Forward and inverse modelling of terrestrial cosmogenic nuclides to detect past glaciations

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Title: Forward and inverse modelling of terrestrial cosmogenic nuclides to detect past glaciations
Author: Ylä-Mella, Lotta
Contributor: University of Helsinki, Faculty of Science
Publisher: Helsingin yliopisto
Date: 2020
Language: eng
Thesis level: master's thesis
Degree program: Geologian ja geofysiikan maisteriohjelma
Master's Programme in Geology and Geophysics
Magisterprogrammet i geologi och geofysik
Specialisation: Kiinteän maan geofysiikka
Solid Earth Geophysics
Den fasta jordens fysik
Discipline: none
Abstract: 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.
Subject: modelling
cosmogenic nuclides
depth profile

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