Browsing by Subject "cosmogenic nuclides"
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(2020)Terrestrial cosmogenic nuclides can be used to date glacial events. The nuclides are formed when cosmic rays interact with atoms in rocks. When the surface is exposed to the rays, the number of produced nuclides increases. Shielding, like glaciation, can prevent production. Nuclide concentration decreases with depth because the bedrock attenuates the rays. The northern hemisphere has experienced several glaciations, but typically only the latest one can be directly observed. The aim of the study was to determine if these nuclides, produced by cosmic rays, can be used to detect glaciations before the previous one by using a forward and an inverse model. The forward model predicted the nuclide concentration with depth based on a glacial history. The longer the exposure duration was, the higher was the number of nuclides in the rock. In the model, it was possible to use three isotopes. Be-10, C-14 and Al-26. The forward model was used to produce synthetic samples, which were then used in the inverse model. The purpose of the inverse model was to test which kind of glacial histories produce similar nuclide concentrations than what the sample had. The inverse model produced a concentration curve which was compared with the concentration of the samples. The misfit of the inverse solution was defined with an “acceptance box”. The box was formed from the thickness of the sample and the corresponding concentrations. If the curve intersected with the box, the solution was accepted. Small misfit values were gained if the curve was close to the sample. The idea was to find concentration curves which have as similar values as the samples. The inverse model was used in several situations, where the number of limitations was varied. If the timing of the last deglaciation and amount of erosion were known, the second last deglaciation was found relatively well. With looser constraints, it was nearly impossible to detect the past glaciations unless a depth profile was used in the sampling. The depth profile provided a tool to estimate the amount of erosion and the total exposure duration using only one isotope.
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(2020)This study reviews the glacial history and the glacial transportation history in Kaarestunturi, Sodankylä in the Finnish Central Lapland interlobate region. The study is conducted on the Aamurusko prospect, where gold bearing quartz vein boulders have been discovered. The local glacial transportation history is studied in order to deduct the likely origin of the boulders. The local portion of the study involves rock and soil sample assays, glacial sedimentological studies, trenching and more unconventional Glacier Dynamic (GD) mapping and Terrestrial Cosmogenic Nuclide (TCN) dating. The local interpretation is set in a regional glacial history framework that is based on wide scaled GD mapping and existing literature. Kaarestunturi is incorporated in multiple regional glacial studies, but no local glacial studies have been conducted in the area. GD mapping indicates that the area has not hosted pronounced ice streams during recent stadials, however, it has experienced different types of glacial transportation. Till fabric analysis shows that the Kaarestunturi and adjacent Kiimajänkä have been subjected to both pronounced and indecisive ice flow directions. The most strongly orientated till fabrics portray repeated flow from NE for the lower till units and a flow from NW for surficial till. The NW flow is also indicated by fans interpreted from soil and rock sample assays. Field studies indicate that the Kaarestunturi fell likely interfered with local glacial flow, making some of the orientations local and till units topographically controlled. However, the till fabric analyses were too distal to confirm the impact the steering had on till deposition. The apparent ages of the four TCN samples are in the range of 27.9 ± 0.9–87.6 ± 2.4 ka BP. The TCN dating based glacial history model indicate that the dated boulders were affected by 2–3 glacial transportation events. The exposure ages together with sedimentological studies argue that at least the dated boulders are allochthonous. Based on this study TCN dating can offer new insight for ore exploration projects in glaciated terrain. Although the sampling has many prerequisites, being able to date exposure time of both boulders and bedrock makes it a versatile technique. However, it is apparent that the potential of the method is elevated in regions with a sound framework of glacial history knowledge. In complicated glaciated regions, more accurate information on erosion rates and reburial may be beneficial. It can be gained by analyzing multiple elements, as oppose to using 10Be only.
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