Scientists use a variety of dating methods to determine the ages of glacial moraines around the world, from the poles where glaciers are at sea level to the tropics where glaciers are high in the mountains. Moraines are sedimentary deposits that mark the past extents of glaciers. Since glaciers respond sensitively to climate, especially at high latitudes and high altitudes, the timing of glacial fluctuations marked by moraines can help scientists to better understand past climatic variations and how glaciers may respond to future changes.
In the tropics, glacial scientists commonly use beryllium surface exposure dating. Beryllium is an isotope of beryllium produced when cosmic rays strike bedrock that is exposed to air. Predictable rates of decay tell scientists how long ago the isotope was generated and suggest that the rock was covered in ice before then. Elevation, latitude and other factors affect the rate at which beryllium is produced, but researchers typically use rates taken from calibration sites scattered around the globe rather than rates locally calibrated at the sites being studied.
The Dartmouth-led team looked at beryllium concentrations in moraine boulders deposited by the Quelccaya Ice Cap, the largest ice mass in the tropics. Quelccaya, which sits 18, feet above sea level in the Peruvian Andes, has retreated significantly in recent decades. The researchers determined a new locally calibrated production rate that is at least 11 percent to 15 percent lower than the traditional global production rate. The new production rate yields beryllium ages that are older than previously reported, which means the boulders were exposed for longer than previously estimated.
Prior studies suggested glaciers in the Peruvian Andes advanced during early Holocene time 8, , years ago, a period thought to have been warm but perhaps wet in the Andes. But the new production rate pushes back the beryllium ages to 11, , years ago when the tropics were cooler and drier.
Also during this time, glaciers expanded in the northern hemisphere, which indicates a relationship between the climate mechanisms that caused cooling in the northern hemisphere and southern tropics. The findings suggest the new production rate should be used to deliver more precise ages of moraines in low-latitude, high-altitude locations, particularly in the tropical Andes. Such precision can help scientists to more accurately reconstruct past glacial and climatic variations, Kelly said. Peruvian glacial retreats linked to European events of Little Ice Age. Scientists at Lamont-Doherty Earth Observatory, with their colleagues, 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.
Many methods of dating glacial sediments
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.
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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.
New study determines more accurate method to date tropical glacier moraines
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: Quaternary Science Reviews , Terrestrial and freshwater carbonates in Hoxnian interglacial deposits, UK: U-Series isochron dating of immature and mature calcretes as a basis for constructing Quaternary landform chronologies; Examples from the Sorbas Basin, southeast Spain.
Quaternary Research , Antarctic Science , 17 Late Neogene to Quaternary environmental changes in the Antarctic Peninsula region: Global and Planetary Change , 45 Share this If you enjoyed this post, please consider subscribing to the RSS feed to have future articles delivered to your feed reader. Using cosmogenic nuclides in glacial geology Sampling strategies cosmogenic nuclide dating Difficulties in cosmogenic nuclide dating Calculating an exposure age Further Reading References Comments How can we date rocks?
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Dating Glacial Sediments
Updates to this particular page are in progress. Right, I have now updated this page to a much more thorough description of CRN dating!
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