Theses defended

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Thomas LELANDAIS - 19th December

"Analog modelling of subglacial water flow : implications on the relations between tunnel valleys and glacial dynamics"

Tunnel valleys are major components of the subglacial meltwater drainage system. The inaccessibility of modern subglacial environments reduces our knowledge on the mechanisms involved in tunnel valleys formation, the parameters controlling their morphology and their influence on ice-sheet dynamics. This work presents a new experimental approach aiming to better assess the processes of tunnel valleys development. This approach relies on the development of a new experimental device simulating a pressurized water flow within a porous and permeable substratum underneath a viscous layer simulating the ice-sheet. The main results of the experiments conducted with this device have demonstrated the influence of both substratum properties and meltwater drainage on tunnel valleys formation and morphology. Using the device, we first manage to reproduce tunnel valley systems experimentally. Analyses conducted on these valleys experimental valleys suggest that the substratum topography and meltwater production play a key role on tunnel valleys genesis and morphology. Two tunnel valleys morphotypes have been identified, each one being charaterized by a unique morphology and mechanism of formation. Monitoring of the experimental ice sheet during tunnel valley formation shows close relationship between tunnel valleys development and "ice streams" dynamics. The evolution of tunnel valley drainage capacity seems to have a strong influence on ice sheet stability by regulating ice flux within "ice stream corridors

Jonas L'HARIDON - 29th November

"Diagenetic processes on Mars: analysis by the ChemCam instrument on board the Curiosity rover"

The exploration of the Martian surface by the rover Curiosity, at Gale crater, has unveiled past habitable conditions on the planet with evidences for sustained period of surface aqueous activity recorded in fluvio-lacustrine sedimentary deposits. In addition, the rocks also bear traces of the complex post-depositional evolution of the sediments during diagenesis, in the form of small-scale fracture-fills, nodules, and concretions that formed by sub-surface fluid circulations. On board, the rover, the Laser-Induced Breakdown Spectroscopy (LIBS) performed by the ChemCam instrument provide remote fine-scale chemical measurements, especially suited to the analysis of such geological items. This study demonstrates that the Ca-sulfate-bearing veins, observed across all geological terrains, show enrichments in Fe at specific locations along the rover traverse, tracing localized oxidizing conditions at the time of their formation in the later stages of diagenesis. Additionally, a key sedimentary structure in Gale crater is characterized by hematite spectral signatures from orbit, suggesting the involvement of redox-driven processes in its formation, either related to primary deposition or post-depositional conditions. There, in situ ChemCam observations highlight the mobility of Fe during diagenesis and the authigenic formation of Fe-oxide in association with Ca-sulfate bearing features. As such, this work attests of significant fluid interactions, and notably redox-driven chemical reactions, affecting the Martian sediments after their deposition and burial, which need to be taken into account in order to avoid any misinterpretations on the environment of deposition and its habitability.

Anaïs SCHMITT - 5th October

"History of the Barrier reefs growth of the south west Pacific overs last 1.5 Ma : link between carbonate production, turbidites deposits and global environmental changes"

Sedimentary record from Australia eastern margin offshore the Great Reef Barrier Reef is used to better understand the dynamics of mixed margins of the Southwest Pacific, in relation to Barrier reef expansion during the Pleistocene. Significant variations are observed in the nature and frequency of the turbidites deposits in the deep sedimentary record, over the last 1.5 Ma. These variations are associated with the barrier reef expansion around 400-500 ka. Comparison with a similar sedimentary record located offshore New Caledonia show similar and nearly synchronous trend, highlights a potential common mechanism for the barrier reefs expansion in the SW Pacific region. This study further explores links between carbonate production and environmental changes which are addressed by the study of the ratio Mg/Ca in surface dwelling planktic foraminifera, a proxy for sea surface temperatures reconstruction. Single foraminifera analysis were also measured to quantify the range of temperature for a given period. Measurements were performed on two G. ruber morphotypes known to calcify at different depths. Mean temperatures and seasonality do not appear to be the main control factor of the barrier expansion in the region. The vertical trend of temperatures over the last 1.5 Ma, offshore the Great Barrier Reef and New Caledonia, seems to be associated with hydrodynamic changes of the Coral Sea.

Morgane LARNICOL - 11th June

"Multispectral and hyperspectral remote sensing of turbid coastal waters"

Spatial remote sensing makes it possible to monitor the variation of Chlorophyll-a concentration (a proxy of microalgae suspended in seawater) at large spatiotemporal scale in the ocean, but its applicability to the coastal zone is a challenge due to atmospheric variability, seawater turbidity, and heterogeneity of suspended colored constituents. The objective of the present study is to improve Chl-a remote sensing in three turbid intertidal sites of the French Atlantic coast: Marennes-Oléron Bay, Bourgneuf Bay, and the Loire estuary. First, an original method using hyperspectral airborne remote sensing was proposed to validate the atmospheric correction of MERIS satellite data. For very turbid waters (suspended particulate matter concentration > 50 g m-3), the FLAASH algorithm appears as the most efficient method. Then, several regional Chl-a algorithms were developed using in situ reflectance measurements acquired in the three study sites. Bio-optical models using a combination of the red and near-infrared spectral bands of the marine reflectance led to satisfactory results, but were variable from one site to another. The implementation of a multiconditional algorithm would therefore be recommended in order to better take into account the optical diversity of nearshore waters. For the most turbid waters, a robust method was validated for the detection of Chl-a. The algorithm is applicable to MERIS (2002-2012) and OLCI (2016-present) data, thus allowing the monitoring of Chl-a during several decades in turbid intertidal waters.

Filipe TERRA-NOVA - 18th May

"The time-dependence of reversed flux patches in archeomagnetic field models and numerical dynamo simulations : implication for the South Atlantic Anomaly evolution "

Dorine BOUQUET - 13th March

"In situ management of urban allotment gardens soils moderately contaminated by lead"

Soil quality is of concern regarding the development of urban allotment gardens. Although lead (Pb) is poorly available in the soil, some vegetables can accumulate it above maximum permitted concentrations. This study focuses on the development of methods for the in situ sustainable management of allotment garden soils moderately Pb-contaminated (200 mg/kg DM), including the phytoextraction option. The first method, in situ assessed on allotment gardens, combines the phytoextraction using Brassica juncea (L.) and the cultivation of metal-excluder vegetables, allowing the crop production during the soil clean-up. Tomato, cabbage, potato tubers, leek and green bean were cultivated either in a crop rotation or co-cropped with B. juncea (vs. only vegetables). The harvested vegetables are indeed Pb-excluders, except for some samples of green beans. Due to a low soil Pb availability (geogenic Pb), Pb phytoextraction was inefficient (max. 2 mg Pb/kg). The second method, tested at the bench scale, aims to increase the phytoextraction by adding chelatants, either organic chemicals (i.e. EDTA) or via a bacterial inoculum (bioaugmentation) notably selected based on its siderophore production, into the soil. Extraction potential of B. juncea and Lycopersicon esculentum was evaluated in hydroponics with a Pb exposure similar to the DTPA extracted fractions. Shoot Pb concentration for L. esculentum cv. "Marmande" (max. 2000 mg/kg) was 20 to 100 times higher than for B. juncea (max. 45 mg/kg) reaching the threashold value for Pb hyperaccumulator. EDTA incorporation into the soil confirmed the high extraction potential of tomato when Pb availability is increased (114 to 615 mg/kg; anthropogenic or geogenic Pb). However, use of the bioaugmentation was not conclusive.

This thesis took place as part of the regional project "POLLUSOLS".