Cosmogenic nuclide exposure (CNE) dating is widely used to directly date .. Contribution to paleoglacial reconstruction in Central Asia Exposure Dating and Glacial Reconstruction at Mt. Field, Tasmania, Australia, Identifies MIS 3 and MIS 2 Glacial Advances and Climatic Variability. Request a. Trimlines can therefore also be used to reconstruct past ice sheet . Chlorine (36 Cl) can also be used to date the exposure age of basalt.
Scientists dating Quaternary glacial sediments in Antarctica most commonly use one of the methods outlined below, depending on what kind of material they want to date and how old it is. It gives an Exposure Age: It is effective on timescales of several millions of years.
Radiocarbon dating dates the decay of Carbon within organic matter. Organic matter needs to have been buried and preserved for this technique.
It is effective for up to the last 40, years. It assumes that organic material is not contaminated with older radiocarbon which, for example, is a common problem with organic material from marine sediment cores around Antarctica. Amino Acid Racemisation dates the decay and change in proteins in organisms such as shells. Optically Stimulated Luminescence dates the radiation accumulated in quartz or feldspar grains within sand.
The radiation emanates from radioactive grains within the sediment, such as zircons. It is effective for hundreds of thousands of years, and dates how long the sediment has been buried. Other methods of dating glacial sediments There are so many other methods of dating Quaternary sediments and organic material that it is impractical to cover them all here in detail. 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.
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.
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. 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.
Dating Glacial Sediments
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.
Cosmogenic nuclide dating
Scientists may also screen for complex exposure by using two different isotopes, such as aluminium and beryllium 26Al and 10Be. 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 26Al and 10Be.
Introduction to dating glacial sediments
Beryillium is used most widely, as it has the best determined production rate and can be measured at low concentrations. Chlorine 36Cl can also be used to date the exposure age of basalt lavas. Extraction of quartz 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.