What is dolomite telling us about ancient seawater chemistry

今天在地球上产生有序白云石的现代环境都有一套关键的物理和化学特征。与此同时，我们对碳酸盐矿化认识的最新进展表明，特定的化学波动对推动高镁碳酸盐的沉淀至关重要。因此，有一个独特的机会来检验与富镁碳酸盐沉淀机制有关的关键假设，这些机制协调了对现代和古代白云石分布的现有限制。该项目将通过实验地球化学、现代高盐度系统环境地球化学、现代和古代富含白云岩沉积物的高分辨率显微构造调查和数值模拟相结合的方法来研究高镁碳酸盐沉积(包括白云岩)的机制。该项目提供了一个机会，将现代高盐度系统和/或保存完好的前寒武纪白云岩序列的野外工作、实验地球化学、高级地球化学和矿物学分析以及理论地球化学结合起来。这些元素可以根据学生的兴趣进行调整。一个关键组成部分将涉及(通过有针对性的实地工作或从合作者那里获取样品)收集各种早期成岩和自生白云岩样品，这些样品代表了广泛的现代和古环境。学生还将设计和实施一个有针对性的实验计划，解构产生高镁碳酸盐的关键过程，并改变它们的晶体化学和结构。此外，利用电子探针、X射线衍射仪、显微拉曼光谱、固体核磁共振、同步加速器-显微XRF和XANES以及高分辨率的透射电子显微镜，对这些机制在合成样品、现代样品和古代样品中的特征进行了研究。最后，有机会通过地球化学模拟和动力学模拟(盒子模拟)来探索生物地球化学对现代和古代系统的意义。

Modern environments that produce ordered dolomite on Earth today all share a key set of physical and chemical characteristics. At the same time, recent advances in our understanding of carbonate mineralisation indicate that specific chemical fluctuations are crucial in driving the precipitation of high-Mg carbonates. As a consequence, there is a unique opportunity to test key hypotheses related to the mechanisms of Mg-rich carbonate precipitation that reconcile available constraints on both the modern and ancient distributions of dolomite. This project will examine the mechanisms of high-Mg carbonate precipitation (including dolomite) by combining experimental geochemistry, the environmental geochemistry of modern hyper-saline systems, high-resolution microstructural investigations of modern and ancient dolomite-rich sediments, and numerical modeling.This project provides an opportunity to combine field work in modern hyper-saline systems and/or well-preserved Precambrian dolomitic successions, experimental geochemistry, advanced geochemical and mineralogical analysis, and theoretical geochemistry. There is scope for these elements to be tailored somewhat to the student's interests. A key component will involve collecting (either through targeted field work or sample acquisition from collaborators) a wide range of early diagenetic and authigenic dolomitic samples that represent a broad spectrum of modern and paleo-environments. The student will also design and implement a targeted program of experimentation that deconstructs the key processes generating high-Mg carbonates and modifies their crystal chemistry and structure. In addition, the signatures of these mechanisms in synthetic, modern and ancient samples using a combination of EPMA, XRD, micro-Raman spectroscopy, solid state NMR, synchrotron-micro XRF and XANES, and high-resolution TEM. Finally, there is an opportunity to explore the biogeochemical significance to modern and ancient systems through geochemical modeling and dynamical simulations (box modeling).