Ancient Environments And The Isotope Geochemistry Of Low Temperature Fe(III) Oxides

古代环境和低温 Fe(III) 氧化物的同位素地球化学

• 批准号: 1250502
• 负责人: Crayton Yapp
• 金额: $ 20.73万
• 依托单位: Southern Methodist University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-09-15 至 2019-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1250502&HistoricalAwards=false
• 关键词: Ancient; Environments; Isotope; Geochemistry; Low

On the continents, increasing modern demands on limited and climatically sensitive "critical zone" resources such as potable water and arable land motivate improved knowledge of past changes in global climate and their manifestation in the critical zone. An overall decline in ocean temperatures over the past 65 million years (Ma) was interrupted at irregular intervals by times of relative warmth. One interruption of special note on the continents was the "middle Miocene climatic optimum" (MMCO) about 15 million years ago. In particular, at a paleo-latitude of ~30°S, western Australia experienced widespread, intense, chemical weathering, which produced ancient soils characterized by pellets of the minerals goethite (alpha-FeOOH) and hematite (alpha-Fe2O3) that are concentrated in vast ore-grade channel iron deposits (CID). Proposed measurements of the H and O isotopes in CID goethite and hematite, as well as radiometric (U-Th)/He crystallization ages of the goethite should produce a well-dated record of ancient subtropical continental temperatures and waters. In addition, measured concentrations and carbon isotope ratios of the Fe(CO3)OH component in goethite would yield soil CO2 concentrations and isotopic characteristics of oxidizing organic matter with their implications for ancient critical zone carbon cycling (e.g., biological activity). These data are also expected to yield the concentration of MMCO atmospheric CO2 and would provide "ground truth" for community use in model simulations of ancient climate.A published model predicts that the O isotope fractionation factor between crystallizing goethite and ambient water will transition from a low-pH (acidic) value to a high-pH (basic) value. A series of goethite synthesis experiments over a range of pH from 3 to 13 at different temperatures are proposed to test this model. Experimental knowledge of the pH at which the transition occurs would minimize the chance of misinterpretation of O isotope data from natural goethites and also yield the relative magnitudes of two rate constants important in the model.The combination of a robust radiometric age of crystallization with stable isotope data from a single mineral (i.e., goethite) is rare and enhances its value as a rich source of paleoenvironmental information. The results from the studies of natural and synthetic goethites would be relevant to climatology, hydrology, and paleoecology, in addition to geochemistry. The proposed research would contribute to the development of human resources through the training and participation of a graduate student in dissertation research. In addition, participation of undergraduate students would provide hands-on experience in the performance of geochemical research and encourage pursuit of scientific careers. Results from this research would be published and integrated into undergraduate and graduate courses taught at SMU by the investigator.

在各大洲，现代社会对饮用水和耕地等有限和气候敏感的“临界区”资源的需求日益增加，促使人们更好地了解过去全球气候的变化及其在临界区的表现。在过去的6500万年（Ma）中，海洋温度的总体下降被相对温暖的时间不规则地打断。大陆上一个特别值得注意的中断是大约1500万年前的“中新世中期气候最佳”（MMCO）。特别是，在约30°S的古纬度，西澳大利亚州经历了广泛，强烈的化学风化，产生了古老的土壤，其特征是针铁矿（α-FeOOH）和赤铁矿（α-Fe 2 O3）的颗粒，集中在巨大的矿石级通道铁矿床（CID）中。建议的CID针铁矿和赤铁矿中的H和O同位素的测量，以及针铁矿的放射性（U-Th）/He结晶年龄应该产生一个古老的亚热带大陆温度和沃茨的良好记录。此外，针铁矿中Fe（CO 3）OH组分的测量浓度和碳同位素比将产生土壤CO2浓度和氧化有机质的同位素特征，以及它们对古临界区碳循环的影响（例如，生物活性）。这些数据也有望产生MMCO大气CO2的浓度，并将为社区在古气候模型模拟中的使用提供“地面实况”。一个已发表的模型预测，结晶针铁矿和环境水之间的O同位素分馏因子将从低pH（酸性）值过渡到高pH（碱性）值。一系列的针铁矿合成实验的pH值从3到13的范围内，在不同的温度下，提出了测试这个模型。对发生转变的pH的实验知识将最小化对来自天然针铁矿的O同位素数据的误解的机会，并且还产生在模型中重要的两个速率常数的相对大小。针铁矿）是罕见的，并提高了其作为古环境信息的丰富来源的价值。天然和合成针铁矿的研究结果将与气候学、水文学和古生态学以及地球化学有关。拟议的研究将有助于通过培训和参与论文研究的研究生开发人力资源。此外，本科生的参与将提供从事地球化学研究的实践经验，并鼓励追求科学事业。这项研究的结果将被发表，并纳入研究人员在SMU教授的本科和研究生课程。