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Mechanisms of physical and chemical erosion and sediment transport

The mechanisms of combined physical and chemical erosion that shape the Earth surface are relatively well known from a qualitative point of view, although many complexities in the biochemical mechanisms are still to be identified. However, in order to better decipher from the sedimentary archive how a landscape responds to tectonic or climatic forcing, quantitative knowledge of the different physico-chemical processes at work in landscape erosion are needed. Therefore several well-suited approaches will be implemented. Our research group will focus on the five projects given below :

• Application for understanding landscape sensitivity to climate variations
• Erosion on hillslope
• Interactions between weathering and physical erosion
• Processes of chemical weathering
• Transport mechanisms in river

Application for understanding landscape sensitivity to climate variations

Response to climate

The response of landscapes to climate will be addressed largely through a comprehensive study of natural systems laboratories that are subjected to contrasting climatic stresses. Geochemistry of the weathering and of dissolved flows supplemented by measurements of cosmogenic isotopes will both help in defining empirical laws and provide test or calibration sites for integrated numerical models.

Modeling

Understanding the interactions between the various processes involved in the landscape evolution require the use of numerical models. Those later should include the downstream evolution of the sediment load, the relationship between hillslope and river system and at last should take into account the soil (collaboration with Y. Goddéris of GET in Toulouse).

Future objectives will therefore be focused on the inclusion of such phenomena, with the goal to study the response and the sensitivity of geomorphic systems to climate or tectonic variations as a function of the amplitude and frequency of climatic cycles particularly at the Quaternary level.

Erosion on hillslope

Glacier

Glacial erosion is essential in shaping the reliefs of many mountain ranges. However, the laws of glacial erosion, especially on subglacial erosion, are still poorly constrained, both in their formal expression and in parameterisation.

The characteric the response of the landscapes to glacial-interglacial cycles during the Quaternary period remain also to be untangled. To progress on these issues, an approach based on cosmogenic isotope dating and measurement of the entrenchment due to glacial cross-over will be conducted.

Landslide

In mountain ranges, physical erosion dominates, in particular through landslides. Because of natural hazard concerns, the mechanical behaviour of landslides are intensively studied. Therefore, to better our approach will primarily focus on the sediment budget combined with precipitation data and the sediment geochemical signatures (major element and cosmogenic isotopes).

Interactions between weathering and physical erosion

Can an active mountain range be "weakened" by a deep chemical weathering ? Beyond the inherited fracturing due to brittle tectonic deformation, deep water circulations (10 m to km) can pervasively affect the bedrock frame if a source of acid is present. Although this process has been identified and partially quantified for hydrothermal circulation, the weathering impact of fluid circulation at low temperatures still remains to be understood. These potential feedback loops have not previously been the subject of extensive studies or of quantification. To understand these interactions, two main approaches will be carried out. First, to constrain flows and residence time of water, sampling of hydrologic systems will be associated to geochemical tracers analyses. In parallel, a systematic study from bibliographic data and fieldwork will be conducted to relate the particle sizes of landslide products (superficial and deep) with the lithology and the degree of weathering.

Processes of chemical weathering

In mountain ranges, if physical erosion dominates, the study of chemical erosion remain critical because dissolved load may impact global cycles (see the study Cycles, Atmosphere, Climates) but also because weathering and geochemical signature can be used as climate proxy in the sedimentary records. General geochemical indicators, such as CIA, provides a first order reading of the weathering intensity. But, a quantitative approach requires more accurate and better-calibrated tracers on a regional basis. Our approach aims at establishing general laws where quantitative tracers of the processes are linked to the degree of weathering, the residence times (derived from the uranium series) and particle size.

Transport mechanisms in river

The sediments shed from the hill slopes are then transported by the river system, where they are actively involved in the erosion of river bedrock and, ultimately, the entrenchment of rivers. During transport, the sediment size distribution evolved due to abrasion in the mountainous segments and to selective transport in depositional areas such as foreland basins.

These processes alter the bedload fraction of the transported sediments, but also the mineralogical and geochemical content of the different size fractions. They impact the geometry and shape of the rivers and their geochemical signature. The studies of bedrock incision will be still considered through experimental approaches but will be also supplemented by in-situ and real-time measurements. In addition to experimental studies on the products of abrasion, field studies that combine sampling and geochemical analysis of particles and velocity measurements by ADCP will be conducted to move forward on these issues.