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Accueil du site → Doctorat → Allemagne → 2018 → Chemical and isotopic (Li, H, O, C) composition of surface waters and sediments – implications on weathering, erosion, and paleoenvironmental reconstructions on the Tibetan Plateau

Freie Universität Berlin (2018)

Chemical and isotopic (Li, H, O, C) composition of surface waters and sediments – implications on weathering, erosion, and paleoenvironmental reconstructions on the Tibetan Plateau

Weynell, Marc

Titre : Chemical and isotopic (Li, H, O, C) composition of surface waters and sediments – implications on weathering, erosion, and paleoenvironmental reconstructions on the Tibetan Plateau

Auteur : Weynell, Marc

Université de soutenance : Freie Universität Berlin

Grade : DOKTOR DER NATURWISSENSCHAFTEN (DR. RER. NAT.) 2018

Résumé partiel
Understanding the natural controls on atmospheric carbon dioxide concentrations (pCO2) is a major goal of climate research, as anthropogenic carbon dioxide (CO2) emissions continue to rise pCO2. Those controls can be partially revealed by investigating the reason for a substantial pCO2 decline throughout the Cenozoic (from 65.5 Ma on), which resulted in a massive climate change. Atmospheric CO2 is mainly balanced by degassing from Earth’s interior and removal during weathering of silicate rocks. Increased silicate weathering rates, mainly triggered by Himalayan and Tibetan Plateau formation, are proposed as reason for the Cenozoic pCO2 decline. This assumption is in opposition to stable CO2 degassing during this era as imbalances result in complete removal of atmospheric CO2. Lithium (Li) is an element, which is almost exclusively hosted in silicate minerals and its two isotopes (7Li/6Li) fractionate during silicate weathering reactions. Thus, studying Li isotope variations are ideal to investigate silicate weathering processes. Coeval to the Cenozoic pCO¬2 decline, oceanic Li isotope ratios (δ7Li) rose. Identifying a potential connection between both events possibly clarifies some controls behind the Cenozoic climate change. Several studies attributed an uplift-driven increase of the global riverine δ7Li value as causal for the rise of seawater δ7Li. The objective of this thesis is to understand the controls on Li isotope fractionation during weathering on the Tibetan Plateau and to use Li isotope variations in rivers and sediments as a silicate weathering tracer across the plateau. Further, the impact of Tibetan Plateau formation on the average global riverine Li isotope composition is considered. For this reason, field work was performed in the catchment of Lake Donggi Cona, located on the semi-humid northeastern Tibetan Plateau and Lake Bangong, located on the hyper-arid western plateau. Bedrock, different types of sediments (loess, fluvial, limnic), and surface waters (lake, stream, spring, and thermal waters) were analyzed. Additionally, the upper Huang He (Yellow River), Yarlung Tsangpo (upper Brahmaputra), and the upper Indus were investigated as these rivers drain large areas with different climate and geomorphic conditions on the eastern, southern, and western plateau. The limited anthropogenic activity on the plateau is ideal to study nearly undisturbed climate and weathering patterns. In contrast, the widespread occurrence of hydrothermal activity has to be considered as this may disturb weathering or climate induced geochemical variations. Average δ7Li values for the bedrock, soil solutions, and secondary weathering products in the catchment of Lake Donggi Cona were deduced from local loess, streams, and lake sediments and are around +1.9 ‰, +16.6 ‰, and 0.8 ‰, respectively. The hydrochemistry of the major inflow, small streams, and the lake is determined by weathering of carbonates.

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