How phosphate is incorporated into carbonate minerals and its dependence on crystal growth conditions

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The interaction of dissolved inorganic phosphate with the carbonate minerals calcite and aragonite is important to many low temperature geochemical and paleoenvironmental systems. Phosphate is an essential, sometimes bio-limiting nutrient, but in soils and natural waters growth of carbonate minerals can remove phosphate. Phosphate is sorbed strongly to the surface of calcite and aragonite and incorporated into these minerals during crystal growth as an incompatible trace element. These processes remove P from the environment, making it unavailable to organisms, but potentially creating a record of the dissolved P content. For example, variation in phosphate concentration in calcite cave deposits at high spatial resolution is used in paleoclimate studies to mark annual growth layers in speleothems and to indicate paleohydrologic conditions. Recently it has been proposed that the P content of certain deep-sea corals can serve as a proxy for the dissolved phosphate concentration in seawater at the time of growth. Although there have been many studies of the sorption and incorporation of phosphate in carbonate minerals, the chemical nature of the incorporated species remains uncertain. Recently, we have shown that solid-state 31P NMR spectroscopy can be used to determine the P speciation in calcite in situ at concentrations found in nature. In calcite speleothems phosphate is incorporated by at least three different processes, involving at least two distinct crystalline inclusions.We propose an experimental program to investigate the pathway for phosphate incorporation in calcite and aragonite and how it varies with crystal growth conditions. Calcite and aragonite will be prepared under carefully controlled conditions with varying dissolved phosphate, growth rate, pH, and other important parameters relevant to precipitation in nature. The distribution of phosphate in the mineral phase will be determined by a combination of solid-state NMR spectroscopy and P K-edge XANES and spectromicroscopic methods, which will also allow identification of any crystalline phosphate phases. Particular emphasis will be placed on determining the conditions under which the phosphate concentration of the solution can be inferred from the P distribution of the mineral which precipitates. The experimental program will be guided by investigation of natural carbonate minerals in collaboration with other researchers, including calcite and aragonite cave deposits and aragonitic deep-sea corals.Intellectual Merit: This project will provide fundamental new knowledge of phosphate incorporation in common minerals, by providing information on the chemical form of the P in carbonate minerals how it varies with crystal growth conditions. An understanding of the P incorporation mechanism is needed to promote informed use of trace P content for paleoenvironmental reconstruction. These data will also be useful for constructing more general models for incorporation of large incompatible oxyanions in carbonates. The experimental data will provide constraints on the conditions that lead to formation of surface precipitates, which is at present poorly understood because such precipitates are very difficult to detect.Broader Impacts: A primary goal of this project is to provide relevant research opportunities for the education of graduate students and to introduce undergraduate students to scientific research in a structured environment. The research plan is designed specifically for students to acquire a diverse set of skills in materials characterization and experimental low-temperature geochemistry, mineralogy, and spectroscopy that are applicable to a wide range of scientific problems. The research will involve undergraduate students in a manner that provides mentoring experience for a graduate student. This project will contribute to research infrastructure in geochemistry through support of solid-state NMR facilities and support for graduate students who operate these spectrometers for research collaborations.