Geochronology

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An artistic depiction of the major events in the history of Earth

Geochronology is the

biostratigraphic
) indicators the precision of the recovered age can be improved.

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

strata
(rock layers) and the time spans utilized to classify sublayers within a stratum.

The science of geochronology is the prime tool used in the discipline of

history of the Earth and extraterrestrial bodies
.

Dating methods

Units in geochronology and stratigraphy[1]
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

Main article: Radiometric dating

By measuring the amount of

radiogenic isotope, which is the decay-product of the radioactive parent isotope.[2][3][4] Two or more radiometric methods can be used in concert to achieve more robust results.[5] 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[6] and into recorded history.[7]

Some of the commonly used techniques are:

Fission-track dating

Main article: Fission track dating

Cosmogenic nuclide geochronology

Main article:
Cosmogenic radionuclide dating

A series of related techniques for determining the age at which a geomorphic surface was created (

surficial materials were buried (burial dating).[10]
Exposure dating uses the concentration of exotic nuclides (e.g. 10Be, 26Al, 36Cl) produced by cosmic rays interacting with Earth materials as a proxy for the age at which a surface, such as an alluvial fan, was created. Burial dating uses the differential radioactive decay of 2 cosmogenic elements as a proxy for the age at which a sediment was screened by burial from further cosmic rays exposure.

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

Main article: 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

Main article: 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

rhyolitic-to-basaltic tephra from Hekla
.

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:

  1. Supereon
  2. Eon
  3. Era
  4. Period
  5. Epoch
  6. Age
  7. Chron

Differences from chronostratigraphy

It is important not to confuse geochronologic and chronostratigraphic units.

Hell Creek
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.

See also

References

  1. ^ Cohen, K.M.; Finney, S.; Gibbard, P.L. (2015), International Chronostratigraphic Chart (PDF), International Commission on Stratigraphy.
  2. ISBN 0-521-59891-5
  3. ISBN 0-471-86412-9
  4. ISBN 0-471-38437-2
  5. doi:10.1016/S0012-821X(99)00223-X
    .
  6. S2CID 83948790. Archived from the original
    on 2013-01-05.
  7. ^ 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.
  8. doi:10.1017/S0033822200037954
    .
  9. ISSN 2662-8449
    .
  10. S2CID 247396585
    .
  11. S2CID 240186965
    .
  12. ^ Hnatyshin, D., and Kravchinsky, V.A., 2014. Paleomagnetic dating: Methods, MATLAB software, example. Tectonophysics, doi: 10.1016/j.tecto.2014.05.013 [1]
  13. doi:10.1139/e97-122
    .
  14. S2CID 140634223
    .
  15. ISBN 0-521-44496-9
  16. ISBN 0-922152-34-9
  17. S2CID 33454060
    .

Further reading

External links

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