geochronology


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Corresponding units
in chronostratigraphy and
geochronology
Chronostratigraphy geochronology
Aeonothem aeon
Arathem era
system period
series epoch
step Age

Geochronology (from ancient Greek : γῆ (ge) "earth" and χρὁνος (chronos) "time, duration" and logic ) is the scientific discipline that dates the events of the earth's history and, secondarily, the time of formation of rocks and sediments (see chronostratigraphy ) in absolute time . Among other things, it uses the determined data to create the geological time scale in which time intervals are identified, named as geochronological units and shown dated in time.

Often geochronological units correspond with the formation time of chronostratigraphic units, i.e. physically existing rock bodies. Geochronology, on the other hand, is essentially immaterial and is therefore not a stratigraphic (rock-dating) discipline in the true sense of the word. The relationships between specific geochronological units are always expressed in an older / younger relationship.

The dating of rocks can be absolute or relative.

Methods

Sedimentation

For a long time there were no direct methods for determining the absolute age of rocks. Estimates were based on erosion rates in mountains and sedimentation rates in lakes and oceans. At the beginning of the 20th century, the Swedish geologist Gerard Jakob De Geer founded varven chronology , i.e. the counting of annual layers ( varves ). Annual layers can also be seen in ice cores .

The establishment of local relative layer sequences and their regional and global allocation is the subject of stratigraphy .

Isotope measurement

Findings on chronometric questions from isotope geology are used here to determine age. With the discovery of radioactivity , various measuring methods were developed based on the investigation of the proportions of natural radioisotopes . The isotope ratios change due to different decay times ( half-life ) or natural radiation (radioactivity of the earth or extraterrestrial radiation).

Today, methods are also used that are based on the quantitative determination of artificially generated radioisotopes, e.g. B. the tritium method to determine the age of near-surface groundwater. After such an isotope is introduced into the water, the content of the isotope decreases due to disintegration and possibly dilution.

The first on the uranium - lead - decay chain based age determination was in 1913 by Arthur Holmes published and at the time was very controversial. In 1953, Friedrich Georg Houtermans published, based on the uranium-lead isotope measurements of meteorites carried out by Clair Cameron Patterson , the now accepted age of about 4.5 billion years. Today, different radioactive isotopes and their decay products are used to determine the age of rocks . The age of a rock is to be interpreted differently depending on the examination method. In the case of igneous rocks , the age of crystallization (the settlement in the earth's crust ) and, depending on the mineral examined , several cooling ages can be determined. The period of a metamorphic event can also be determined in metamorphic rocks . In some sediments , certain minerals are formed during deposition (for example glauconite in many marine (green) sandstones ), the age of which can be determined by measuring radioactive isotopes. This age is then interpreted as the sedimentation age.

Rubidium-Strontium Method

Rubidium 87 Rb decays into 87 Sr. with a half-life of 47 billion years. Radiometric dating is suitable for very old rocks. Since the stable 86 Sr also occurs in addition to 87 Sr, the isochronous method can be used to obtain very precise data for, for example, feldspars , hornblende or mica in the order of 1000 million years with an error of several 10 million years.

Uranium-lead method

The uranium-lead method uses two series of decays :

  1. Decay of the radioisotope 235 U with a half-life of 703.8 million years via various daughter isotopes to stable 207 Pb ( uranium-actinium series )
  2. Decay of the radioisotope 238 U with a half-life of 4.468 billion years via various daughter isotopes to a stable 206 Pb ( uranium-radium series )

The age of uranium-containing minerals can now be determined from the ratio of the daughter isotopes to the remaining portion of the mother isotope (here: U) with knowledge of the half-life of the mother isotope. In doing so, the content of the lead isotopes 207 Pb and 206 Pb existing before the radioactive decay must be taken into account; this is done by measuring the content of 204 Pb that was not created by radioactive decay, i.e. that is already present : The unchanged ratios of 207 Pb / 204 Pb and 206 Pb / 204 Pb are known from the measurement of meteorite material, so the 204 Pb- Content also the original content of 207 Pb or 206 Pb can be calculated; this must be subtracted from the measured content - the rest was then caused by radioactive decay.

A great advantage of the uranium-lead method is that you can usually use both decay series and thus secure your result. Because of the long half-lives, the method is best suited to determine age from one million years.

Potassium-argon and argon-argon method

The potassium-argon method uses the decomposition products of potassium. Potassium itself occurs in nature in the form of three isotopes : 39 K (93.26%), 40 K (0.012%), 41 K (6.73%).

The radioactive 40 K decays to 40 Ar and 40 Ca with a half-life of 1.277 · 10 9 years . The rarely occurring 40 Ar is used to determine the age. 40 Ca occurs very frequently as an isotope of calcium , so that the formation of additional 40 Ca from the decay of potassium is hardly measurable and is therefore not suitable for age determinations.

To determine the 40 Ar content of a rock, the rock must be melted. The noble gas 40 Ar is determined in the gas escaping in the process . If the 40 K content of the rock has also been determined, the age of the rock can be calculated from the change in the ratio of 40 K to 40 Ar between the time the rock was formed or solidified and the time the ratio was determined in the laboratory.

Due to the relatively long half-life of 1.28 · 10 9 years, these methods are suitable for rocks that are more than 100,000 years old.

The 40 Ar / 39 Ar method uses the formation of 39 Ar from 39 K by neutron bombardment of a rock sample in a reactor. After the bombardment, the ratio of the isotopes 40 Ar and 39 Ar released during the subsequent melting of a rock sample is determined.

