The effects of different types and amounts of dissolved organic matter (DOM) on the isotope fractionation of Hg isotopes during aqueous photo-reduction of monomethylmercury (MMHg) were investigated to assess whether mass-independent fractionation (MIF) signatures can be used to track photo-degradation of MMHg in natural waters. From experiments conducted with different amounts of reduced organic sulfur (Sred-DOM), it appears that MIF during photo-reduction may be dependent on whether MMHg is dominantly bound to Sred-DOM. Similar fractionation factors were observed for experiments where Sred-DOM was in far excess of MMHg, while significantly lower fractionation factors were observed with lower Sred-DOM. We also characterized the signature of MIF (i.e. Δ199Hg/Δ201Hg) during MMHg photo-degradation to assess if it was similar in different matrices. The experimental Δ199Hg/Δ201Hg was very similar for different matrices. However, the experimental slope is slightly but statistically different than the slope observed in freshwater fish, which preserve MMHg in nature.

The effects of different types and amounts of dissolved organic matter (DOM) on the isotope fractionation of Hg isotopes during aqueous photo-reduction of monomethylmercury (MMHg) were investigated to assess whether mass-independent fractionation (MIF) signatures can be used to track photo-degradation of MMHg in natural waters. From experiments conducted with different amounts of reduced organic sulfur (Sred-DOM), it appears that MIF during photo-reduction may be dependent on whether MMHg is dominantly bound to Sred-DOM. Similar fractionation factors were observed for experiments where Sred-DOM was in far excess of MMHg, while significantly lower fractionation factors were observed with lower Sred-DOM. We also characterized the signature of MIF (i.e. Δ199Hg/Δ201Hg) during MMHg photo-degradation to assess if it was similar in different matrices. The experimental Δ199Hg/Δ201Hg was very similar for different matrices. However, the experimental slope is slightly but statistically different than the slope observed in freshwater fish, which preserve MMHg in nature.

L'atmosphère terrestre a subi, au cours de son histoire précoce, de très violents changements dans sa composition chimique. L'émergence des premiers êtres vivants photosynthétique a provoqué la mise en place et la modification des grands cycles biogéochimiques. L'usage des isotopes comme traceurs des processus physico-chimiques actifs pendant ces périodes s'est développé depuis les années 80. L'observation de compositions isotopiques anormales dans certaines molécules atmosphériques (ozone notamment), similaires à celles observées dans les météorites par Clayton (1973), pose question. De nombreuses hypothèses ont été proposées pour expliquer ce phénomène "anormal". L'objectif principal de cette thèse de doctorat est de développer, par l'expérimentation, l'étude d'un certain nombre de réactions en phase plasma du point de vue isotopique. Seront particulièrement étudiées des réactions appelées réactions à trois corps de formation de l'ozone, où la formation d'un complexe intermédiaire excité entre en jeu dans les mécanismes de génération de ces fractionnements isotopiques anormaux. / The Earth's atmosphere has undergone, during its early history, very violent changes in its chemical composition. The emergence of the first photosynthetic living beings has led to the establishment and modification of major biogeochemical cycles. The use of isotopes as tracers of active physico-chemical processes during these periods has been developed since the 1980s. The observation of abnormal isotopic compositions in certain atmospheric molecules (ozone in particular), similar to those observed in meteorites by Clayton (1973), raises questions. Many hypotheses have been proposed to explain this "abnormal" phenomenon. The main objective of this doctoral work is to develop, through experimentation, the study of plasma reactions from an isotopic point of view. In particular, ozone formation third-body reactions, where the formation of an excited intermediate complex is involved in the mechanisms of generating these abnormal isotopic fractionations, will be studied.

Le mercure (Hg) est un métal lourd ubiquiste et très toxique. Présent à l’état de traces dans la colonne d’eau des milieux aquatiques, il peut cependant atteindre des concentrations très élevées en fin de chaine alimentaire car il a la particularité d’être bioaccumulé et bioamplifié dans les organismes sous forme de méthylmercure (MeHg). L’identification et la caractérisation des transformations amenant à la formation de MeHg (méthylation) ou à sa dégradation (déméthylation) sont donc de première importance pour évaluer son devenir dans les milieux aquatiques. L’utilisation du comportement des isotopes stables du Hg, à la fois en laboratoire mais aussi dans des échantillons environnementaux, a permis d’évaluer l’influence des processus biotiques et abiotiques mis en jeu dans les système aquatiques sur les transformations et donc la spéciation du Hg dans de tels environnements. Le fractionnement isotopique du Hg, pouvant être dépendant et/ou indépendant de la masse, s’est également avéré être un outil performant pour étudier sa bioaccumulation dans plusieurs membres de la chaîne alimentaire endémique du Lac Baikal (Russie). Les signatures isotopiques mesurées dans ces échantillons ont permi d’améliorer la connaissance sur la distribution, les sources et les transformations affectant les espèces du Hg dans l’écosystème de ce grand lac faisant partie du patrimoine mondial de l’UNESCO depuis 1996 et étant la plus grande réserve d’eau douce liquide de surface mondiale. / Mercury (Hg) is a toxic and ubiquitous heavy metal. Only present at trace levels in the water column of aquatic systems, it can reach high amounts in food web end-members because of its ability to bioaccumulate and biomagnify in organisms as methylmercury (MeHg). Hence, the characterization of the transformations leading to the formation (methylation) and the degradation (demethylation) of MeHg is of great concern to evaluate its fate in aquatic environments. The use of the Hg stable isotopes, during laboratory experiments or in environmental samples, allowed to identify and characterize several biotic and biotic pathways involved in Hg transformations and speciation in aquatic systems. The study of Hg mass-dependent and/or mass-independent fractionation was also a competitive tool to assess its bioaccumulation process in several members of the Lake Baikal endemic food chain (Russia). Measured Hg isotopic signatures in such samples provided insight about Hg species fate and sources within the ecosystem of this lake, which has been nominated as a world heritage site by UNESCO in 1996 and constitutes the world’s largest freshwater lake in terms of volume.