Ancient environments and the geochemistry of low temperature Fe(III) and Al oxides

Southern Methodist University, Dallas TX

Investigators

Abstract

ABSTRACT Environmental change on Earth's surface occurs on a wide range of temporal and spatial scales. These changes are expressed as variations within, and interactions among, the Earth's atmosphere, hydrosphere, and lithosphere. Goethite (-FeOOH) and gibbsite (Al(OH)3) are common products of the weathering that results from the combination of moderate surface temperatures, abundant liquid water, and (for goethite) atmospheric O2 that makes the surface of the Earth unique in this solar system. The D/H and 18O/16O ratios of goethite have been shown to preserve information on ancient temperatures and waters. Goethite also contains small amounts of the component Fe(CO3)OH, which is a proxy for the partial pressure and 13C/12C ratio of CO2 locally present at the time of goethite crystallization. Under the right circumstances, Fe(CO3)OH can also be used to infer concentrations of ancient tropospheric CO2 whose role in Phanerozoic climate change is debated. Published research suggests that gibbsite contains similar information. The proposed research is divided into two broad categories: (1) experimental studies of the crystal chemical controls of the isotopic composition of goethite; and (2) studies of selected systems that contain geochemical information on wet continental environments and ambient CO2 from two notably warm intervals in the Phanerozoic. (1) Significant isotopic effects of Al substitution for Fe in goethite are predicted by thermodynamic models, which have been used with apparent success to interpret D/H and 18O/16O data from natural goethites. However, as yet, there is no experimental confirmation of the model predictions. The proposed research includes a series of synthesis experiments under closed system conditions at different temperatures and pH in aqueous solutions to produce goethites with varying degrees of Al substitution. The resulting D/H and 18O/16O fractionations between the goethite and water would be measured and related to degree of Al substitution, temperature, and pH. These results in turn would be compared with the predictions of the thermodynamic models. (2) The very warm intervals of the mid-Cretaceous and early- to mid-Eocene as manifested in selected continental weathering systems are the focus of the proposed research. 18O/16O, D/H, 13C/12C, chemical, XRD, SEM, and petrographic data would be obtained from Cretaceous and Eocene systems that contain goethite, gibbsite, and/or kaolinite to obtain information on ancient ambient temperatures, waters, CO2, and organic matter. An important new component of this research would be the acquisition of radiometric ages of goethite crystallization as determined by the (U-Th)/He method. This would be the first combined use of stable isotope and radiometric analyses of goethite to study paleoenvironments. Broader impact: Concerns about future global warming and the complexity of the climate system drive a persistent need for new quantitative data including data from times in the past, when Nature may have already "run the warming experiment". Paleoenvironmental data representing a variety of spatial and temporal scales are needed to better describe the diverse surface environments that manifest paleoclimates and to understand the interactions among the parts of the system. The proposed geochemical/isotopic research would contribute to the effort with new information from ancient, warm, wet, continental, weathering environments for which there is currently a relative dearth of isotopic data. In addition, the combination of radiometric and stable isotope analyses in this work should enhance not only our knowledge of paleoclimate, but also of diagenesis and the degree of preservation of paleoenvironmental information in low temperature systems such as paleosols. The proposed work would support student research at SMU. Graduate and undergraduate students would receive geochemical training as part of this work and apply it to their research on these geological proxies of paleoenvironmental change. Results of paleoenvironmental research performed in the P.I.'s laboratory have been, and will be, integrated where appropriate into courses in the SMU geology graduate and undergraduate curriculum.