Intitulé du sujet: Iron organo-mineral association in the dissolved and suspended load of the black river streams along the hydrological cycle and during storm events.

Sujet

Codirection: Dr T Allard (IMPMC) et Dr P Tucq (GET)

Nombre de mois: 48 mois

Ecole Doctorale: ED 560 - Sciences de la Terre et de l'environnement et Physique de l'Univers

Unité de recherche et équipe:

Coordonnées de l’équipe:

1 rue jussieu 75005 Paris 

Phone tel: 33 (0) 1 83 95 76 95 

email: benedetti@ipgp.fr

Secteur: Sciences Physiques et Ingénierie / Physical sciences and Engineering

Langue attendue: Anglais

Niveau de langue attendu: C1

Description

Description du sujet:

Project supported by ANR TICAR.

The 2019 IPCC Special Report on Climate Change and Land highlights the importance of the inter-tropical zone in the carbon (C) cycle, both as a source and sink of greenhouse gases, but also as a region of significant uncertainty. Key issues include: 1) The C flux from tropical rivers to oceans may be underestimated by 70%, potentially representing 10% of anthropogenic emissions (Resplandy et al., 2018). 2) Remote sensing of C fluxes from tropical forests is challenging due to uncertainties in biomass estimates, respiratory fluxes, and the changing nature of soil C compounds. 3) The role of tropical wet zones in C storage is unclear, especially regarding the mineralization of organic matter through bacterial activity and iron (Fe) reduction (Calabrese and Porporato, 2019; Huang and Hall, 2017). These uncertainties complicate future predictions in global climate models, underscoring the need for improved in situ understanding of C stabilization, export, and transformation processes in soils and waters.

The general objective of the project is therefore to improve our knowledge on the coupled dynamics of organic matter, Fe-oxyhydroxides and water in tropical wet zones.

The specific objective of the PhD is to evaluate the change in composition of organo-mineral association in the dissolved and suspended load of the black river streams along the hydrological cycle and during storm events.

Rationale and Methodology: The composition of suspended and dissolved organic matter in Amazonian black waters is very dependent on the level of the aquifers which will leach different soil horizons in response to the variation in precipitation during the year, and which will then supply the tributaries of the Rio Negro (Alasonati et al., 2010; Do Nascimento et al., 2008). If small organic molecules (fulvic acids), more oxygenated and less reactive, are present in the lower parts of the podzols and migrate into rivers via groundwater, larger organic molecules (humic acids), rich in aromatic compounds, would be found in the intermediate horizons of podzols (Bh) and would be leached during periods of high waters. These large humic acids would therefore be transferred to the hydrographic network during the rainy season. In these black waters, the iron in the particulate matter is complexed with lignin, coming from plant debris (Rose et al., 1998). As seasonal iron isotopic variations were observed on the particulate fraction (dos Santos Pinheiro et al., 2014), we hypothesized that this was linked to preferential leaching during the rainy season from the more superficial horizons of the podzols, which have also been shown to be isotopically lighter than deeper horizons (Fekiacova et al., 2013; Wiederhold et al., 2007). However, this hypothesis remains to be verified on the basis of the joint characterization of the speciation and isotopic compositions of iron and particulate organic matter of black water thanks in particular to UV-visible fluorescence or absorbance, to the Rock-Eval pyrolyzer making it possible to measure the rate degradation of humic substances (e.g., Simonneau et al., 2013), the C/H/O/N/S analyzer, as well as infrared spectroscopy (FT- IR) and organic mass spectrometry GC-MS to identify major organic groups (alkanes, acids, phenols, pentacyclic triterpenes, etc.) and Electron Paramagnetic Resonance (EPR) for the organo-metallic speciation of iron. Mössbauer spectroscopy will also be used, as it allows distinguishing the degree of oxidation and the spin of iron, as well as the nature and symmetry of its environment. Thus, it provides constraints for the interpretation of iron isotope fractionation. Water samples devoted to this extensive chemical characterization will be taken at key moments, related to seasonal variations and storm events, determined from the semi-automated probes survey.

In addition to studies on organic matter, the solid speciation of Fe down to ~100 ppm wt. detection limit will be determined by X-ray absorption spectroscopy (XANES and EXAFS) on synchrotron beamlines (e.g. Samba SOLEIL, BM30, BM16, BM23 at ESRF) based on proposal allocation. Photoreduction linked to the organic matter will be minimized by measuring data at cryogenic temperature (using a liquid He cryostat) and by shortening sample exposure to X-rays (quickscan mode). XANES analysis will reveal redox composition at ~5-10% relative accuracy and will give unique information on the molecular environment of iron. In addition, when iron concentration will be favorable (≥ 1wt%), even better accuracy (2% relative). Characterization tests of iron nanoparticles by SP-ICP-MS or SP-ICP-TOF-MS will be conducted to evaluate their possible relationships with the iron isotopic signatures also measured by MC- ICP-MS on corresponding samples. Even if soils/sediments will not be the focus of this study for reasons of necessary limitation of the scope of this project to within a reasonable scale, we will supplement the information extracted from the literature on the possible signatures of soils by the analysis of the Fe-organic matter associations and minerals from some samples of soils/sediments from source sites to support our hypotheses on the characteristics of the sources. To obtain relevant and consistent results with water samples, we will sample and keep these soil cores under anoxic conditions before laboratory processing.

Once the mechanisms at play will be better understood, we expect to incorporate them into a time-integrated hydrochemical modelling at the scale of the small, black water, Rio Negro sub-watershed investigated (Cuieiras Basin). This quantitative modeling will be an objective of the PhD project. We will use an updated version of the physically-based SWAT(+) code for this, which has predictive capacities once the system is properly characterized and understood. Reference (equilibrium) water composition, elemental and molecular speciation will be calculated with the MINTEQ or PHREEQC thermodynamic codes using an updated database. This modeling will offer a holistic perspective of the small watershed aimed at presenting a contemporary understanding of the iron-carbon (Fe-C) fluxes within the riverine system, and consequently, narrowing the uncertainties of/facilitating future climatic predictions in response to environmental changes

Compétences requises:

• University education/engineering school in Geosciences, Geochemistry, Environment with an honour degree or an outstanding track record ;
• Knowledge in Geosciences, use of modelling software, solid knowledge in mineralogy or geochemistry is desirable ;
• The candidate must be autonomous and rigorous ;
• Very good knowledge of spoken, read and written English is essential

Références bibliographiques: