Surface exposure dating
Surface exposure dating is a collection of
Cosmogenic radionuclide dating
The most common of these dating techniques is cosmogenic radionuclide dating.[2] Earth is constantly bombarded with primary cosmic rays, high energy charged particles – mostly protons and alpha particles. These particles interact with atoms in atmospheric gases, producing a cascade of secondary particles that may in turn interact and reduce their energies in many reactions as they pass through the atmosphere. This cascade includes a small fraction of hadrons, including neutrons. When one of these particles strikes an atom it can dislodge one or more protons and/or neutrons from that atom, producing a different element or a different isotope of the original element. In rock and other materials of similar density, most of the cosmic ray flux is absorbed within the first meter of exposed material in reactions that produce new isotopes called cosmogenic nuclides. At Earth's surface most of these nuclides are produced by neutron spallation. Using certain cosmogenic radionuclides, scientists can date how long a particular surface has been exposed, how long a certain piece of material has been buried, or how quickly a location or drainage basin is eroding.[3] The basic principle is that these radionuclides are produced at a known rate, and also decay at a known rate.[4] Accordingly, by measuring the concentration of these cosmogenic nuclides in a rock sample, and accounting for the flux of the cosmic rays and the half-life of the nuclide, it is possible to estimate how long the sample has been exposed to the cosmic rays. The cumulative flux of cosmic rays at a particular location can be affected by several factors, including elevation, geomagnetic latitude, the varying intensity of the Earth's magnetic field, solar winds, and atmospheric shielding due to air pressure variations. Rates of nuclide production must be estimated in order to date a rock sample. These rates are usually estimated empirically by comparing the concentration of nuclides produced in samples whose ages have been dated by other means, such as radiocarbon dating, thermoluminescence, or optically stimulated luminescence.
The excess relative to natural abundance of cosmogenic nuclides in a rock sample is usually measured by means of
Chlorine-36 nuclides are also measured to date surface rocks. This isotope may be produced by cosmic ray spallation of calcium or potassium.[6]
See also
- Climate proxy
- Lichenometry, measurement of exposure time based on lichen growth
Notes
- S2CID 247396585.
- S2CID 247396585.
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- ISBN 978-0-521-87380-2.
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References
- Geomorphology and in situ cosmogenic isotopes. Cerling, T.E. and Craig, H. Annual Review of Earth and Planetary Sciences, 22, 273-317, 1994.
- Terrestrial in situ cosmogenic nuclides: theory and application. Gosse, J.C. and Phillips, F.M. Quaternary Science Reviews, 20, 1475–1560, 2001. [1]
- A complete and easily accessible means of calculating surface exposure ages or erosion rates from 10Be and 26Al measurements. Balco, Greg; Stone, John O.j Lifton, Nathaniel A.; Dunaic, Tibor J.; Quaternary Geochronology Volume 3, Issue 3, August 2008, Pages 174-195.[2]
- Geological calibration of spallation production rates in the CRONUS-Earth project. Borchers, Brian; Marrero, Shasta; Balco, Greg; Caffee, Marc; Goehring, Brent; Lifton, Nathaniel; Nishiizumi, Kunihiko; Phillips, Fred; Schaefer, Joerg; Stone, John. Quaternary Geochronology Volume 31, February 2016, Pages 188–198.
External links
- Online system for exposure age calculations
- Cosmogenic Isotope Laboratory, University of Washington
- Surface exposure dating[permanent dead link]
- Cosmogenic dating of the foothills erratics train Archived 2011-06-07 at the Wayback Machine
- Dating rockslides
- Cosmogenic isotope laboratory
- New techniques for surface exposure dating