h1

%0 Thesis
%A Bodenburg, Sascha Barbara
%T On the role of geothermal feedback mechanisms on tunnel valley genesis above salt domes
%I RWTH Aachen University
%V Dissertation
%C Aachen
%M RWTH-2023-09361
%P 1 Online-Ressource : Illustrationen, Diagramme, Karten
%D 2023
%Z Veröffentlicht auf dem Publikationsserver der RWTH Aachen University 2024
%Z Dissertation, RWTH Aachen University, 2023
%X Physical feedback mechanisms between the subsurface and an overlying ice sheet are manifold. They lead to the development of special landforms, e.g. tunnel valleys. Tunnel valleys are widespread in (formerly) glaciated areas. They are eroded by subglacially flowing water. To investigate their genesis, it is necessary to model feedback mechanisms between the ice sheet and the subsurface. In the North German Basin, a possible spatial correlation between tunnel valleys and underlying salt domes was often observed. Partly, this was explained mechanically with the presence of faults. We want to investigate a different hypothesis for tunnel valley genesis based on the following geothermal argumentation: As salt better conducts heat than the surrounding rocks, the geothermal heat flux is augmented above. Hydrothermal groundwater flows through crestal faults enhance this effect. The resulting subglacial melting leads to subglacial rivers eroding the tunnel valleys. In order to determine the subglacial melting rate, a holistic computational model is needed. Usually, the different regimes of the overlying ice sheet and the underlying subsurface are investigated separately. The other domain is only included by a boundary condition. In this study, we developed a coupled computational model comprising both the heterogeneous subsurface and the dynamic ice sheet including subglacial phase change processes to allow for feedback mechanisms. The basing physical assumptions are the following: The subsurface is influenced by heat conduction and advection due to groundwater flow through the different rocks. The ice sheet melts at its base, while ice is accumulated at its surface. Therefore, it moves as a whole ice sheet. In our approach, we attached great value on the energy balance at the ice sheet's base. First, very simplified models show that the subglacial temperature should be increased at locations of increased geothermal heat fluxes as above salt domes. However, the temperature at the glacier's base must be assumed to be the melting temperature. Therefore, the effect of the geothermal energy has to be equalised. This happens by the overlying ice sheet, which cools the subsurface. Because of the higher geothermal heat flux above salt domes, the subglacial melting rate increases. This subglacial melting rate, together with snowfall on the glacier, leads to a vertical motion of the ice. At locations of increased subglacial melting rates, the ice moves faster downwards, so that the glacier base gets colder. These two effects, the warming effect from the subsurface and the cooling effect from the ice sheet, balance, so that in the end, the subglacial temperature meets the melting temperature. This process model was first developed in one dimension. The computational model bases on the apparent heat capacity method and on finite differences. As the investigation of the heterogeneous subsurface needs more spatial dimensions, the computational model was subsequently transferred to two dimensions, which allowed to implement it into the already existing software SHEMAT-Suite. This finally enabled us to conduct two-dimensional case studies, in which we investigated the development of tunnel valleys above subsurface salt structures: first the Gorleben salt dome, and second a profile derived from seismic data from the southern North Sea, where additionally crestal faults were taken into account. We modelled heat conduction and groundwater flows. We could conclude that the modelled hydrothermal processes reinforce the formation of tunnel valleys above salt structures. This example shows the necessity of a coupled model of the subsurface and the ice sheet, as the geothermal heat flux strongly influences the ice sheet dynamics whereas the isolating effect of the ice sheet determines the temperature of the subsurface and thereby erosional processes.
%F PUB:(DE-HGF)11
%9 Dissertation / PhD Thesis
%R 10.18154/RWTH-2023-09361
%U https://publications.rwth-aachen.de/record/969814