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Cosmogenic nuclide dating
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Using certain cosmogenic radionuclidesscientists 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. 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 fieldsolar winds, and atmospheric shielding due to air pressure variations.
Rates of nuclide production must be estimated in order to date a rock sample.
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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 datingthermoluminescenceor optically stimulated luminescence. The excess relative to natural abundance of cosmogenic nuclides in a rock sample is usually measured by means of accelerator mass spectrometry. Cosmogenic nuclides such as these are produced by chains of spallation reactions.
Using cosmogenic nuclides in glacial geology Sampling strategies cosmogenic nuclide dating Difficulties in cosmogenic nuclide dating Calculating an exposure age Further Reading References Comments How can we date rocks?
Geologists taking rock samples in Antarctica for cosmogenic nuclide dating. They use a hammer and chisel to sample the upper few centimetres of the rock. Cosmogenic nuclide dating can be used to determine rates of ice-sheet thinning and recession, the ages of moraines, and the age of glacially eroded bedrock surfaces.
It is an excellent way of directly dating glaciated regions. It is particularly useful in Antarctica, because of a number of factors: The lack of terrestrial marine organisms makes radiocarbon dating difficult; High winds make burial by snow less likely; Burial and cover by vegetation is unlikely. Cosmogenic nuclide dating is effective over short to long timescales 1,, yearsdepending on which isotope you are dating. Different isotopes are used for different lengths of times. This long period of applicability is an added advantage of cosmogenic nuclide dating.
Cosmogenic nuclide dating is effective for timescales from , years. What are cosmogenic nuclides? Cartoon illustrating cosmogenic nuclide exposure ages.
A glacier transports an erratic boulder, and then recedes, exposing it to cosmic rays. Spallation reactions occur in minerals in the rocks upon bombardment by cosmic rays. Cosmogenic nuclides are rare nuclides that form in surface rocks because of bombardment by high-energy cosmic rays .
These cosmic rays originate from high-energy supernova explosions in space.
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Wherever we are on Earth, when we are outside, we are constantly bombarded by these cosmic rays. When particular isotopes in rock crystals are bombarded by these energetic cosmic rays neutronsa spallation reaction results. Spallation reactions are those where cosmic-ray neutrons collide with particular elements in surface rocks, resulting in a reaction that is sufficiently energetic to fragment the target nucleus.
These spallation reactions decrease with depth.
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This is important for glacial geologists, as it means that surfaces that have had repeated glaciations with repeated periods of exposure to cosmic rays can still be dated, as long as they have had sufficient glacial erosion to remove any inherited signal.
Using cosmogenic nuclides in glacial geology Reconstructing past ice sheet extent Cosmogenic nuclide samplng an erratic granite boulder with hammer and chisel on James Ross Island, January Glacial geologists use this phenomenon to date glacial landforms, such as erratics or glacially transported boulders on moraines or glacially eroded bedrock.
Dating glacial landforms helps scientists understand past ice-sheet extent and rates of ice-sheet recession. The basic principle states that a rock on a moraine originated from underneath the glacier, where it was plucked and then transported subglacially. When it reaches the terminus of the glacier, the boulder will be deposited.
Glacial geologists are often interested in dating the maximum extents of glaciers or rates of recession, and so will look for boulders deposited on moraines. Once exposed to the atmosphere, the boulder will begin to accumulate cosmogenic nuclides. Assuming that the boulder remains in a stable position, and does not roll or move after deposition, this boulder will give an excellent Exposure Age estimate for the moraine.
Rates of ice-sheet thinning We can use cosmogenic nuclide dating to work out how thick ice sheets were in the past and to reconstruct rates of thinning.
This is crucial data for numerical ice sheet models. As well as using cosmogenic nuclide dating to work out the past extent of ice sheets and the rate at which they shrank back, we can use it to work out ice-sheet thicknesses and rates of thinning[5, 6].