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Freie Universität Berlin (2021)

Discharge variability and river incision along a climate gradient in central Chile

van Dongen, Renee

Titre : Discharge variability and river incision along a climate gradient in central Chile

Auteur : van Dongen, Renee

Université de soutenance : Freie Universität Berlin

Grade : Doktors der Naturwissenschaften (doctor rerum naturalium) 2021

Résumé partiel
River incision into bedrock is an important process in the context of landscape evolution. Climate can affect the river incision process in several ways. Firstly, precipitation is the main source of river discharge that is required to initiate the motion of river sediment and expose the riverbed to erosion. However, the relationship between river incision and river discharge is often non-linear, in that river discharge has to exceed a certain erosional threshold to mobilize bedload sediment and expose the riverbed to erosion. Whether this erosion threshold plays a significant role, or not, depends on the flux and grain size distribution of river sediment. Besides its effect on discharge, the climate’s influence on bedrock weathering also affects river incision, by changing bedrock erodibility as well as the grain size distribution of river sediment. Finally, climate sets discharge variability, which determines how often and by how much river discharge exceeds the erosion threshold. Discharge variability is typically high in arid regions, whereas river discharge is less variable in humid regions. Although it is evident that climate has an effect on river incision, studies that have investigated the effect of climate on 10Be-derived erosion rates – which in a steady state landscape equal river incision rates – have often found ambiguous relationships. This is most likely because other non-climatic factors (e.g., tectonic uplift rates, lithology, biota) interplay to obscure potential climatic trends.

In this PhD thesis, I investigate the role of climate on various aspects of the river incision process in the Coastal Cordillera of central Chile. I focus on regions that are underlain by similar granodioritic lithology, but are exposed to contrasting climate regimes (arid, semi-arid, mediterranean, and humid-temperate). Using this approach, I aimed to reduce the variations in non-climatic factors that may obscure the climatic effect on erosion and river incision processes. I used in situ cosmogenic-10Be in river sediment to quantify erosion and river incision rates. In situ cosmogenic 10Be is produced in quartz grains in the upper few meters of the earth’s surface by high energy cosmic rays. The 10Be concentration reflects the time that grains are exposed to cosmic rays, which is proportional to the residence time of grains in the surface layer (i.e., inversely to the erosion rate). As a result of this, cosmogenic-10Be is frequently measured in river sediment to constrain catchment average erosion rates, which in steady state landscapes should equal river incision rates.

Mots clés  : River incision Discharge variability Landscape evolution Cosmogenic-10Be Erosion rates ENSO Chile

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