(M/F) Origin of seasonal iron isotope fractionation in the Rio Negro, Brazil: spectroscopic and isotopic approaches to suspended particle analysis

Job Description

Thesis Subject

Our ability to predict future climate depends, in particular, on our understanding of the cycles of elements and especially carbon at the Earth's surface. However, IPCC reports highlight gaps in this knowledge in intertropical wetlands. In the environment, transported particles, and in particular colloids conventionally defined as ranging in size from 1 µm to 1 nm, are now widely recognized as important vectors in the transport of various trace metals and carbon. They are most often composed of organic matter and iron oxides and oxyhydroxides, or aluminum hydroxides and oxyhydroxides, in various tropical, boreal, and temperate contexts. The study of the role of colloids as vectors is relevant not only for the study of trace metal pollution but also for revealing species resulting from the coupling of the iron and carbon cycles in various environmental compartments, from soils to sediments. It is recognized that the carbon cycle has an impact on climate change, and iron could affect the stability of organic matter and thus influence CO2 production and therefore climate change by contributing to carbon storage in soils or sediments. Laboratory geochemical studies have highlighted elementary mechanisms of the iron-organic matter association, such as complexation or the formation of iron nano-oxyhydroxides by oxidation of Fe(II) in the colloidal fraction. However, despite their role in surface waters, colloids are rarely characterized directly in natural samples, mainly due to the difficulty in concentrating and separating them. Studies have been conducted on isolated particles, for example, using time-of-flight mass spectrometry to perform elemental correlations, or electron microscopy to reveal shapes, sizes, structures, and compositions. However, these approaches remain very limited in terms of the statistical data available for the particles observed. Colloids can also be analyzed using spectroscopic tools that integrate large particle populations, providing access to the nature of the solid phases and chemical species at the molecular level involved in biogeochemical processes. We have thus been able to show that colloids are particularly important in the case of blackwater rich in organic matter, such as that of the Rio Negro (Brazil), which drains a major sub-basin of the Amazon basin and where colloids contain a significant portion of the transported iron. Furthermore, seasonal fluctuations in the isotopic fractionation of iron in suspended particles have been observed in this river. Some recent laboratory studies indicate, in particular, a role for organic complexation in this fractionation. However, the understanding of the natural context of the Rio Negro's watercourses in terms of processes has not yet been fully explored, even though they represent significant carbon fluxes. This environment provides an ideal setting for better understanding the coupling of iron and carbon cycles in the hydrological continuum, between soils and sediments. Thus, to interpret in particular the origin of fluctuations in iron isotopic ratios in terms of elementary mechanisms of iron speciation associated with organic matter (redox reactions, complexation, formation of nano-oxyhydroxides), this PhD project aims to combine spectroscopic and isotopic approaches on natural suspended particles (particulate fractions greater than 0.2 µm, colloidal fractions between 0.2 µm and 5 kDa, dissolved fractions less than 5 kDa) separated by tangential ultrafiltration in streams of different orders within the Rio Negro watershed. For the first time in these environments, and using a device set up at the IMPMC, samples will be collected in the field under controlled conditions to preserve the oxidation state of the iron (in a glove bag during ultrafiltration and in a glove box in the laboratory). Two field missions will then focus on four watercourses of varying sizes, from an upstream stream to the Rio Negro in the Manaus region of Brazil, and will take place during both low and high water seasons. The quantitative dimension of the spectroscopic approach will be enhanced by the laboratory preparation of standards that best represent the observed iron species.

Your Work Environment

This project is part of the ANR TICAR project (Tracing the Iron-Carbon interactions in the Amazonian black River headwaters). It will be managed by the CNRS (Paris Centre 02 Delegation). The doctoral school will be Geosciences, Natural Resources and Environment (ED 398). The thesis will take place in two laboratories, the IMPMC and the GET:
(i) The Institute of Mineralogy, Materials Physics and Cosmochemistry (http://impmc.sorbonne-universite.fr/fr/equipes.html), is a joint research unit between the CNRS, Sorbonne University (SU), and the National Museum of Natural History (MNHN). It is located on the Pierre and Marie Curie campus in Paris, Jussieu. The unit is a laboratory with approximately 120 permanent staff members, distributed across research teams that conduct multidisciplinary research in the fields of physics, biology, geosciences, and chemistry. At the IMPMC, the doctoral student will join the environmental mineralogy team. He/she will work under the supervision of T. Allard (CNRS Research Director, Habilitation) and interact with other members involved in the ANR TICAR project. He/she will have access to the laboratory's analytical instruments.
(ii) (ii) The Géosciences Environnement Toulouse laboratory (https://www.get.omp.eu/) is a joint research unit between the CNRS, the IRD, the University of Toulouse 3, and the CNES, specializing in Earth and Environmental Sciences. It is part of the Midi-Pyrénées Observatory (OSU OMP), which comprises approximately 226 people, including 156 permanent staff. The doctoral student will work under the supervision of F. Poitrasson (CNRS Research Director, Habilitation), coordinator of the ANR TICAR project. The student will have access to the cleanroom and plasma mass spectrometers necessary for the project.

Compensation and benefits

Compensation

2300 € gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

CNRS

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