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

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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.