Life on Earth could have existed already 3.8 Ga ago, and yet, more complex, multicellular life did not evolve until over three billion years later, about 700 Ma ago. Many have searched for the reason behind this apparent delay in evolution, and the dominating theories put the blame on the hostile Precambrian environment with low oxygen levels and sulphide-rich oceans. There are, however, doubts whether this would be the full explanation, and this thesis therefore focuses on a new hypothesis; the levels of the redox sensitive element arsenic increased in the oceans as a consequence of the change in weathering patterns that followed the upcome of free oxygen in the atmosphere at about 2.4 billion years ago. Given its toxicity, this could have had negative effects upon the life of the time. To test the hypothesis, 66 samples from drill cores coming from South Africa and Gabon with ages between 2.7 and 2.05 Ga were analysed for their elemental composition, and their arsenic content were compared with carbon isotope data from the same samples. These confirmed that a rise in arsenic concentration following the upcome of free oxygen in the atmosphere and the onset of oxidative weathering of continental sulphides. Arsenic, which is commonly found in sulphide minerals, was weathered together with the sulphide and delivered into the oceans, where it in the Palaeoproterozoic increased to over 600% compared to the older Archaean levels, at least locally. Iron had the strongest control over the arsenic levels in the anoxic (ferruginous and sulphidic) oceans, probably due to its ability to remove arsenic through adsorption. During oxygenated conditions, sulphur instead had the strongest influence upon arsenic, likely because of the lack of dissolved iron. The highest arsenic levels were found in samples recognised as coming from oxygenated conditions, although this might be due to the oxygenation state of arsenic affecting its solubility. Arsenic is toxic already at low doses, especially if the necessary arsenic detoxification systems had not yet evolved. However, the lack of correlation between arsenic and changes in δ13C indicated that the increase of arsenic did not affect the primary production between 2.7 and 2.05 Ga. Thus, whether arsenic could have affected the evolution of life during the Mesoproterozoic remains to be shown.

L’émergence des bactéries photosynthétiques oxygéniques (BPO), ou cyanobactéries, est probablement l’évènement le plus important de l’histoire de la Terre, depuis l’apparition de la vie elle-même. Par la libération d’O2 dans l’environnement, les BPO ont conduit à l’oxygénation de notre planète, jusqu’alors anoxique, et au développement de la vie complexe. Cependant, cette évolution n’est toujours pas datée. Dans cette étude, j’ai cherché à identifier des signatures chimiques spécifiques aux BPO, in situ à l’échelle du μm, dans des tapis microbien fossiles datant de 3,45 à 1,88 Ga, recouvrant ainsi une période allant de la Terre anoxique à la Terre oxygénée après le « Great Oxidation Event » (GOE). Nous avons utilisé la microscopie optique, la spectroscopie Raman, le MEB/EDX, l’EPMA, la μ-XRF à rayonnement synchrotron (SR-XRF), et des analyses isotopiques. Une nouvelle méthode de quantification élémentaire pour SR-XRF, ainsi qu’une nouvelle méthodologie de préparation d’échantillons ont été developpés. Les résultats obtenus par EPMA et μ-XRF montrent que, dans certains contextes de déposition, un enrichissement en lanthanides (par exemple La, Sm, Gd) de cellules fossiles, et un enrichissement en Cu de pyrites diagénétiques formées en association avec des BPO, pourraient représenter des signatures chimiques spécifiques aux BPO. Suite à ces résultats, je propose que les BPO ont évolué entre 3,33 et 2,98 Ga. Je propose que les techniques élémentaires telles que l’EPMA et la μ-XRF sont les techniques les plus appropriées pour trouver des signatures chimiques spécifiques aux BPO et contraindre leur émergence dans le temps. / The evolution of oxygenic photosynthetic bacteria (OPB) is probably the most important biological event of Earth’s history since the emergence of life itself. The release of their by-product O2 in the environment, which was globally anoxic, fundamentally changed the face of the Earth and led to the development of complex life. However, the specific timing of this evolutionary step remains unclear. This study is based on the search for in situ chemical signatures of OPB at the microbial (μm) scale, within fossilized microbial photosynthetic mats in Archean and Paleoproterozoic sediments dated between 3.45 Ga and 1.88 Ga, i.e. spanning the anoxic Earth to the aftermath of the GOE. We used optical microscopy, Raman spectroscopy, SEM/EDS, EPMA, synchrotron radiation μ-XRF, and isotope analytical techniques. The μXRF results were improved by the use of a new sample preparation method and a new quantification method, both developed during this study.Results obtained by EPMA and μXRF show that, under certain depositional contexts, enrichment in lanthanides (such as Sm, La and Gd) in individual OPB cells, as well as a Cu enrichment in diagenetic pyrites formed in association with OPB, may represent chemical signatures of OPB. I propose that OPB evolved sometime between 3.33 Ga and 2.98 Ga. Also, I argue that elemental techniques such as EPMA and μ-XRF are the most suitable techniques to find chemical signatures of OPB and constrain the timing of their emergence.