As with the potassium-argon method, 40 Ar is the daughter isotope. Since the isotope ratios of K are known, 39 Ar, which is formed during the decay of 39 K through neutron bombardment, can be used as a substitute for the K parent isotope.

Only the ratio of 40 Ar to 39 Ar in the exiting gas needs to be determined. Analyzes of other isotopes by further analytical methods are not required.

Radiocarbon method

The radiocarbon method, which is particularly suitable for determining the age of organic material, geologically more recent, uses the decay of the 14 C generated by cosmic radiation in the higher atmosphere (half-life: 5730 years). It is only suitable for geological purposes if carbon-containing objects are to be dated that are less than approx. 50,000 years old. It is thus limited to the Quaternary .

The main areas of application of the radiocarbon method are archeology , archaeological stratigraphy and historical climatology .

Aluminum beryllium method

Formation of radionuclides (e.g. 26 Al, 10 Be) due to cosmic radiation on rock surfaces

The age determination with the help of surface exposure dating via the aluminum isotope 26 Al and the beryllium isotope 10 Be in the mineral quartz (SiO 2 ) is based on the (known) ratio of 26 Al and 10 Be, both of which are caused by cosmic radiation (neutron spallation , muon capture ) arise on the surface of stones / minerals. The ratio depends u. a. on the altitude, the geomagnetic latitude, the radiation geometry and a possible weakening of the radiation through shielding (movement, covering). The specific radiation conditions and thus the ratio of 26 Al to 10 Be must be able to be determined or estimated before the age is determined.

From the point in time at which the material in question was shielded from cosmic radiation (e.g. by storing it in a cave), the proportion of the two radionuclides decreases at different rates due to radioactive decay, so that the ratio of these radionuclides to The time of the examination and the assumed (known) equilibrium ratio under irradiation and knowledge of the respective half-lives (see also nuclide map ) allows the age to be estimated.

This method was also used to determine the age of hominid fossil bones. However, the bones cannot be examined directly, but the sediments containing quartz are used.

Samarium neodymium method

Samarium 146 converts to neodymium 142 via alpha decay . The long half-life of the samarium allows age determinations of geological time periods. Before 2012 it was assumed that the samarium had a half-life of 103 million years. Presumably it is significantly shorter at 68 million years. From the measured ratio to the stable isotope 144 Sm of 0.007 and an initial ratio of 0.0094, an age for the earth of 4.57 billion years follows.

Tritium method

Tritium ( 3 H) is a natural isotope of hydrogen and decays with a half-life of 12.32 years. The atmospheric nuclear weapons tests in the 1950s and early 1960s released large amounts of tritium into the atmosphere .

Tritium then found its way into surface waters and near-surface groundwater through precipitation . The decrease in the tritium concentration through dilution and radiological decay enables the age of a water to be determined, ie its entry via precipitation or its retention time in the aquifer, provided that possible dilution can be estimated from existing water or other inflows.

With the tritium method, it is possible to determine the residence times of the groundwater from a few years to several decades. The prerequisite is that this entry did not take place before the atmospheric nuclear weapons tests.

Other methods

  • Neodymium Strontium Method
  • Lutetium-Hafnium Method
  • Rhenium-Osmium Method

See also

literature

  • I. Wendt: Radiometric Methods in Geochronology. Clausthaler Tektonische Hefte, 13, EPV Clausthal-Zellerfeld 1972 ( pdf 7 MB ).
  • D. Brookins: Geochemical Aspects of Radioactive Waste Disposal ; Springer-Verlag, New York Inc .; 1984; ISBN 3-540-90916-8
  • G. Faure: Principles of Isotope Geology ; John Wiley & Sons; 1986; ISBN 0-471-86412-9
  • G. Faure, D. Mensing (2005): Isotopes - Principles and applications . Third edition. J. Wiley & Sons. ISBN 0-471-38437-2
  • JM Mattinson (2013): Revolution and evolution: 100 years of U-Pb geochronology . Elements 9, 53-57.
  • Mebus A. Geyh: Handbook of physical and chemical age determination ; Scientific Book Society; 2005; ISBN 3-534-17959-5
  • H. Murawski, W. Meyer: Geological dictionary. 10th edition: 278 p., 82 fig., 7 tab .; Stuttgart (Enke); 1998.
  • St. M. Stanley: Historical Geology. 2nd edition: XIII + 710 p., 632 ill .; Heidelberg u. Berlin (Spectrum Academic Publishing House); 2001.
  • FF Steininger, WE Piller (Ed.): Recommendations (guidelines) for handling the stratigraphic nomenclature. Courier Forschungsinstitut Senckenberg, Vol. 209: 19 p., 11 fig., 3 tab .; Frankfurt am Main 1999.

Web links

swell

  1. Wilhelm Gemoll : GEMOLL, Greek-German school and hand dictionary , G. Freytag Verlag, Munich.
  2. Murawski & Meyer 1998: 74.
  3. ^ Stanley 2001: 145.
  4. Steininger & Piller 1999: 4.
  5. Isotope Geochemistry and Isotope Geology , Institute of Earth Sciences of the University of Heidelberg 25 October 2012 call on Jan. 15, 2017.
  6. Heuel-Fabianek, B. in: Radiation Protection Practice, 3/2003, p. 69.
  7. Norikazu Kinoshita et al. (2012) (PDF; 4.3 MB) Literature values ​​so far: 103 + -5 · 10 6 a.