Theses defended

Hyperspectral and full-waveform LiDAR synchronized remote-sensing applied to the monitoring of the Pays-de-la-Loire region’s sandy coasts

Giovanni FRATI - 16/12/2021

Remote sensing allows an accurate monitoring of coastal sand dune ridges. These latter protect the retrolittoral areas against marine submersions. In the context of the Pays-de-la-Loire region sandy coast monitoring, two main parameters are analyzed: 1) morphology, giving access to sedimentary stocks and movements, and 2) vegetation cover, allowing to assess the dune ridges stability. Dune morphologies and their végétation cover are studied using laser ranging (LiDAR and their chemical and pigment compositions are obtained using hyperspectral imaging (HIS). Full waveform LiDAR and hyperspectral imaging data were both acquired via bihatch aircraft. The acrosstrack scanning (whiskbroom) of the 1064 nm laser beam were synchronized with the alongtrack scanning (pushbroom) of the camera. The dune typology were determined by using hyperspectral image hierarchical classification, based on the vegetation cover spectral properties. The mapping of Ammophila arenaria (marram grass), white dune stabilisation proxy, is difficult because of its proximal spectral signature with other plants of the dune and because of its high phenological variability from a year to another. Ground ranging is only possible when the vegetation cover allows it, which is not the case with the marram grass, because of its foliar component density. However, full-waveform analysis shows that this particular leaf structure induces a caracteristical deformation of the signal, which allows to identify the marram grass between other plants. As a result, this deformation can now be used to map the marram grass and to correct dune topographical determination errors.

Impact of partial melting on tidal dissipation in the silicate mantle of Jupiter’s moons Io and Europa

Mathilde KERVAZO - 09/12/2021

The presence of a subsurface ocean on Europa in direct contact with the silicate mantle, along with thespectacular tidally driven volcanic activity exhibited by its neighbour satellite Io raise the possibility of seafloor volcanic activity, which has significant implications for Europa’s ocean habitability. One way to generate such an activity is tidal dissipation in a partially molten silicate (or rocky) mantle. The aim of this thesis is toinvestigate the role of melt presence in the mantle of Io and Europa on both shear and bulk dissipation, the contribution of the latter having never been quantified before in a silicate mantle. I model the viscoelastic deformation of Io’s and Europa’s mantle and quantify the influence of a partially molten layer, assuming rheological laws parameterizing the effect of partial melting on anelastic properties of rocks. For Io, I determine the rheological structure of the silicate mantle in terms of partial melt distribution required to explain the present-day heat budget. For Europa, I estimate the effect of melt accumulation on the local dissipation rate of its mantle. The results are discussed in terms of heat flux, distribution and amplitude of tidal deformation, so that they can be compared with future spacecraft measurements. This work shows that the effect of melt presence on both shear andbulk dissipation must be taken into account in order to characterize rocky planetary bodies in the galilean system, the Solar System or in exoplanetary systems.

Ecology of benthic foraminifera, geochemical and biological interactions: multidisciplinary approach at different scales

Constance CHOQUEL - 09/07/2021

The overall aim of this PhD thesis was to investigate sedimentary micro-environments and ecosystem functioning of two coastal areas. We combined different high spatial resolution methods and multivariate analyses at different spatio-temporal scales to reveal interactions between benthic faunal and geochemical compartments. Firstly, we investigated two stations with contrasted oxygen, nitrate and manganese conditions in the Gullmar Fjord (Sweden). We revealed the high contribution (50–100 %) of denitrifying benthic foraminifera to the nitrogen cycle in oxygenated and nitrate-rich micro–environments. Nitrogen and manganese cycles are closely related to oxygenation conditions of the ecosystem. Our results highlighted the high contribution (87 %) of macrofaunal bioirrigation to Mn release to the water column under hypoxic conditions. Secondly, we focused on a monthly monitoring of two ecological bioindicators groups; microphytobenthos (MPB) and foraminifera in the Bourgneuf Bay mudflat (France). We showed that foraminiferal reproduction events were modulated by unfavorable conditions (high hydrodynamic and winter conditions) versus favorable conditions (low hydrodynamic and summer conditions). We also demonstrated that foraminiferal species fed preferentially on diatom species based on their shape, size and life-forms. We further compared, with high spatial resolution methods, geochemical conditions at two contrasted months, which allowed to clarify the behavior of redox species and nutrients. Then, foraminiferal micro-distributions indicated the state of sediment instability versus stability. Finally, this doctoral research opens new perspectives in the use of high spatial resolution in 2D/3D to solve complex benthic ecology problems.

Role of bacterial siderophore in biogeochemical processes controlling the mobility of metals in vineyard soils and their transfer to the plant

Sitraka RANDRIAMAMONJY - 17/03/2021

Incorporation of iodine in nuclear glasses by vitrification under high pressure

Valentin JOLIVET - 27/01/2021

129I is a product of the fission of uranium in nuclear power plants. It is radiotoxic, very mobile in the environment, and has a long half-life (15.6 Ma). The conditioning of 129I in nuclear glasses for geological storage is complicated by its high volatility at high temperature. The vitrification of nuclear glasses under pressure is a solution that can overcome this problem, because the solubility of volatile elements in liquids increases with pressure. Nuclear glass analogues have been vitrified under high pressure (0.5-2 GPa) to determine the solubility of iodine in glasses, as a function of thermodynamic and compositional parameters. The solubility of iodine in glasses depends on pressure, boron content, but also on the content of non-network forming cations. Polymerized borosilicate glasses such as ISG nuclear glass simulant incorporate ~1 mol.% iodine, while depolymerized glasses such as “Low Activity Waste Glass” incorporate ~2 mol.%. Iodine is incorporated into glasses in the vicinity of the non-network-forming cations. In doing so, it changes the polymerization state of the lattice. Iodine has a depolymerizing effect on polymerized glass, and vice versa for depolymerized glass. The solubility of iodine is also strongly influenced by its oxidation state, with I5+ being much more soluble in glass than I-.