Development of gallium-aluminum ratios as a tracer of the Critical Zone behavior of Al

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Aluminum (Al) is a major component of common primary and secondary rock-forming minerals, and forms the "backbone" of major soil minerals and clays. Mineral transformations involving aluminum are fundamental to weathering and soil formation processes. Aluminum is not known to perform any beneficial role in organisms, and can be toxic to aquatic life at elevated concentrations. Aluminum can inhibit plant uptake of nutrients in soils, and high Al availability in soils has been shown to limit crop productivity. The chemical behavior of aluminum in the environment is complex, making it difficult to investigate. One of the tools that geochemists have used to study the behavior of many other elements is to use variations in naturally occurring isotopes as tracers, but aluminum only has one isotope. Instead, the investigator proposes to develop the ratio of gallium (Ga) to aluminum as a tracer for Al behavior. Ga has similar chemistry to Al, and shows overall coherent behavior. He can use Ga/Al ratios to identify key mineral sources and reaction pathways for Al in the environment. Further, gallium itself is a trace element with critical technological applications, used in key electronic components in common consumer products such as cellular telephones. As such, gallium is classified as a "strategic metal" by the US Department of Energy. The planned research should provide additional insight into processes that can enrich Ga to levels that are economically recoverable in bauxite ores, for example. Proposed research will work in conjunction with the Critical Zone Observatories National Office (CZO-NO) to create outreach and educational materials related to Critical Zone science. The applied aspects of Al toxicity and Ga as a critical resource provide an excellent opportunity to develop curricular resources linking basic research and societal impacts. The investigator will work with the CZO-NO to develop curricular resources for secondary school and undergraduate science programs in support of Next Generation Science Standards (NGSS, 2013). He will continue fostering outreach to HBCUs via engineering and biology departments to demonstrate intellectual excitement and societal impacts of Critical Zone science. Building connections at HBCUs is especially important for building geoscience diversity since most do not have geoscience departments and thus do not have a traditional pipeline to Earth science graduate programs.The investigator proposes to develop gallium-aluminum ratios (Ga/Al) as an effective geochemical tracer for the behavior of Al in the Critical Zone. The overarching goal is to develop the necessary fundamental understanding to support the use of Ga/Al as an effective tool for mass balance and tracer studies of Al in the Critical Zone and beyond; a ?pseudo-isotope? for Al. He will focus on four of the processes that control the distribution of Al in the Critical Zone and may cause fractionation of Ga/Al in order to answer the following questions: 1) Is the neoformation of clay minerals and oxyhydroxides responsible for Ga/Al fractionation during weathering? 2) Is the fractionation of Ga and Al in the Critical Zone driven by solution chemistry (i.e. hydrolysis or ligand complex formation)? 3) Do differences in the incorporation of Ga and Al in colloids result in fractionation of Ga from Al during stream export? He will use a combination of field and laboratory-based studies on materials from granitoid catchments across four Critical Zone Observatories (Calhoun, Boulder Creek, Catalina-Jemez, and Luquillo). He will analyze rock, soil, plant, and water samples from each to characterize the behavior of Ga/Al ratio under different conditions of climate, weathering and biological activity. Mineral synthesis and soil column leaching experiments in the laboratory will provide understand partitioning of Ga in oxyhydroxides and the impact of organic ligands on Ga and Al in soils, supported by geochemical modeling. Development of an effective tracer for Al in the environment should be of broad interested to the environmental and geoscience community. Aluminum plays a central role in many geochemical processes and is associated with multiple issues in human health, water quality, and crop productivity. As a strategic metal, an improved understanding of gallium geochemistry is also of broad importance for society. The proposed work will provide insight into Ga geochemistry that will be of interest to the mineral exploration community, especially given that CZ processes are responsible for the formation of the main economic source of Ga, bauxites.