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

在各大洲，对饮用水和耕地等有限的、对气候敏感的“临界区”资源的现代需求日益增加，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6500万年中，海洋温度的总体下降(Ma)被相对温暖的时期以不规律的间隔中断。关于大陆的一个特别注解是大约1500万年前的“中中新世气候最佳时期”(MMCO)。特别是，在古纬度约30°S，西澳大利亚经历了广泛的、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-Fe2O)颗粒矿物集中在巨大的矿石级水道铁矿中。对CID针铁矿和赤铁矿中氢和氧同位素的拟议测量，以及针铁矿的放射性(U-Th)/He结晶年龄，应该会产生古代亚热带大陆温度和水的良好年代记录。此外，测量针铁矿中Fe(CO3)OH组分的浓度和碳同位素比值，可以得出土壤CO2浓度和氧化有机质的同位素特征，以及它们对古代临界区碳循环(如生物活性)的意义。这些数据还有望得出MMCO大气中二氧化碳的浓度，并将为社区在古代气候模型模拟中使用提供“地面事实”。一个发表的模型预测，结晶针铁矿和环境水之间的氧同位素分馏系数将从低pH(酸性)值过渡到高pH(碱性)值。在不同温度下，在pH为3~13的范围内进行了一系列针铁矿合成实验，以验证该模型。对发生转变的pH值的实验知识将最大限度地减少对来自天然针铁矿的氧同位素数据的误解，并获得在模型中重要的两个速率常数的相对大小。稳健的放射性结晶年龄与来自单一矿物(即针铁矿)的稳定同位素数据相结合是罕见的，并增强了其作为丰富的古环境信息来源的价值。天然和合成针铁矿的研究结果除了与地球化学有关外，还与气候学、水文学和古生态学有关。拟议的研究将通过对研究生的培训和参与论文研究来促进人力资源的开发。此外，本科生的参与将提供在地球化学研究方面的实践经验，并鼓励追求科学事业。这项研究的结果将被公布，并纳入新加坡管理大学的本科生和研究生课程，由调查员教授。

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.