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

古代环境与低温 Fe(III) 和 Al 氧化物的地球化学

• 批准号: 0616627
• 负责人: Crayton Yapp
• 金额: $ 23.18万
• 依托单位: Southern Methodist University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-10-01 至 2010-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0616627&HistoricalAwards=false
• 关键词: Ancient; environments; geochemistry; low; temperature

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.

地球表面的环境变化具有广泛的时空尺度。这些变化表现为地球大气圈、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）是风化的常见产物，其来自适度的表面温度、丰富的液态水和（针铁矿）大气中的O2的组合，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18 O/16 O比值已被证明保留了古代温度和沃茨的信息。针铁矿还含有少量的组分Fe（CO 3）OH，其是针铁矿结晶时局部存在的CO2的分压和13 C/12 C比率的代表。在适当的情况下，Fe（CO 3）OH也可以用来推断古对流层CO2的浓度，其在中生代气候变化中的作用存在争议。已发表的研究表明，三水铝石包含类似的信息。拟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）研究选定的系统，包含潮湿的大陆环境和环境CO2的地球化学信息，从两个显着温暖的间隔在中生代。(1)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，该模型已成功地用于解释天然针铁矿的D/H和18 O/16 O数据。然而，到目前为止，还没有实验证实模型的预测。建议的研究包括一系列的合成实验，在封闭系统条件下，在不同的温度和pH值的水溶液中，以产生针铁矿与不同程度的铝取代。由此产生的D/H和18 O/16 O分馏针铁矿和水之间的测量和相关的Al取代度，温度和pH值。这些结果反过来将与预测的热力学模型进行比较。(2)白垩纪中期和始新世早期至中期的非常温暖的间隔，表现在选定的大陆风化系统是拟议的研究的重点。18 O/16 O、D/H、13 C/12 C、化学、XRD、SEM和岩相学数据将从包含针铁矿、三水铝石和/或高岭石的白垩纪和始新世系统获得，以获得关于古环境温度、沃茨、CO2和有机物的信息。这项研究的一个重要的新组成部分将是收购的针铁矿结晶的放射性年龄测定（U-Th）/He方法。更广泛的影响：对未来全球变暖和气候系统复杂性的担忧推动了对新的定量数据的持续需求，包括过去时代的数据，当时大自然可能已经“运行变暖实验”。需要代表各种空间和时间尺度的古环境数据，以更好地描述表现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从目前相对缺乏同位素数据的古老、温暖、潮湿、大陆、风化环境中获得新的信息。此外，在这项工作中的放射性和稳定同位素分析的结合，不仅应该提高我们的古气候知识，但也成岩作用和古环境信息的保存程度在低温系统，如paleosol.The拟议的工作将支持学生在SMU的研究。作为这项工作的一部分，研究生和本科生将接受地球化学培训，并将其应用于他们对这些古环境变化的地质代理的研究。PI进行的古环境研究结果的实验室已经，并将在适当的情况下整合到SMU地质学研究生和本科生课程中。