to use the APWP in order to date a pole obtained from rocks or sediments of unknown age by linking the paleopole to the nearest point on the APWP. Two methods of paleomagnetic dating have been suggested: (1) the angular method and (2) the rotation method.[12] The first method is used for paleomagnetic dating of rocks inside of the same continental block. The second method is used for the folded areas where tectonic rotations are possible.
Magnetostratigraphy
Magnetostratigraphy determines age from the pattern of magnetic polarity zones in a series of bedded sedimentary and/or volcanic rocks by comparison to the magnetic polarity timescale. The polarity timescale has been previously determined by dating of seafloor magnetic anomalies, radiometrically dating volcanic rocks within magnetostratigraphic sections, and astronomically dating magnetostratigraphic sections.
Chemostratigraphy
Global trends in isotope compositions, particularly carbon-13 and strontium isotopes, can be used to correlate strata.[13]
Correlation of marker horizons
Marker horizons are stratigraphic units of the same age and of such distinctive composition and appearance that, despite their presence in different geographic sites, there is certainty about their age-equivalence. Fossil faunal and floral assemblages, both marine and terrestrial, make for distinctive marker horizons.[14] Tephrochronology is a method for geochemical correlation of unknown volcanic ash (tephra) to geochemically fingerprinted, dated tephra. Tephra is also often used as a dating tool in archaeology, since the dates of some eruptions are well-established.
Geological hierarchy of chronological periodization
Geochronology, from largest to smallest:
- Supereon
- Eon
- Era
- Period
- Epoch
- Age
- Chron
Differences from chronostratigraphy
It is important not to confuse geochronologic and chronostratigraphic units.
See also
- Astronomical chronology
- Chronological dating, archaeological chronology
- Absolute dating
- Relative dating
- Phase (archaeology)
- Archaeological association
- Geochronology
- General
- Consilience, evidence from independent, unrelated sources can "converge" on strong conclusions
References
- ^ Cohen, K.M.; Finney, S.; Gibbard, P.L. (2015), International Chronostratigraphic Chart (PDF), International Commission on Stratigraphy.
- ISBN 978-0-521-59891-0.
- ISBN 978-0-471-86412-7.
- ISBN 978-0-471-38437-3.
- .
- S2CID 83948790. Archived from the originalon 2013-01-05.
- ^ Renne, P. R., Sharp, W. D., Deino. A. L., Orsi, G., and Civetta, L. 1997. Science, 277, 1279-1280 "40Ar/39Ar dating into the historical realm: Calibration against Pliny the Younger" (PDF). Archived from the original (PDF) on 2008-10-30. Retrieved 2008-10-25.
- .
- ISSN 2662-8449.
- S2CID 247396585.
- S2CID 240186965.
- .
- doi:10.1139/e97-122.
- S2CID 140634223.
- ISBN 978-0-521-44496-5.
- ISBN 978-0-922152-34-6.
- S2CID 33454060.
Further reading
- Smart, P.L., and Frances, P.D. (1991), Quaternary dating methods - a user's guide. Quaternary Research Association Technical Guide No.4 ISBN 0-907780-08-3
- Lowe, J.J., and Walker, M.J.C. (1997), Reconstructing Quaternary Environments (2nd edition). Longman publishing ISBN 0-582-10166-2
- Mattinson, J. M. (2013), Revolution and evolution: 100 years of U-Pb geochronology. Elements 9, 53–57.
- Geochronology bibliography Talk:Origins Archive