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

在大陆上，现代对有限和气候敏感的“临界带”资源（如饮用水和耕地）的需求日益增加，促使人们对过去全球气候变化及其在临界带的表现有了更深入的了解。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期以不规则的间隔中断。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期”（MMCO）。特别是，在古纬度~30°S，西澳大利亚经历了广泛、强烈的化学风化作用，产生了以矿物针铁矿（α - feooh）和赤铁矿（α - fe2o3）颗粒为特征的古土壤，这些颗粒集中在巨大的矿级河道铁矿（CID）中。对CID针铁矿和赤铁矿中H和O同位素的测量，以及针铁矿的辐射(U-Th)/He结晶年龄的测定，应该能提供一个准确的古代亚热带大陆温度和水域的记录。此外，针铁矿中Fe(CO3)OH组分的测量浓度和碳同位素比值将产生土壤CO2浓度和氧化有机质的同位素特征，及其对古代临界带碳循环（例如生物活性）的影响。这些数据还有望产生MMCO大气二氧化碳的浓度，并将为社区在古代气候模型模拟中使用提供“基础真相”。一个已发表的模型预测，结晶针铁矿与周围水之间的O同位素分馏因子将从低ph（酸性）值转变为高ph（碱性）值。提出了一系列的针铁矿合成实验，在pH从3到13的范围内，在不同的温度下，以验证这一模型。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O同位素数据的误读的机会，并产生模型中重要的两个速率常数的相对大小。从单一矿物（如针铁矿）获得的稳定同位素数据与可靠的结晶放射性年龄相结合是罕见的，这增强了其作为丰富的古环境信息来源的价值。天然针铁矿和人工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。建议的研究将通过培训和参与研究生的论文研究，促进人力资源的发展。此外，本科生的参与将提供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，并由研究者整合到新大的本科和研究生课程中。

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.