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Exploration des compositions isotopiques en magnésium des carbonates marins comme traceurs paléoenvironnementaux

Abstract : Studying the Mg isotopic composition in marine carbonate can help to understand the biogeochemical cycle of this element. Indeed, carbonates may record seawater Mg isotopic composition during their precipitation. However, it is necessary to understand the possible control of some environmental factors (e.g. temperature, pH, Mg/Ca of the solution) on those compositions. Therefore, the first goal of this thesis was to determine parameters impacting carbonate Mg isotopic composition with experimental precipitations under controlled conditions. Thus, it has been shown, in restricted ranges, that temperature, pH and solution Mg/Ca have no influence on Mg isotopic fractionation during calcite precipitation. Equilibrium Mg isotopic fractionation between solution and calcite has been evaluated to -2.13 ± 0.24 per thousand (2sigma) from this study, combined with published data. These results were then applied to a study of Mg isotopic compositions in modern and past brachiopod shells. For this, a quantification of vital effects with respect to Mg isotopes during the brachiopod growth was realized. The zones in isotopic equilibrium for Mg, O and C, and thus susceptible to be used for paleoenvironnemental reconstruction, are in the inner calcite at the edge of the shell. The first measurements of Mg isotopic compositions for the last 60 Ma suggest variations of Mg isotopic compositions of the seawater which could be linked to changes of carbonate flux in the ocean
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Submitted on : Thursday, March 29, 2018 - 12:16:53 PM
Last modification on : Tuesday, October 19, 2021 - 7:01:26 PM
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Ségolène Saulnier. Exploration des compositions isotopiques en magnésium des carbonates marins comme traceurs paléoenvironnementaux. Paléontologie. Université de Lorraine, 2012. Français. ⟨NNT : 2012LORR0267⟩. ⟨tel-01749461⟩



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