Ancient Atmospheric Pco2, Paleoclimates and the Stable Isotope Geochemistry of Low Temperature Iron Oxides

古代大气 Pco2、古气候和低温氧化铁的稳定同位素地球化学

• 批准号: 9614265
• 负责人: Crayton Yapp
• 金额: $ 22.48万
• 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: 1997
• 资助国家: 美国
• 起止时间: 1997-02-01 至 2001-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9614265&HistoricalAwards=false
• 关键词: Ancient; Atmospheric; Pco2; Paleoclimates; Stable

9614265 Yapp The geologic record reveals that the history of climatic change on the Earth is one of complexity on a wide range of time scales. As a result, questions about causality and the ways in which natural systems are linked through the media of the Earth's fluid "spheres" (the atmosphere and hydrosphere) have become more complex. The search for answers to such questions has required new sources of data on the abundance of atmospheric "greenhouse" gases (e.g., COO) and corresponding information on surfical temperatures at various times in the history of the Earth. The oxygen isotope systematics of the common, low temperature iron oxyhydroxide, goethite (a-FeOOH), make it a valuable source of information on ambient temperature and water at the time of mineral formation. Goethite forms in moist, oxidizing environments such as soils and can preserve a quantitative record of climatic conditions on the continents. It was discovered that there is a small amount of CO 2 "trapped" in the crystal structure of goethite as an Fe(CO3) OH in goethites from ancient soils can be used to determine the partial pressure of CO2 in the Earth's atmosphere. There appear to have been large fluctuations in atmospheric CO2 over the past 440 million years, but it seems that the Earth's climate has not always responded in a conventionally anticipated manner. A continuation of the efforts to acquire and verify information on long-term changes in climate and atmospheric chemistry during the Phanerozoic is proposed here. The diffusive mixing model for soil CO2 will be applied to carbon isotope data from ancient oolitic ironstones to obtain additional information on atmospheric CO2 pressures in the Earth's past-particularly at two interesting and puzzling times in the "age of the dinosaurs" (the Mesozoic era). Oxygen isotope data from appropriate mineral pairs in these same samples would be used to evaluate temperatures and sources of water in a search for apparent links with the CO2 content of the Eart h's atmosphere. Carbon, hydrogen and oxygen isotope and elemental analyses will be performed on young, active, goethite-bearing deposits to further define the isotopic systematics of natural goethites (and possibly ferrihydrites). The carbon isotope composition of "refractory" organic carbon in these young goethites will be analyzed and compared with coexisting "accessible" organic carbon as part of the study of this newly recognized source of information in ancient samples. As part of the overall effort to understand and utilize the isotopic systematics of low temperature iron oxides in studies of ancient environments, the following experimental work will be undertaken: (I) an experimental study of the carbon isotope effects associated with the adsorption of CO2 onto ferrihydrites; (ii) an assessment of the potential for admixed ferrihydrite to interfere with the recovery of CO2 from the Fe(CO3)OH component in goethites from young natural systems: and (iii) a comparison at various temperatures of the carbon isotope fractionation (relative to gaseous CO2) of the Fe(CO3)OH in the interior of goethite with that of CO2 adsorbed onto the surface of goethite. Such experiments are important, because of the important role of ferrihydrite as the common initial Fe(III) hydroxide precipitate in natural systems (and therefore a precursor to goethite), and because of a need to better understand the mechanisms by which Fe(CO3)OH in goethite acquires its carbon isotope composition.

9614265 Yapp 地质记录揭示了地球气候变化的历史在广泛的时间尺度上是复杂的。 因此，关于因果关系以及自然系统通过地球流体“圈”（大气圈和水圈）媒介联系起来的方式的问题变得更加复杂。 寻找这些问题的答案需要有关大气“温室”气体（例如COO）丰度的新数据来源以及地球历史上不同时期地表温度的相应信息。 常见的低温氢氧化铁、针铁矿 (a-FeOOH) 的氧同位素系统学使其成为矿物形成时环境温度和水的宝贵信息来源。 针铁矿形成于土壤等潮湿的氧化环境中，可以保存大陆气候条件的定量记录。 人们发现，在针铁矿的晶体结构中“捕获”了少量的CO 2 ，​​因为来自古代土壤的针铁矿中的Fe(CO3) OH可用于确定地球大气中CO2的分压。 过去 4.4 亿年来，大气中的二氧化碳浓度似乎出现了大幅波动，但地球气候似乎并不总是以传统预期的方式做出反应。 这里建议继续努力获取和验证显生宙期间气候和大气化学长期变化的信息。 土壤二氧化碳的扩散混合模型将应用于来自古代鲕粒铁岩的碳同位素数据，以获得有关地球过去大气二氧化碳压力的更多信息，特别是在“恐龙时代”（中生代时代）的两个有趣且令人费解的时期）。 来自这些相同样本中适当矿物对的氧同位素数据将用于评估温度和水源，以寻找与地球大气中二氧化碳含量的明显联系。 将对年轻的、活跃的含针铁矿矿床进行碳、氢和氧同位素和元素分析，以进一步确定天然针铁矿（可能还有水铁矿）的同位素系统。 作为研究古代样品中这一新认识的信息源的一部分，将对这些年轻针铁矿中“难熔”有机碳的碳同位素组成进行分析，并与共存的“可接近”有机碳进行比较。 作为在古代环境研究中理解和利用低温氧化铁的同位素系统学整体努力的一部分，将进行以下实验工作：（I）与 CO2 吸附相关的碳同位素效应的实验研究水铁矿； (ii) 评估混合水铁矿干扰从年轻自然系统的针铁矿中 Fe(CO3)OH 组分中回收 CO2 的可能性：以及 (iii) 不同温度下碳同位素分馏的比较（相对于针铁矿内部的 Fe(CO3)OH 与针铁矿表面吸附的 CO2 的气态 CO2）。 这些实验很重要，因为水铁矿作为自然系统中常见的初始 Fe(III) 氢氧化物沉淀物（因此是针铁矿的前体）的重要作用，并且因为需要更好地了解 Fe(CO3)针铁矿中的 OH 获得其碳同位素组成。