EAR-PF: Mechanisms and Isotopic Signatures of Iron-Silica Adsorptive Interactions in the Precambrian Ocean

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Dr. Steven Chemtob has been awarded an NSF Earth Sciences Postdoctoral Fellowship to carry out a research and education plan at Washington University in St. Louis. Dr. Chemtob will perform laboratory experiments to characterize the structures, chemistries and stable isotope compositions of primary iron-silica precipitates that contributed to banded-iron formation (BIF). The presence of BIFs, iron- and silica-rich chemical sediments found in Precambrian successions, is tied to the oxygenation of Earth's early oceans and the development of the photosynthesizing biosphere. An outstanding issue in the study of BIF genesis is the mechanism for transport of iron and silica to depositional centers. Silica could have been transported to the BIF deposition site by efficient adsorption on iron oxide surfaces; alternatively, Fe and Si could have coprecipitated as a single oxyhydroxide phase. The impacts of specific mechanisms of iron-silica interaction on observable geochemical signatures, such as Fe and Si isotope composition, are unknown. The objectives of the proposed work are to characterize the molecular mechanisms of silica adsorption on iron oxide surfaces using X-ray scattering techniques and to identify site-specific silicon isotope fractionation factors for each adsorptive mechanism. This research will reveal fundamental mechanisms of aqueous Fe-Si interaction in Precambrian oceans, assist in the interpretation of chemical and isotopic analyses of BIFs, and place quantitative constraints on the role of silica adsorption in BIF deposition. The proposed studies will provide insight into the chemistry of ancient oceans but are also applicable to problems relevant to water chemistry and human health. The silicon isotope composition of modern natural waters is frequently used as a proxy for chemical weathering; these results will allow for improved quantification of the weathering process. Additionally, aqueous silica competes with other aqueous chemical species for adsorptive sites, impacting the capacity of iron oxide to adsorb nutrients, trace elements and toxic contaminants. Understanding silica adsorption mechanisms in Fe- and Si-rich waters is important for predicting trace element behavior and for developing effective remediation techniques. Dr. Chemtob's education plan will include mentoring high school and undergraduate students on authentic research experiences, giving public lectures in the St. Louis area, and participating in a teaching-as-research program, culminating in the development and presentation of new course content using novel pedagogical techniques.