Ancient environments and the geochemistry of low temperature Fe(III) and Al oxides

地球表面的环境变化在广泛的时间和空间尺度上发生。这些变化表现为地球大气层、水圈和岩石圈内部的变化和相互作用。针铁矿(-FeOOH)和三水铝石(Al(OH)3)是常见的风化产物，是中等地表温度、丰富的液态水和(对于针铁矿)大气O2的组合导致的，这使得地球表面在这个太阳系中是独一无二的。针铁矿的D/H和18O/16O比值已被证明保存了有关古代温度和水的信息。针铁矿还含有少量的Fe(CO3)OH组分，它代表了针铁矿结晶时局部存在的二氧化碳的分压和13C/12C比。在适当的情况下，Fe(CO3)OH也可以用来推断古代对流层二氧化碳的浓度，其在显生界气候变化中的作用尚有争议。已发表的研究表明，三水铝石含有类似的信息。建议的研究分为两大类：(1)针铁矿同位素组成的晶体化学控制的实验研究；(2)所选系统的研究，其中包含显生界两个显著温暖区间的潮湿大陆环境和环境二氧化碳的地球化学信息。(1)热力学模型预测了针铁矿中铝替代铁的显著同位素效应，并已明显成功地解释了天然针铁矿的D/H和18O/16O数据。然而，到目前为止，还没有实验证实模型的预测。该研究包括在封闭体系条件下，在不同温度和水溶液的pH值下进行一系列合成实验，以制备不同程度Al取代的针铁矿。由此得到的针铁矿和水之间的D/H和18O/16O分馏分数将被测量，并与铝的取代度、温度和pH有关。这些结果将反过来与热力学模型的预测进行比较。(2)部分大陆风化系统所显示的白垩纪中期和始新世早期至中期的非常温暖的时间段是本研究的重点。从白垩纪和始新世含针铁矿、三水铝石和/或高岭石的系统中获得18O/16O、D/H、13C/12C、化学、X射线衍射、扫描电子显微镜和岩石学数据，以获得有关古代环境温度、水体、二氧化碳和有机质的信息。这项研究的一个重要新组成部分将是获得(U-Th)/He方法测定的针铁矿结晶的放射年龄。这将是第一次联合使用稳定同位素和针铁矿的放射性分析来研究古环境。广泛的影响：对未来全球变暖和气候系统复杂性的担忧推动了对新的量化数据的持续需求，包括过去的数据，当时自然可能已经进行了变暖实验。需要代表各种空间和时间尺度的古环境数据，以更好地描述显示古气候的不同地表环境，并了解系统各部分之间的相互作用。拟议的地球化学/同位素研究将有助于利用来自古老、温暖、潮湿、大陆、风化环境的新信息，目前关于这些环境的同位素数据相对匮乏。此外，放射性同位素和稳定同位素分析的结合不仅可以增强我们对古气候的认识，而且还可以增强我们对成岩作用的认识，以及对低温系统(如古土壤)中古环境信息的保存程度。研究生和本科生将接受地球化学培训，作为这项工作的一部分，并将其应用于他们对这些古环境变化的地质指标的研究。在P.I.S实验室进行的古环境研究成果已经并将适当地纳入新加坡国立大学地质学研究生和本科生课程。

ABSTRACTEnvironmental change on Earth's surface occurs on a wide range of temporal and spatial scales. These changes are expressed as variations within, and interactions among, the Earth's atmosphere, hydrosphere, and lithosphere. Goethite (-FeOOH) and gibbsite (Al(OH)3) are common products of the weathering that results from the combination of moderate surface temperatures, abundant liquid water, and (for goethite) atmospheric O2 that makes the surface of the Earth unique in this solar system. The D/H and 18O/16O ratios of goethite have been shown to preserve information on ancient temperatures and waters. Goethite also contains small amounts of the component Fe(CO3)OH, which is a proxy for the partial pressure and 13C/12C ratio of CO2 locally present at the time of goethite crystallization. Under the right circumstances, Fe(CO3)OH can also be used to infer concentrations of ancient tropospheric CO2 whose role in Phanerozoic climate change is debated. Published research suggests that gibbsite contains similar information.The proposed research is divided into two broad categories: (1) experimental studies of the crystal chemical controls of the isotopic composition of goethite; and (2) studies of selected systems that contain geochemical information on wet continental environments and ambient CO2 from two notably warm intervals in the Phanerozoic.(1) Significant isotopic effects of Al substitution for Fe in goethite are predicted by thermodynamic models, which have been used with apparent success to interpret D/H and 18O/16O data from natural goethites. However, as yet, there is no experimental confirmation of the model predictions. The proposed research includes a series of synthesis experiments under closed system conditions at different temperatures and pH in aqueous solutions to produce goethites with varying degrees of Al substitution. The resulting D/H and 18O/16O fractionations between the goethite and water would be measured and related to degree of Al substitution, temperature, and pH. These results in turn would be compared with the predictions of the thermodynamic models.(2) The very warm intervals of the mid-Cretaceous and early- to mid-Eocene as manifested in selected continental weathering systems are the focus of the proposed research. 18O/16O, D/H, 13C/12C, chemical, XRD, SEM, and petrographic data would be obtained from Cretaceous and Eocene systems that contain goethite, gibbsite, and/or kaolinite to obtain information on ancient ambient temperatures, waters, CO2, and organic matter. An important new component of this research would be the acquisition of radiometric ages of goethite crystallization as determined by the (U-Th)/He method. This would be the first combined use of stable isotope and radiometric analyses of goethite to study paleoenvironments.Broader impact: Concerns about future global warming and the complexity of the climate system drive a persistent need for new quantitative data including data from times in the past, when Nature may have already "run the warming experiment". Paleoenvironmental data representing a variety of spatial and temporal scales are needed to better describe the diverse surface environments that manifest paleoclimates and to understand the interactions among the parts of the system. The proposed geochemical/isotopic research would contribute to the effort with new information from ancient, warm, wet, continental, weathering environments for which there is currently a relative dearth of isotopic data. In addition, the combination of radiometric and stable isotope analyses in this work should enhance not only our knowledge of paleoclimate, but also of diagenesis and the degree of preservation of paleoenvironmental information in low temperature systems such as paleosols.The proposed work would support student research at SMU. Graduate and undergraduate students would receive geochemical training as part of this work and apply it to their research on these geological proxies of paleoenvironmental change. Results of paleoenvironmental research performed in the P.I.'s laboratory have been, and will be, integrated where appropriate into courses in the SMU geology graduate and undergraduate curriculum.