Geochronology is thescience ofdetermining the age ofrocks,fossils, andsediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished throughradioactive isotopes, whereas relative geochronology is provided by tools such aspaleomagnetism andstable isotope ratios. By combining multiple geochronological (andbiostratigraphic) 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 notdirectly provide an absolute age determination of a rock, but merely places it within aninterval 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 namingstrata (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 ofchronostratigraphy, which attempts to derive absolute age dates for all fossil assemblages and determine the geologichistory of the Earth andextraterrestrial bodies.
| Segments of rock (strata) inchronostratigraphy | Time spans ingeochronology | 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 | 38 defined, tens of millions of years |
| Stage | Age | 101 defined, millions of years |
| Chronozone | Chron | subdivision of an age, not used by the ICS timescale |
By measuring the amount ofradioactive decay of aradioactive isotope with a knownhalf-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 theradiocarbon method, most of these techniques are actually based on measuring an increase in the abundance of aradiogenic 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 the40Ar/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:
A series of related techniques for determining the age at which a geomorphic surface was created (exposure dating), or at which formerlysurficial 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 techniques observe 'light' emitted from materials such as quartz, diamond, feldspar, and calcite. Many types of luminescence techniques are utilized in geology, includingoptically stimulated luminescence (OSL),cathodoluminescence (CL), andthermoluminescence (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 techniques allow the construction of year-by-year annual chronologies, which can be fixed (i.e. linked to the present day and thuscalendar orsidereal time) or floating.
A sequence ofpaleomagnetic poles (usually called virtual geomagnetic poles), which are already well defined in age, constitutes anapparent 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 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.
Global trends in isotope compositions, particularly carbon-13 and strontium isotopes, can be used to correlate strata.[13]
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 floralassemblages, 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, datedtephra. Tephra is also often used as a dating tool in archaeology, since the dates of some eruptions are well-established.
Geochronology, from largest to smallest:
It is important not to confuse geochronologic and chronostratigraphic units.[15] Geochronological units are periods of time, thus it is correct to say thatTyrannosaurus rex lived during the LateCretaceous Epoch.[16] Chronostratigraphic units are geological material, so it is also correct to say that fossils of the genusTyrannosaurus have been found in the Upper Cretaceous Series.[17] In the same way, it is entirely possible to go and visit an Upper Cretaceous Series deposit – such as theHell Creek deposit where theTyrannosaurus fossils were found – but it is naturally impossible to visit the Late Cretaceous Epoch as that is a period of time.
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