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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. For a rock to be suitable for cosmogenic nuclide dating, quartz must occur in the rock in sufficient quantities and in the sufficient size fraction. They want to sample a rock that they are sure has undergone subglacial transport. It is an excellent way of directly dating glaciated regions. This is typically characterised by spread of exposure ages across a single landform.
The first stage in the calculation of a cosmogenic nuclide exposure age is to extract the quartz from a rock. 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. This is called inheritance.
These cosmic rays originate from high-energy supernova explosions in space. The more durable quartz is left behind. Sampling and dating boulders in a transect down a mountain will rapidly establish how thick your ice sheet was and how quickly it thinned during deglaciation.
Accounting for variable production rates Bethan Davies cosmogenic nuclide sampling a sandstone boulder on a moraine. Topographic shielding, for example by a nearby large mountain, also affects the production rate of cosmogenic nuclides. The first stage is to crush the rock or rock fragments in a jaw crusher.
The Production Rate of cosmogenic nuclides varies spatially, but is generally assumed to have remained constant at a particular location. Cosmogenic nuclide dating can also be used in this context to understand past ice-sheet thicknesses and changes in subglacial thermal regime.
Rock samples may be collected with a hammer and chisel or with a rock saw. These factors must be measured by the scientist, and are accounted for in the calculation of the exposure age.
Stable position Frost heave in periglacial environments can repeatedly bury and exhume boulders, resulting in a complex exposure age. Scientists must therefore carefully measure the horizon line all for degrees all around their boulder.
As mentioned above, sampling strategy is the most import factor in generating a reliable cosmogenic nuclide age. Ian Hey Cosmogenic nuclide production rates vary according to latitude and elevation.
Wherever we are on Earth, when we are outside, we are constantly bombarded by these cosmic rays. Many mountains have trimlines on them, and are smoothed and eroded below the trimline, and more weathered with more evidence of periglaciation above the trimline. This can result in a complex exposure history. This can be a particular problem in Antarctica, where cold-based ice may repeatedly cover a boulder, preventing the accumulation of cosmogenic nuclides, without eroding or even moving the rock.
The basic principle states that a rock on a moraine originated from underneath the glacier, where it was plucked and then transported subglacially. Dating glacial landforms helps scientists understand past ice-sheet extent and rates of ice-sheet recession. Published production rates are available for different parts of the Earth. This is crucial data for numerical ice sheet models.
Cartoon illustrating cosmogenic nuclide exposure ages. This long period of applicability is an added advantage of cosmogenic nuclide dating. Spallation reactions occur in minerals in the rocks upon bombardment by cosmic rays. Further reading The video below, produced by Science Bulletins, National Centre for Science Library, nicely and simply illustrates the core concepts in cosmogenic exposure age dating.
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