Cosmogenic isotope surface exposure dating
Information on the shape and size of the Antarctic Ice Sheets over the past 20, years is contained within rocks deposited on the surface of Antarctica as the ice sheet has retreated and thinned since that time. Surface exposure dating involves collecting such rocks and measuring the abundance of an isotope concentrated within their upper surfaces, which acts as a chemical signal for the length of time since the rock was last covered by ice. As well as establishing the history of this part of the WAIS, this approach will also give us insight into the significance of ice sheet changes recorded and widely publicised over the past decade. By comparing the retreat history of glaciers in the western and eastern parts of the Amundsen Sea Embayment, we will learn how different parts of the region are likely to respond to future environmental change. This technique involves measuring the abundance of isotopes that are produced within rock surfaces when they are exposed to cosmic radiation.
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How can we date rocks? Using cosmogenic nuclides in glacial geology Sampling strategies cosmogenic nuclide dating Difficulties in cosmogenic nuclide dating Calculating an exposure age Further Reading References Comments. 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:. Cosmogenic nuclide dating is effective over short to long timescales 1,,, years , depending 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. 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 neutrons , a 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.
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. 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 strategy is the most important factor in generating a reliable exposure age.
Several factors can affect cosmogenic nuclide dating: Photo credit: 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. Bethan Davies sampling a boulder for cosmogenic nuclide dating in Greenland. Dr David Roberts. Rock samples may be collected with a hammer and chisel or with a rock saw. This can take a very long time! 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. 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 boulders , and has not been covered with large amounts of soil, snow or vegetation.
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. 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. Solifluction lobes on the Ulu Peninsula. Solifluction is common in periglacial environments, and can result in rolling, burial and movement of boulders on slopes. As mentioned above, sampling strategy is the most import factor in generating a reliable cosmogenic nuclide age. Post-depositional processes, such as rolling, burial, exhumation or cover with vegetation can result in interruption of the accumulation of cosmogenic nuclides and a younger than expected age.
Alternatively, if the boulder has not undergone sufficient erosion to remove previously accumulated cosmogenic nuclides, it will have an older than expected age. This is called inheritance. 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.
Rocks can therefore be left in a stable position or moved slightly, without having suffiicient erosion to remove cosmogenic nuclides from a previous exposure. This can result in a complex exposure history. This is typically characterised by spread of exposure ages across a single landform. Dating just one boulder from a moraine may therefore be an unreliable method to rely on. Scientists may also screen for complex exposure by using two different isotopes, such as aluminium and beryllium 26 Al and 10 Be.
The Production Rate of cosmogenic nuclides varies spatially, but is generally assumed to have remained constant at a particular location. Published production rates are available for different parts of the Earth. Glacial geologists target elements that only occur in minerals in rocks, such as quartz, through cosmic-ray bombardment, such as aluminium and beryllium 26 Al and 10 Be. Beryillium is used most widely, as it has the best determined production rate and can be measured at low concentrations.
Chlorine 36 Cl can also be used to date the exposure age of basalt lavas. Bethan Davies using HF to dissolve rocks for cosmogenic nuclide dating. Note the personal protection equipment! The first stage in the calculation of a cosmogenic nuclide exposure age is to extract the quartz from a rock. This is quite an involved process and means using some quite dangerous chemicals, such as HF Hydrogen Flouride.
HF is an acid with a pH of about 3, but the small molecule is easily absorbed by your skin. Once absorbed, it reacts vigorously with the calcium in your bones, forming Calcium Flouride which may then be deposited in your arteries. All in all, not a substance you want to get on your skin! Scientists must therefore take strong precautions before using this chemical. The first stage is to crush the rock or rock fragments in a jaw crusher. The crusher must be perfectly clean to avoid contamination.
The crushed rock is then sieved to the right size. Magnetic seperation removes particles with lots of iron such as micas , leaving you if you sampled granite, for example with a g sample of sand, comprising mostly feldspar and quartz. Feldspar is removed by placing the sample in Hexafloursilicic acid or HF on a shaking table for around 2 weeks. The acids are changed daily. The more durable quartz is left behind. A series of chemical precipitations leaves you with Beryllium Oxide BeO , a white powder.
It is mixed with Niobium NB and pressed into a copper cathode. Once the ratio of cosmogenic to naturally occuring isotopes has been calculated, the production rate is used to calculate an exposure age. This varies with altitude and latitude.
Abstract: In the last decades surface exposure dating using cosmogenic . Production rate. Advantages/minerals used. Disadvantages isotopes method. The shortlived 36 Cl isotope (t 1/2 = ka) is used in dating groundwater and cosmogenic exposure dating of landforms formed by lava flows or glacial.
