Surface exposure dating

Formation of radionuclides due to cosmic radiation on rock surfaces

The surface exposure dating , also known as TCN dating (Terrestrial Cosmogenic nuclide) and under its English name Surface Exposure Dating is a geochronological method for studying landscape developments and processes at the surface. It is used, for example, to determine the age of lava flows , meteorite impacts , landslides , erosion processes and glacier movements . Compared to other methods of geochronology, which allow an absolute age determination, with the help of quantitatively determined radionuclides it is determined when a rock or a rock surface was exposed for the first or last time or for what period of time to cosmic radiation at or near the earth's surface and the period calculated under certain assumptions.

In the case of the investigation of prehistoric glacier movements, after a glacier has released the rock surface, it can be determined how long the new surface was exposed to cosmic rays, i.e. since when the glacier has retreated.

basis

By determining the concentration of cosmogenic radionuclides (e.g. ¹⁰ Be, ²⁶ Al, ³⁶ Cl) in surface rocks, it is possible to determine how long the surface under investigation was exposed to cosmic radiation. With this, in the simplest case, a minimum exposure age or, with suitable measurement combinations, the exposure age and the erosion rate can be determined.

The method can be used for periods of time that a rock was exposed to cosmic rays, up to 5,000,000 years.

Aluminum beryllium method

Skull dated by the aluminum beryllium method of “Mrs. Ples “( Australopithecus africanus ) from Sterkfontein / South Africa

The age determination via the aluminum isotope ²⁶ Al and the beryllium isotope ¹⁰ Be in the mineral quartz is based on the (known) ratio of ²⁶ Al and ¹⁰ Be, both of which are caused by cosmic radiation (neutron spallation , muon capture) on the surface of stones / minerals arise. 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 fossil hominid bones from Sterkfontein . However, the bones cannot be examined directly, but the sediments containing quartz are used.

¹⁰ Be was also used to determine the time of the final retreat of the Ice Age glaciers in Mecklenburg-Western Pomerania in the Vistula Ice Age about 14,000 to 15,000 years ago.

Chlorine method

Lava flow dated by the chlorine method west of Carrizozo / New Mexico, USA

The usability of the chlorine isotope ³⁶ Cl is based on the fact that ³⁶ Cl is almost completely created by the action of cosmic radiation on the rock surface. The main formation processes are the spallation of ³⁹ K and ⁴⁰ Ca and the activation of ³⁵ Cl by thermal neutrons . Based on an initial ratio of ³⁶ Cl to stable Cl at the beginning of the exposure of the rock on the earth's surface, the calculation is made back from the real ratio of the two chlorine isotopes, taking into account the half-life of ³⁶ Cl.

The applicability of the chlorine method also depends on the accuracy of the determination of the initial ratio of the two isotopes. I.a. Altitude and geographical latitude must be taken into account.

It is possible to determine the age of young volcanic rocks from several hundred to several million years. For example, the age of a lava flow in New Mexico has been dated to 5,200 years.

Individual evidence

↑ B. Heuel-Fabianek: Natural radioisotopes: the "atomic clock" for determining the absolute age of rocks and archaeological finds. In: Radiation Protection Practice. 1/2017, pp. 31–42.
↑ N. Akcar, S. Ivy-Ochs, C. Schlüchter: Application of in-situ produced terrestrial cosmogenic nuclides to archeology: A schematic review. In: Ice Age and the Present / Quaternary Science Journal. Vol. 57, No. 1-2, 2008, pp. 226-238.
↑ B. Heuel-Fabianek: Age determination with silicon? In: Radiation Protection Practice. 3/2003, p. 69.
^ TC Partridge, DE Granger, MW Caffee, RJ Clarke: Lower Pliocene Hominid Remains from Sterkfontein. In: Science. Vol. 300, No. 5619, April 25, 2003, pp. 607-612.
↑ A. Börner, V. Rinterknecht, D. Bourles, R. Bübers: First results of surface exposure dating on large glacial debris through in-situ formed cosmogenic beryllium-10 in Mecklenburg-Western Pomerania (Northeast Germany)? In: Z. geol. Knowledge (Berlin). Vol. 41, 2013, pp. 123-143.

↑ FM Phillips, BD Leavy, NO Jannik, D. Elmore, PW Kubik: The accumulation of cosmogenic chlorine-36 in rocks: A method for surface exposure dating. In: Science. Vol. 231, 1986, pp. 41-43.

^ NW Dunbar: Cosmogenic ³⁶ Cl -determined age of the Carrizozo lava flows, south-central New Mexico. In: New Mexico Geology. Volume 21, No. May 2, 1999.

literature

G. Balco, JO Stone, NA Lifton, TJ Dunai: A complete and easily accessible means of calculating surface exposure ages or erosion rates from ¹⁰ Be and ²⁶ Al measurements. In: Quaternary Geochronology. Volume 3, 2008, pp. 174-195.

Mark D. Kurz, Edward J. Brook: Surface exposure dating with cosmogenic nuclides. In: Dating in exposed and surface contexts. 1994, pp. 139-159.

D. Lal, JR Arnold: Tracing quartz through the environment. In: Proceedings of the Indian Academy of Sciences-Earth and Planetary Sciences. Volume 94, No. 1, 1985, pp. 1-5.

TE Cerling, H. Craig: Geomorphology and in situ cosmogenic isotopes. In: Annual Review of Earth and Planetary Sciences. Volume 22, 1994, pp. 273-317.

S. Tschudi: Surface exposure dating: A geologist's view with examples from both hemispheres. Dissertation from the Philosophical and Natural Sciences Faculty of the University of Bern, 2000. (ams.ethz.ch , PDF; 1.8 MB).

MG Zreda, FM Phillips, D. Elmore, PW Kubik, P. Sharma, RI Dorn: Cosmogenic chlorine-36 production rates in terrestrial rocks. In: Earth and Planetary Science Letters. Volume 105, 1991, pp. 94-109.

Web links

[1] B. Heuel-Fabianek: Natural radioisotopes: the "atomic clock" for determining the absolute age of rocks and archaeological finds. In: Radiation Protection Practice. 1/2017, pp. 31–42.

[2] N. Akcar, S. Ivy-Ochs, C. Schlüchter: Application of in-situ produced terrestrial cosmogenic nuclides to archeology: A schematic review. In: Ice Age and the Present / Quaternary Science Journal. Vol. 57, No. 1-2, 2008, pp. 226-238.

[3] B. Heuel-Fabianek: Age determination with silicon? In: Radiation Protection Practice. 3/2003, p. 69.

[4] TC Partridge, DE Granger, MW Caffee, RJ Clarke: Lower Pliocene Hominid Remains from Sterkfontein. In: Science. Vol. 300, No. 5619, April 25, 2003, pp. 607-612.

[5] A. Börner, V. Rinterknecht, D. Bourles, R. Bübers: First results of surface exposure dating on large glacial debris through in-situ formed cosmogenic beryllium-10 in Mecklenburg-Western Pomerania (Northeast Germany)? In: Z. geol. Knowledge (Berlin). Vol. 41, 2013, pp. 123-143.

[6] FM Phillips, BD Leavy, NO Jannik, D. Elmore, PW Kubik: The accumulation of cosmogenic chlorine-36 in rocks: A method for surface exposure dating. In: Science. Vol. 231, 1986, pp. 41-43.

[7] NW Dunbar: Cosmogenic ³⁶ Cl -determined age of the Carrizozo lava flows, south-central New Mexico. In: New Mexico Geology. Volume 21, No. May 2, 1999.