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

摘要地球表面的环境变化具有广泛的时间和空间尺度。这些变化表现为地球大气、水圈和岩石圈内部的变化和相互作用。针铁矿（-FeOOH）和三水石（Al(OH)3）是风化作用的常见产物，这种风化作用是由温和的地表温度、丰富的液态水和（针铁矿）大气中的O2共同作用的结果，这使得地球表面在太阳系中独一无二。针铁矿的D/H和18O/16O比值已被证明保存了古代温度和水域的信息。针铁矿中还含有少量的Fe(CO3)OH组分，这是针铁矿结晶时局部存在的分压和13C/12C比值的代表。在适当的情况下，Fe(CO3)OH也可以用来推断古代对流层CO2的浓度，其在显生宙气候变化中的作用是有争议的。已发表的研究表明，三水铝石含有类似的信息。拟开展的研究分为两大类：(1)针铁矿同位素组成的晶体化学控制实验研究；(2)对包含显生宙两个显著暖期的湿大陆环境和环境CO2地球化学信息的选定系统进行研究。(1)利用热力学模型预测了针铁矿中Al取代Fe的显著同位素效应，并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。本研究包括在不同温度和pH的封闭体系条件下，在水溶液中进行一系列合成实验，以制备不同程度Al取代的针铁矿。将测量针铁矿和水之间的D/H和18O/16O分馏，并将其与Al取代度、温度和ph相关联。这些结果将与热力学模型的预测结果进行比较。(2)中白垩世和早始新世至中始新世的极暖期是本文研究的重点。18O/16O, D/H, 13C/12C，化学，XRD， SEM和岩石学数据将从白垩纪和始新世系统中获得，其中包含针铁矿，三水石和/或高岭石，以获得有关古代环境温度，水，CO2和有机质的信息。这项研究的一个重要的新组成部分将是获得针铁矿结晶的放射性年龄，由(U-Th)/He法确定。这将是首次将针铁矿的稳定同位素和放射性分析结合起来研究古环境。更广泛的影响：对未来全球变暖和气候系统复杂性的担忧，促使人们持续需要新的定量数据，包括过去的数据，那时大自然可能已经“进行了变暖实验”。为了更好地描述反映古气候的地表环境的多样性，了解系统各部分之间的相互作用，需要具有不同时空尺度的古环境数据。拟议的地球化学/同位素研究将有助于这项工作，因为它提供了来自古代、温暖、潮湿、大陆和风化环境的新信息，而目前同位素数据相对缺乏。此外，在本研究中，放射性分析和稳定同位素分析的结合不仅可以提高我们对古气候的认识，还可以提高我们对成岩作用和古土壤等低温系统中古环境信息保存程度的认识。拟议的工作将支持新大的学生研究。研究生和本科生将接受地球化学训练，并将其应用于古环境变化地质指标的研究。古环境研究结果在P.I.在适当的情况下，该实验室已经并将被整合到新大地质学研究生和本科生的课程中。

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.