cosmogenic isotope dating: Topics by rozamira.info
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.
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. 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.
Cosmogenic nuclide dating
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.I GOT EXPOSED ON A MUSLIM DATING SITE!!
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].
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.
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. Trimlines can therefore also be used to reconstruct past ice sheet thickness. However, this can be difficult, as thermal boundaries within the ice sheet may mean that it is more erosive lower down than higher up, and that cold, non-erosive ice on the tops of mountains may leave in tact older landscapes. Cosmogenic nuclide dating can also be used in this context to understand past ice-sheet thicknesses and changes in subglacial thermal regime.
Sampling strategies cosmogenic nuclide dating Sampling strategy is the most important factor in generating a reliable exposure age. Several factors can affect cosmogenic nuclide dating: Mike Hambrey Geologists must ensure that they choose an appropriate rock. Granite and sandstone boulders are frequently used in cosmogenic nuclide dating, as they have large amounts of quartz, which yields Beryllium, a cosmogenic nuclide ideal for dating glacial fluctuations over Quaternary timescales.
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. A general rule of thumb is that you should be able to see the quartz crystals with the naked eye. Attenuation of cosmic rays Bethan Davies sampling a boulder for cosmogenic nuclide dating in Greenland.
Rock samples may be collected with a hammer and chisel or with a rock saw. This can take a very long time! Stable position Frost heave in periglacial environments can repeatedly bury and exhume boulders, resulting in a complex exposure age.
One of the largest errors in cosmogenic nuclide dating comes from a poor sampling strategy. Because cosmic rays only penetrate the upper few centimetres of a rock, movement of a boulder downslope can result in large errors in the age calculated.
Surface exposure dating
Before sampling a rock, geologists must take detailed and careful measurements of the landsurface, and satisfy themselves that the rock is in a stable position, has not rolled, slipped downslope, been repeatedly buried and exhumed during periglacial rock cycling within the active layer frequently a problem with small bouldersand has not been covered with large amounts of soil, snow or vegetation. Signs of subglacial transport Scratches striations on a sandstone boulder show that it has undergone subglacial transport and erosion.
They want to sample a rock that they are sure has undergone subglacial transport. They will therefore sample boulders that are subrounded, faceted, bear striations, or show other signs of subglacial transport.
- 10Be for Surface exposure dating (SED)
Accounting for variable production rates Bethan Davies cosmogenic nuclide sampling a sandstone boulder on a moraine. Ian Hey Cosmogenic nuclide production rates vary according to latitude and elevation.
These factors must be measured by the scientist, and are accounted for in the calculation of the exposure age. Topographic shielding, for example by a nearby large mountain, also affects the production rate of cosmogenic nuclides. This is because the cosmic rays, which bombard Earth at a more or less equal rate from all sectors of the sky, will be reduced if the view of the sky is shielded — for example, by a large mountain that the rays cannot penetrate.
Scientists must therefore carefully measure the horizon line all for degrees all around their boulder. Difficulties in cosmogenic nuclide dating Solifluction lobes on the Ulu Peninsula.
Decay rates are given by the decay constants of the nuclides. These equations can be combined to give the total concentration of cosmogenic radionuclides in a sample as a function of age.
The two most frequently measured cosmogenic nuclides are beryllium and aluminum These nuclides are particularly useful to geologists because they are produced when cosmic rays strike oxygen and siliconrespectively.
The parent isotopes are the most abundant of these elements, and are common in crustal material, whereas the radioactive daughter nuclei are not commonly produced by other processes.
As oxygen is also common in the atmosphere, the contribution to the beryllium concentration from material deposited rather than created in situ must be taken into account.
Each of these nuclides is produced at a different rate. Both can be used individually to date how long the material has been exposed at the surface. Because there are two radionuclides decaying, the ratio of concentrations of these two nuclides can be used without any other knowledge to determine an age at which the sample was buried past the production depth typically 2—10 meters.
Chlorine nuclides are also measured to date surface rocks. This isotope may be produced by cosmic ray spallation of calcium or potassium.