The physics of decay and origin of carbon 14 for the radiocarbon dating 1: We can indirectly date glacial sediments by looking at the organic materials above and below glacial sediments. Radiocarbon dating provides the age of organic remains that overly glacial sediments. It was one of the earliest techniques to be developed, during the s.
Can We Fix It? Carbon fluxes on glacier surfaces
Radiocarbon dating works because an isotope of carbon, 14 C, is constantly formed in the atmosphere by interaction of carbon isotopes with solar radiation and free neutrons. Living organisms absorb carbon for example, we breathe it in. This carbon is therefore present in their bodies and bones. In the figure right, the production of radio-active carbon is demonstrated.
Here, 7 protons and 7 neutrons N plus one neutron form an isotope of carbon, with 8 neutrons and 6 protons. These 14 C atoms are rapidly oxidised into carbon dioxide 12 CO 2 , and are then absorbed by living organisms and oceans. In Antarctica, where organic remains are rare, this usually means dating microscopic marine organisms in glaciomarine muds that overly glacial tills and sediments on the continental shelf.
Radiocarbon dating marine organisms has added complications in Antarctica, because around the Antarctic continent old deep ocean currents up well. Rates of radiocarbon production vary through time, in a quasi-periodic manner.sturpunfide.tk
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It is therefore necessary to distinguish between radiocarbon years 14 C and calendar years. These two ages can be reconciled using calibration against a chronology of calendar years. Tree ring data has been widely used to calibrate the timescales, as tree rings provide an annual calendar year, and the wood can be radiocarbon dated to provide a calibration. 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: 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. 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. 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.
Dating Glacial Sediments
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.
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- Carbon flux on glacier ice.
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.
About Bethan Davies
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.