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.
Explaining the science of Antarctic glaciers
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.
Surface exposure 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. 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[3].
Chlorine 36 Cl can also be used to date the exposure age of basalt lavas[4]. 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.
Cosmogenic nuclide dating
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.
Surface exposure dating - Wikipedia
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. Topographic shielding and shielding by snow, vegetation or soil is also taken into account. There are a number of online calculators that can be used to calculate the exposure age. The video below, produced by Science Bulletins, National Centre for Science Library, nicely and simply illustrates the core concepts in cosmogenic exposure age dating. Quaternary Science Reviews , 31 0: Contributions and unrealized potential contributions of cosmogenic-nuclide exposure dating to glacier chronology, Quaternary Science Reviews , 30 Global and Planetary Change , 69 4: Exposure ages from mountain dipsticks in Mac.
Geology , 35 6: Constraints on past ice volume change. Geology , 38 5: Holocene deglacial history of the north east Antarctic Peninsula — a review and new chronological constraints. Closed-system behaviour of the intra-crystalline fraction of amino acids in mollusc shells. Quaternary Geochronology , 3: Amino acids from the intra-crystalline fraction of mollusc shells: Terrestrial cosmogenic nuclide surface exposure dating of moraine boulders and alluvial fan sediments define the timing of five glacial advances over at least the last five glacial cycles in the Ladakh Range of the Transhimalaya.
The glacial stages that have been identified are: The exposure ages of the Indus Valley moraines are the oldest observed to date throughout the Himalayan orogen. We observe a pattern of progressively more restricted glaciation during the last five glacial cycles, likely indicating a progressive reduction in the moisture supply necessary to sustain glaciation.
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Alternatively, this pattern of glaciation may reflect a trend of progressively less extensive glaciation in mountain regions that has been observed globally throughout the Pleistocene. Sign In or Create an Account. Close mobile search navigation Article navigation. Volume , Number Previous Article Next Article. Research Article March 01, Terrestrial cosmogenic nuclide surface exposure dating of the oldest glacial successions in the Himalayan orogen: Ladakh Range, northern India Lewis A.
GSA Bulletin Abstract Terrestrial cosmogenic nuclide surface exposure dating of moraine boulders and alluvial fan sediments define the timing of five glacial advances over at least the last five glacial cycles in the Ladakh Range of the Transhimalaya. You do not currently have access to this article.