The SFMC jury has decided to award the 2025 Haüy-Lacroix Prize jointly to Jeanne Caumartin and Timmo Weidner.
Jeanne Caumartin‘s thesis, entitled ‘Study of environmental determinants and anoxia in the formation of microbialites,’ was carried out at the Institute of Mineralogy, Materials Physics and Cosmo-Chemistry (Sorbonne Université) and at the Ecology, Society and Evolution Laboratory (Université Paris-Saclay) under the supervision of Karim Benzerara and Purificaciόn Lόpez-García. This work demonstrated the existence of modern microbialites, rocks formed by microorganisms, in seasonally anoxic environments, in contrast to most of those known to date. This is particularly interesting since these modern objects are often used as analogues for fossil microbialites, the oldest of which appeared billion years ago and thrived in anoxic conditions. An approach combining mineralogy, microbial diversity analysis and solution geochemistry studies on natural and laboratory-incubated samples was used. This allowed (i) demonstrating the existence of a critical saturation threshold for solutions with respect to carbonate mineral phases, close to the solubility of amorphous phases, necessary for the formation of microbialites, (ii) identifying mineralogical and geochemical signatures of anoxia, also leading to microbial adaptations, and (iii) detecting early diagenetic mineral changes in these structures, inducing the formation of carbonate phases such as huntite. These results will improve our knowledge of the geographical and environmental distributions of current microbialites and provide a better understanding of the extent to which major environmental changes, particularly those linked to anthropogenic activities, could influence the mineral composition and microbial ecology of these ecosystems.
Timmo Weidner‘s thesis, entitled ‘Dislocation Electron Tomography – Applications and Association to Continuum Mechanics and Dislocation Dynamics’, was conducted within the Plasticity Group at Lille University as part of the TimeMan project, under the supervision of Alexandre Mussi, Karine Gouriet and Patrick Cordier. The aim of his research was to improve understanding of plastic deformation in minerals by using dislocation electron tomography, a technique that enables 3D reconstructions of dislocation microstructures using transmission electron microscopy. Going beyond simple characterisation, his work combined tomography with continuum mechanics, opening new perspectives for integrating electron microscopy with dislocation dynamic modelling. A key focus of his thesis was the role of dislocation climb under natural strain rates. Using dislocation dynamic modelling on periclase, he demonstrated that, at lower mantle pressures, dislocation climb acts as the rate-limiting mechanism for creep since anionic diffusion becomes extremely slow. Consequently, periclase deforms more slowly than bridgmanite, suggesting that bridgmanite, which deforms predominantly by pure climb, controls mantle rheology. Applying electron tomography to naturally deformed quartz revealed a significant contribution from ‘mixed climb’ dislocations, whereby climb works not only as a recovery process, but also actively contributes to natural strain. Similar configurations observed in olivine indicate that this mechanism is not unique to quartz. Overall, his thesis highlights the limitations of laboratory-based data alone, emphasising the need for advanced 3D characterisation techniques to fully understand deformation processes in the Earth’s interior.
