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Theses
Evolution de l’altération continentale au cours du Crétacé supérieur sur les marges sud atlantiques de l’Afrique et de l’Amérique du sud : apports d’un nouveau traceur géochimique (systèmes isotopiques combinés Lu-Hf et Sm-Nd) couplé à la minéralogie des argiles.
Evolution de l’altération continentale au cours du Crétacé supérieur sur les marges sud atlantiques de l’Afrique et de l’Amérique du sud : apports d’un nouveau traceur géochimique (systèmes isotopiques combinés Lu-Hf et Sm-Nd) couplé à la minéralogie des argiles.
Abstract : The late Cretaceous records a pronounced decrease in temperatures on a global scale between 90 and 65 million years that marks the first step of the progressive climatic decline ultimately leading to our modern climate mode. This first cooling step is concomitant to a major tectonic uplift of the east South American and west African margins. Relief formation on the African ans South American continent, enhancing continental weathering, can have induced a climatic cooling at a global scale through atmospheric CO2 consumption linked to silicate weathering reactions. The main objective of this project is to explore the potentially determinant impact of this tectonic uplift on the long-term cooling of the late Cretaceous. A new proxy, based on the coupled Lu-Hf and Sm-Nd isotope systems in clays, will be used to track the late Cretaceous continental weathering evolution on those margins, associated to clay mineralogy that tracks the hydrolysis and mechanical erosion variations. The three IODP sites studied have thus made it possible to show that in the late Cretaceous, the chemical alteration of silicates was intensified in response to the uplifts of the margins but that this response depended on the climatic and tectonic context and illustrated the competition between tectonics and climate on the continental chemical weathering of silicates. Finally, the comparison of the data with the climatic evolution shows concomitant evolution between the intensification of the weathering of the margins and the acceleration of the cooling suggests a contribution of the uplift to the late Cretaceous global climate cooling through an increased CO2 consumption engendered by silicates chemical weathering.
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