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.

摘要地球表面的环境变化发生在广泛的时间和空间尺度上。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) 是风化的常见产物，是由于适中的表面温度、丰富的液态水和（对于针铁矿）大气中的 O2 的结合而产生的，这使得地球表面在这个太阳系。针铁矿的 D/H 和 18O/16O 比率已被证明可以保存有关古代温度和水域的信息。针铁矿还含有少量的 Fe(CO3)OH 成分，它是针铁矿结晶时局部存在的 CO2 分压和 13C/12C 比的代表。在适当的情况下，Fe(CO3)OH 还可用于推断古代对流层 CO2 的浓度，其在显生宙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) 对选定的系统进行研究，这些系统包含来自显生宙两个显着温暖区间的潮湿大陆环境和周围二氧化碳的地球化学信息。(1) 通过热力学模型预测了针铁矿中 Al 取代 Fe 的显着同位素效应，这些效应已被用于解释天然针铁矿的 D/H 和 18O/16O 数据取得了明显的成功。然而，到目前为止，还没有实验证实模型的预测。拟议的研究包括在封闭系统条件下、在不同温度和pH值的水溶液中进行一系列合成实验，以生产具有不同Al取代度的针铁矿。将测量所得的针铁矿和水之间的 D/H 和 18O/16O 分馏，并将其与 Al 取代度、温度和 pH 值相关联。这些结果又将与热力学模型的预测进行比较。(2)选定的大陆风化系统中所体现的白垩纪中期和始新世早至中期的非常温暖的区间是本研究的重点。 18O/16O、D/H、13C/12C、化学、XRD、SEM 和岩相数据将从含有针铁矿、三水铝石和/或高岭石的白垩纪和始新世系统中获得，以获得有关古代环境温度、水、CO2 的信息和有机物。这项研究的一个重要的新组成部分是获取通过 (U-Th)/He 方法测定的针铁矿结晶的辐射年龄。这将是首次结合使用稳定同位素和针铁矿辐射分析来研究古环境。更广泛的影响：对未来全球变暖和气候系统复杂性的担忧推动了对新定量数据的持续需求，包括过去时期的数据，当大自然可能已经“进行变暖实验”时。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表面环境，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代、温暖、潮湿、大陆、风化环境的新信息，目前这些环境的同位素数据相对缺乏。此外，这项工作中放射性和稳定同位素分析的结合不仅可以增强我们对古气候的了解，还可以增强我们对成岩作用的了解以及古土壤等低温系统中古环境信息的保存程度。拟议的工作将支持学生的研究在新加坡管理大学。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，并将其应用于对古环境变化的地质代理的研究。 P.I. 实验室进行的古环境研究的结果已经并将在适当的情况下纳入 SMU 地质学研究生和本科课程的课程中。