Post-glacial geomorphic and erosion response in the western European Alps and Pyrenees (W/M)

CNRS PhD offer

PhD project description

Quantifying landscape dynamics has proven challenging in the context of current climate change because geomorphic processes operate at interrelated timescales. Alpine landscapes were progressively shaped during the Quaternary under oscillating glacial-interglacial conditions, but the net impact of glaciation on mountains is spatially variable. Furthermore, modern mountain erosion integrates both (1) the recent impact of climate change (i.e. 10-100 yr) as a result of glacier shrinking and permafrost thawing, and (2) the long-term response (i.e. 1-10 kyr) to glacial retreat and glacial conditioning of topography. Predictions of the geomorphic response to future climatic scenarios have consequently remained limited. All these observations point toward the need for a better assessment of mountain landscape sensitivity to climate forcing over a range of timescales and processes. In this PhD project, we will investigate the geomorphological record of the last glacial-interglacial transition in two natural settings (western European Alps and eastern Pyrenees). We aim at quantifying the postglacial erosion and sediment transfer since the Last Glacial Maximum (LGM, ca. 20 ka), addressing the following question: what are the interactions, and response times between climate change, glacier retreat and landscape erosion dynamics in alpine settings?

The landscape response to the last glacial-interglacial transition was different between the western Alps and eastern Pyrenees. In the western Alps, post-LGM glacier retreat has resulted in strong landscape rejuvenation through river incision and slope movements, with high rates of sediment production over modern to millennial timescales. In the eastern Pyrenees, the post-glacial geomorphic response has been limited, with scarce slope instabilities and relatively low sediment production. Catchment-wide erosion rates reveal contrasting erosion regimes in the western Alps and eastern Pyrenees, with proposed apparent slope, lithology or elevation control on post-glacial geomorphic processes.

This raises the issue of disentangling the respective contribution of glacial pre-conditioning, glacier retreat and current climate change in mountainous erosion regimes, which is the central question of this PhD project. We aim to quantify the spatio-temporal variations in postglacial erosion coupled to sediment provenance and topographic rejuvenation, with the following objectives : (1) quantify the spatial (western Alps vs. eastern Pyrenees) and temporal (since the LGM to the late Holocene) variability in mountain erosion rates using catchment-wide denudation data, (2) track sediment production and transfer over lateglacial to postglacial timescales, with potential source changes over time, and finally (3) better understand how geomorphic processes and timescales interact to regulate the landscape response in a changing climate system.
The PhD project will be organized around a multi-method approach. First, the PhD student will use in-situ cosmogenic nuclides (10Be-26Al in quartz) in modern river sands of the eastern Pyrenees. This work will allow to establish a database for the eastern Pyrenees for geomorphic comparison with the western European Alps[11]. In addition, the PhD student will acquire new in-situ cosmogenic nuclide data (10Be/26Al, potentially 14C) in Lateglacial and Holocene sediment archives (alluvial fan, lacustrine and floodplain deposits) to be used as proxy for paleo-erosion or paleo-environmental conditions. Sediment provenance will also be investigated using mineralogical and geochemical analyses to quantify temporal changes in river-sediment sources since the LGM. Sediment transfer times will finally be evaluated for lateglacial to postglacial deposits with bulk sediment geochemistry to derive weathering indices and U-Th series to date the time of sediment production (i.e. the comminution age). With this approach, we want at investigating the budget contribution of re-mobilized glacial sediments during the Lateglacial/Holocene, as well as potentially complex sediment transfer from mountains to forelands. Ultimately, these results will be incorporated in surface-process modelling using the state-of-the-art models (e.g. Landlab) which predicts landscape evolution and sediment production (river and hillslope processes) under climate forcing.

Work context

The PhD student will develop his/her project at the Institute of Earth Sciences (ISTerre, Univ. Grenoble Alpes) under the supervision of Pierre Valla and in close collaboration with researchers from ISTerre and other partners (national/international). ISTerre is part of the Observatoire des Sciences de l’Univers de Grenoble (OSUG) and fosters multidisciplinary research in Earth Sciences, particularly focusing on the combined approach between natural observations, experimental research and modeling of the Earth system. Grenoble is a nice city within the Alpine massifs, with many nature/sport activities and active student life. The PhD candidate should have knowledge in quantitative geomorphology, Quaternary geology and/or sediment geochemistry. Some skills in topographic analysis, GIS mapping and numerical modelling would be advantageous to lead this project and establish planned collaborations. The doctoral project is financially supported by the ANR-PIA MAGICLIM project ("MountAin Glacier fluctuations and landscape dynamIcs under a changing CLIMate" - PI P. Valla) that will cover all financial expenses for fieldwork missions, analyses and planned travels (collaborations and conferences) within the framework of the PhD project. The PhD student will also be part of the MOPGA program and will participate to associated conferences and workshops.

Applicants should have a MSc degree in geology, geophysics, geography, or any related quantitative discipline in natural sciences, with strong interests in earth surface dynamics. Successful applicant should have strong written and oral communication skills in English (French skills would be advantageous for social interactions and daily life in Grenoble).