The relatively new technique of surface exposure dating SED utilises primarily the build-up of 10 Be in rock materials over time rather than its radiometric decay: Its amount and that of other cosmogenic isotopes e. Analytical results may only be interpreted geologically if the 10 Be production rate is carefully calibrated, for example by correcting for partial attenuation and complete shielding effects.
This study analyzes the cosmogenic isotope surface exposure dating method for its feasibility of use on Mount Rainier because, while this is a powerful method commonly used to determine glacial history, many factors that make cosmogenic isotopes difficult to use converge on Mount Rainer.
How can we date rocks? Using cosmogenic nuclides in glacial geology Sampling strategies cosmogenic nuclide dating Difficulties in cosmogenic nuclide dating Calculating an exposure age Further Reading References Comments.
10Be for Surface exposure dating (SED)
Quartz band on sliding surface bombarded by a cosmic ray and producing here the nuclide 10Be. Earth is constantly bombarded with cosmic rays that are high-energy charged particles. These particles interact with atoms in atmospheric gases and thereby producing northern lights and the surface of Earth. 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. 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.
Surface exposure dating
Unlike other dating methods, which tell us how long it is since a rock was formed, cosmogenic surface dating tells us how long a rock has been exposed on the surface. In some cases, as when the rock is a lava flow , this amounts to the same thing. But there are other ways in which a rock can become exposed, as for example when a glacier erodes the sediment covering bedrock: In the article on radiocarbon dating we have already introduced one cosmogenic isotope , 14 C , which is produced by cosmic rays from 14 N. For cosmogenic surface dating, the two most commonly used isotopes are the cosmogenic isotopes 10 Be , which is produced from 16 O and which has a half-life of 1. Because the isotopes we're using have a short half-life , it follows that if a rock has been buried for a few million years the quantities of these isotopes will be negligible. But when the rock becomes exposed on the surface, and so exposed to cosmic rays, these cosmogenic isotopes will begin to accumulate in the rock. If we take all the relevant factors into account, and calculate, estimate, or simply measure the amount of cosmic rays a given rock is exposed to per year, and if we measure the quantities of the cosmogenic isotopes in a sample of the rock, then we can figure out how long the rock has been exposed.
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Some cosmic ray particles reach the surface of the earth and contribute to the natural background radiation environment. It was discovered about a decade ago that cosmic ray interaction with silica and oxygen in quartz produced measurable amounts of the isotopes Beryllium and Aluminium Researchers suggested that the accumulation of these isotopes within a rock surface could be used to establish how long that surface was exposed to the atmosphere.
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Surface exposure dating is a collection of geochronological techniques for estimating the length of time that a rock has been exposed at or near Earth's surface. Surface exposure dating is used to date glacial advances and retreats , erosion history, lava flows, meteorite impacts, rock slides, fault scarps , cave development, and other geological events. It is most useful for rocks which have been exposed for between 10 years and 30,, years [ citation needed ]. The most common of these dating techniques is Cosmogenic radionuclide dating [ citation needed ]. 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. By the time the cosmic ray cascade reaches the surface of Earth it is primarily composed of neutrons. 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. 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.
Surface Exposure Dating
Radiocarbon dating is abundantly used and offers very high precision dates, but we often want to date an event that is either too far in the past, or without the right type of organic matter, to be dated by 14 C. If we are particularly interested in the timing of the uncovering of a surface—say, bedrock that had been covered by ice, or sediments that had been revealed by the incision of a stream—we can employ cosmogenic nuclide surface exposure dating to study that uncovering process. Super high energy particles—mostly protons— are produced by our Sun, supernovae, and probably other extraterrestrial sources. These particles continuously enter the Earth system at incredible rates and are often, but misleadingly, called cosmic rays. Depending on the elements that a particle collides with, it produces different end products. Because 10 Be is only produced by interaction with cosmogenic radiation, measuring the concentration of 10 Be in a sample can allow you to determine how long it has been exposed to cosmic radiation.
Historical Geology/Cosmogenic surface dating
Advancements in cosmogenic 38Ar exposure dating of terrestrial rocks. Cosmogenic exposure dating of Ca-rich minerals using 38Ar on terrestrial rocks could be a valuable new dating tool to determine timescales of geological surface processes on Earth. Although apatite shows much larger 38Ar abundances than pyroxene, our modelling and analyses of unirradiated apatite suggest that apatite suffers from both natural and reactor-derived chlorogenic as well as natural nucleogenic contributions of 38Ar. Hence, we suggest that cosmogenic 38Ar exposure dating on irradiated Ca-rich and eventually K-rich , but Cl-free, terrestrial minerals is a potential valuable and accessible tool to determine geological surface processes on timescales of a few Ma. Considerations for successful cosmogenic 3He dating in accessory phases.
Terrestrial cosmogenic nuclide dating
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