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PhD position in isotope geochemistry of the Critical Zone

This offer is available in the following languages:
Français - Anglais

Date Limite Candidature : lundi 30 mai 2022

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General information

Reference : UMR5204-MATDEL-001
Date of publication : Friday, April 29, 2022
Scientific Responsible name : Mathieu Dellinger
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 1 October 2022
Proportion of work : Full time
Remuneration : 2 135,00 € monthly gross

Description of the thesis topic

Overview: The Earth's “Critical Zone” (CZ) is the thin layer between the top of the vegetation canopy and the bottom of the groundwater beneath soil that sustains nearly all terrestrial life and human resources such as food, water and energy. Chemical weathering, i.e. the dissolution of primary minerals from rocks and the formation of secondary minerals (clay minerals, iron oxides), is a key process within the Critical Zone (CZ) that gives rise to soils, delivers nutrients to ecosystems and regulates the long-term carbon cycle. Quantifying chemical weathering fluxes (the chemical erosion) and controls is therefore fundamental for understanding and predicting the dynamics of the CZ and its ecosystems.
Despite recognition of this, it remains difficult to decipher the influence of the climatic, tectonic and anthropic drivers on chemical erosion. This limits our ability to assess how soil weathering will respond to future climate and land-use changes, particularly in rapidly-eroding mountains which dominate the global chemical erosion budget. All these uncertainties fundamentally reflect the challenges associated with i) robust fingerprinting of weathering sources, processes and signatures, ii) teasing apart the effects of mineral vs. water residence time on chemical weathering and iii) integrating the data and observations into a modelling framework for coherent weathering processes. Through this project, we will improve our understanding of the functioning of the Critical Zone using isotope tracers in soils and streams of small mountainous catchments.

Methodology: The student will use field work, isotopic geochemistry and modeling to study river catchments in order to quantify chemical weathering processes and controls. The project will focus on the European Alps, a temperate climate mountain region with a long history of human exploitation and where the soils are thin and vulnerable to rapid environmental changes. They will work on and collect a large range of samples (soils, rocks, river water and sediments, colloids, rainwater) from a variety of Alpine catchments (~30 rivers) to characterize the spatial variability of chemical weathering in the Alps. S/he will also use existing time-series sample collections (20-30 samples per year) from eLTER-France OZCAR instrumented river catchments (e.g. Draix-bléone observatory) to decipher the role of hydrological flowpath and transit time on chemical weathering. Isotopic ratios of the trace elements lithium (7Li/6Li) and strontium (87Sr/86Sr) are powerful tools to fingerprint chemical weathering and will be the two main geochemical tracers used in this project. Results will be implemented in mass balance and reactive-transport models (from the soil to catchment scales) to characterize the main controls on chemical weathering.

Goals: Quantify the sources, fluxes and controls on chemical weathering in the modern European Alps. Characterize the main parameters controlling Li isotope fractionation at catchment scale. Better understand present-day weathering processes in order to reconstruct long-term weathering trajectories in the European Alps (the main objective of the LAKE-SWITCH project).

Doctoral training: S/he will receive specialist training in field river sample collection and cutting-edge geochemical methods (e.g. Li-Sr isotopic measurements by MC ICP-MS, major ion analysis by ion chromatography, trace element measurement by Q-ICP-MS). The student will work in clean laboratory environment and learn standard sample preparation methods, i.e. sample digestion, purification by column chemistry.

Work Context

This PhD project is fully-funded as part of the European Research Council (ERC) Starting Grant “LAKE-SWITCH: Using lake sediments to reconstruct soil weathering trajectories over the Holocene”. The supervisory team will be composed of Dr. Mathieu Dellinger (CNRS researcher, EDYTEM), Dr. Julien Bouchez (CNRS researcher, IPGP) and Dr. Pierre Sabatier (Lecturer, EDYTEM). The National Center for Scientific Research (CNRS) is the largest fundamental science research organization in Europe, with laboratories located throughout France. The student will be primarily based at the CNRS joint research unit EDYTEM (“Environments, Dynamics and Mountain Territories”; Le Bourget-du-lac) but will also spend time at the CNRS research unit Institut de Physique du Globe de Paris (IPGP). They will be integrated into the “Critical Zone” research team at EDYTEM and the “G2E” research team at IPGP. They will be enrolled in the SIE (“École doctorale Sciences, ingénierie et environnement”) graduate programme of the University Savoie Mont-Blanc (USMB). EDYTEM and IPGP have a large and vibrant postgraduate communities and host regular seminars and social events. The student will join the ERC-funded LAKE-SWITCH team (1 PDRA and 1 Assistant Engineer) and will interact with international collaborators in the EU and the UK.
The EDYTEM research unit is part of the University Savoie Mont-Blanc (USMB), one of the major academic institutions in the Auvergne Rhone Alpes region. The Bourget-du-lac campus (where EDYTEM is located) is perfectly located for outdoor activities, close to the Bourget Lake between Chambéry and Aix-les-Bains, with a wide selection of sports, leisure and cultural events offered by the University. IPGP, located at the heart of Paris 5th "arrondissement" and is the largest academic Earth Science research institution in France, with ~100 PhD students. The project will benefit from the world-class facilities of the PARI analytical platform. IPGP is an integral part of Université Paris-Cité, the largest university in France, offering a range of services and facilities to its students

Constraints and risks

The project is open to French and international students that have a Master's degree in the area of geochemistry and/or Earth Sciences. Further evidence for outstanding academic pedigree and previous geochemical experience in working in a clean laboratory will be welcomed. Geographical mobility will be needed for conducting the isotopic measurements at IPGP in Paris as part as doctoral co-supervision. Finally, appreciation of fieldwork will be valued as the project includes several sampling trips.

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