Tectonic modulation of climate cycle impact on orogens

The project Tectonic Modulation of Climate Cycle Impact on Orogens (conneCt) aims at understanding the role that tectonics play in modulating mountain topography under a variable climate. Funding is based on a Schrödinger Fellowship of the Austrian Science Fund (FWF), which allows conducting analyses at the University of Lausanne and benefiting from local expertise and infrastructure for thermochronometric dating and landscape evolution modelling. The last two million years of Earths history were characterized by alternating cold and warm phases. These climate variations affected the processes that erode Earths surface and led to a cyclic change between fluvial and glacial process regimes. Erosive processes are generally driven by tectonic forces that elevate the Earths surface and generate gradients along which gravity can act. Different processes lead to different patterns of erosion and hence change the shape of the landscape, which in turn affects the forming processes via variables such as elevation and slope. While changes in process type and erosion rate are often attributed to climate change, the feedbacks with tectonics remain unresolved. This project helps closing this knowledge gap by constraining the relevant mechanisms and feedbacks that occur between tectonics and periodically changing process regimes, based on three main hypotheses: Imbalances between landscape shape and the prevailing erosional processes lead to intensive reshaping of mountain landscapes and associated pulses of sediment flux during landscape response to climate transitions. Tectonics controls relief turnover time (time required to renew valley-scale relief) and hence the time needed for a landscape to go back to equilibrium in response to climate transitions. Tectonics controls relief production in response to climate transitions in high-uplift mountain ranges. The hypotheses are tested using state-of-the-art numerical landscape evolution modeling, quantitative analyses of landscape shape and constraints on spatial variations of process rates derived from new thermochronometric dating techniques. Numerical modelling takes advantage of a newly developed graphics processing unit (GPU) cluster at the University of Lausanne in conjunction with a novel GPU-based fluid solver to efficiently model ice flow. The project uses the Southern Alps of New Zealand as a natural laboratory. The mountain range is ideally suited for analyzing and modelling surface process rates over a wide range of climatic conditions and tectonically controlled uplift rates. The project makes key contributions to the scientific fields of geomorphology and geodynamics by (a) deciphering an unexplored set of feedbacks in cold-climate landscape evolution that coined the current state of worlds mountain ranges, and (b) providing an alternative approach to inferring erosion from sedimentary and thermochronometric data, the reliability of which is in the focus of an intense debate.

The project "Tectonic modulation of climate cycle impact on orogens" (conneCt) investigates the influence of tectonic uplift on the shape and height of mountain ranges under different climatic conditions. For this, new theoretical computer models were developed during the project. The most important findings from applying these models are: 1) The height of glaciated mountain ranges is controlled by the influence of climate AND tectonics. The relevance of climate and tectonics for mountain range height and shape has so far only been shown for mountain ranges conditioned by fluvial erosion, while in glaciated mountain only the influence of climate has been assumed to be relevant. Therefore, this finding contributes substantially to the understanding of mountain range evolution, particularly in a cold climate. It further implies that the tectonic activity of a mountain range contributes to controlling the impact of oscillating cold and warm phases on erosion, mountain range height and shape. 2) Glaciers erode more efficiently than rivers. The height and shape of mountain ranges are controlled by the competition between constructive (tectonic uplift) and destructive (erosion on the Earth's surface) processes. The newly developed computer models allow to directly compare erosional processes of glaciers and rivers in the same tectonic and climatic environment. The simulations show that mountain ranges eroded by glaciers are systematically lower than those eroded by rivers under the same environmental conditions. This finding contributes to a better understanding of long-term climate change. For decades, a possible feedback between climate and erosion is discussed: increased erosion and weathering in a cold climate could lead to increased absorption of CO2 from the atmosphere and thus to further climate cooling. In this scenario, it is crucial whether decreasing temperatures (and the spreading of of glaciers) do indeed lead to increasing erosion. Our simulations say: yes.