PhD project, role of peatlands on trace metal cycles, Toulouse, France

An excellent PhD candidate is being sought within the Toulouse, France, doctoral school annual competition studying the role of peatlands on trace metal cycles in mountains watersheds: fluxes and speciation.

Keywords

Trace metals, atmospheric deposition, Organic matter, Pb isotopes, colloids, C isotopes

Main scientific fields

Contaminant Fate in the Mountain Critical Zone
Isotope Biogeochemistry, Organic matter geochemistry

Requirements

Master degree in biogeochemistry, geochemistry, hydrology, Environmental Sciences.

Description

The mountain critical zone is highly sensitive to current environmental changes, in relation to human activities (e.g. mining, peat stubble burning, forest cutting) and climate change. This rapidly evolving context questions the fate of these carbon and Trace Metal stocks. In relation to their sharp topography and harsh climate, mountain watersheds are characterized by extreme hydrological events, which strongly control biogeochemical cycles. In the case of organic carbon transfer to surface waters, it has been shown that about half of the dissolved organic carbon fluxes occur during less than 10% of the time, during storm events (Rosset et al. 2017). Preliminary studies show that peaks of dissolved organic carbon during storm events to the fluvial networks are paired with trace metal peaks. This lead to the hypothesis of strongly dynamic exports of TM from peatland, characterized by ‘hot moments’ of TM release to the fluvial network. Trace metal exports, and their potential impact on biota depend on their speciation in water, which is mainly controlled by pH, redox status, as well as interaction with dissolved organic matter (DOM) and aluminum and iron hydroxides. In mountain watersheds, these parameters are strongly influenced by peatlands and sharply evolve during storm events. The complex dynamic of TM in mountain watersheds is poorly constrained.

Objectives

This doctoral project aims to answer the following questions:

- Are peatlands currently sources or sinks of Trace Metals in mountain watersheds?
- Do trace metal exported from peatlands originate from ancient mining legacy or from recent pollutions?
- What is the influence of peatlands on trace metal speciation after their export to the fluvial network?

In European mountains, valuable metallic ores have been mined since the Bronze Age, inducing metallic dispersion in the atmosphere. In some areas, the intensity of the dispersion of trace metal (TM) during medieval mining activities even exceed the one measured after the industrial revolution. In addition to local inputs from mining activities, mountain watersheds are specifically impacted by long range transboundary air pollution. Atmospheric deposition of TM can be increased compared to neighboring valleys. Higher precipitation occurs on hills and mountains due to several orographic effects like the feeder-seeder effect or canopy interception of low altitude clouds and fogs. Peatlands are common ecosystems in mountainous environments and act as reservoirs of organic matter. Peat has the ability to retain trace metals and radionuclides and peatlands are considered to have accumulated contaminants since the beginning of metallurgy. Indeed in the Pyrenees, it has been shown that 85% of Pb accumulation in peatlands occurred before AD 1800, highlighting the importance of the ancient mining activity legacy in TM stocks (Hansson et al. 2017). In mountain watersheds, peatlands are then ‘hotspots’ of both carbon and TM accumulation.

Methods

In order to establish TM budgets, data gathered on instrumented sites will be compiled. Trace metal fluxes will be modeled using in situ high temporal resolution of discharge and DOC concentrations. The amount of dry deposition as well as the contribution of occult deposition will be assessed using specific collecting devices deployed during the PhD project (fog collector, wet and dry collector). Current TM fluxes and budgets will be compared to established historical fluxes. A coupled isotopic approach (210Pb, stable Pb isotopes, 14C-DOM) will be developed to estimate the origin (recent vs historical deposition) of exported TM. The speciation of TM will be assessed in relation with hydrological conditions combining physical fractionation (dissolved, colloidal and particulate), chemical analysis (focusing on TM interaction with DOM and hydroxides) and geochemical modelling approaches.

Expected results

This doctoral project will contribute to significant improvements in the understanding of the role of peatlands in the critical zone biogeochemical cycles. Trace Metal budgets in remote ecosystems, and speciation studies in the context of low contamination are extremely rare. It will use and develop innovative geochemical tools to further investigate TM speciation and the complex link between organic and trace metal cycles.

Framework

The project will be conducted in the Pyrenees. The main study sites (peatlands) are located within the OHM (Observatoire Homme Milieu) Haut Vicdessos-Vallée des Gaves. The sites present contrasted mining history and are influenced by representative practices, including peat stubble burning. The Bernadouze peatland has been instrumented since 2012 (meteorological, hydrological and geochemical in situ sensing). Trace Metal deposition have been recorded since 2012. Additional sites in the Pyrenees, as well as international sites close to active mines (Andes) will be investigated in order to consider a wide range of physicochemical conditions and contamination ranges. The doctoral project is part of the ANR TRAM (Trace Metal Legacy in Mountain Environment). It will benefit from the analytical environment of the EcoLab Laboratory and OMP (ETV, ICP OES, ICP MS, HR-ICPMS...). It will benefit from regional collaboration within the Observatoire Midi Pyrenees, and from the international collaboration within the REPLIM network (a Pyrenean network of research labs working on lakes and peatlands in the Pyrenes).

References

T Rosset, L Gandois, JM Antoine, R Teisserenc, S Binet. 2017. Hydrological controls on dissolved organic carbon exports from a-French Pyrenean-mountainous peatland. EGU Confence, Vienna.

Hansson, S., Claustres A., Probst A., Vleeschouwer F., Baron S., Galop D., Mazier F., Le Roux G. (2017). Atmospheric and terrigenous metal accumulation over 3000 years in a French mountain catchment: Local vs distal influences. Anthropocene. 19. 10.1016/j.ancene.2017.09.002.

Le Roux, Gaël and Hansson, Sophia V. and Claustres, Adrien: Inorganic Chemistry in the Mountain Critical Zone : Are the mountain water towers of contemporary society under threat by trace contaminants ? (2016) In: Mountain Ice and Water - Investigations of the Hydrologic Cycle in Alpine Environments. (Developments in Earth Surface Processes). Elsevier, p. 131-154. ISBN 978 0 444 63787 1

Further information

Send questions about applying to Laure Gandois (This email address is being protected from spambots. You need JavaScript enabled to view it.)) and Gael Le Roux (This email address is being protected from spambots. You need JavaScript enabled to view it.)

http://summits.cnrs.fr/