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.

地质记录表明，地球上气候变化的历史是一个在大范围时间尺度上复杂的历史。因此，关于因果关系和自然系统通过地球流体“球体”（大气和水圈）这一媒介联系起来的方式的问题变得更加复杂。为了寻找这些问题的答案，需要关于大气“温室”气体（如COO）丰度的新数据来源和关于地球历史上不同时期地表温度的相应资料。常见的低温氢氧化铁针铁矿（a- feooh）的氧同位素系统，使其成为矿物形成时环境温度和水的宝贵信息来源。针铁矿形成于潮湿、氧化的环境中，如土壤，可以保存大陆气候条件的定量记录。研究发现，在针铁矿的晶体结构中有少量的CO2被“捕获”，因为古土壤针铁矿中的Fe(CO3) OH可以用来测定地球大气中CO2的分压。在过去的4.4亿年里，大气中二氧化碳的波动似乎很大，但地球的气候似乎并不总是以传统预期的方式作出反应。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的长期变化信息。土壤二氧化碳的扩散混合模型将应用于古鲕状铁矿的碳同位素数据，以获得地球过去大气二氧化碳压力的额外信息，特别是在“恐龙时代”（中生代）的两个有趣而令人困惑的时期。这些样品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、含针铁矿的矿床进行碳、氢、氧同位素和元素分析，以进一步确定天然针铁矿（可能还有水合铁）的同位素系统。将分析这些年轻针铁矿中“难熔”有机碳的碳同位素组成，并将其与共存的“易溶”有机碳进行比较，作为研究这一新认识的古代样品信息来源的一部分。作为了解和利用低温氧化铁同位素系统在古环境研究中的整体努力的一部分，将进行以下实验工作：(1)碳同位素效应与铁水合物吸附CO2的相关实验研究；（ii）对混合水合铁干扰针铁矿中从年轻的自然系统中Fe(CO3)OH组分中回收CO2的可能性的评估；（iii）在不同温度下针铁矿内部Fe(CO3)OH的碳同位素分馏（相对于气态CO2）与针铁矿表面吸附的CO2的碳同位素分馏的比较。这些实验很重要，因为水合铁在自然系统中作为常见的初始铁（III）氢氧化物沉淀（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针铁矿中铁（CO3）OH获得碳同位素组成的机制。