Natural Variations in Silicon Isotopes in Seawater Silicic Acid and Marine Diatoms: A New Tool for Studying the Marine Silica Cycle

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9729996 Brzezinski In this project, researcher will test the utility of an entirely new tracer for chemical oceanography. Analytical tools for measuring silicon isotope natural abundance in biogenic silica from diatoms and dissolved silicic acid from seawater have been developed and have been used to demonstrate for the first time silicon isotope fractionation by marine diatoms during opal biomineralization. The extent of fractionation is constant among species, and does not vary measurably with temperature, suggesting a single fractionation factor can be used to describe variations in silicon isotopic ratios due to diatom growth in the sea. We propose to obtain data sets I) to assess rigorously the role of biological fractionation in controlling the distribution of silicon isotopes in the sea and ii) to evaluate silicon isotope natural abundance as a measure of silica production in the surface ocean. The first goal will achieved by analyzing samples of dissolved and particulate silica gathered from a number of ocean basins. Additional samples will be obtained to define the isotopic signature of the Si entering the sea through deep-sea hydrothermal vents and rivers. The resulting data will be combined with numerical modeling to test the hypothesis that the interplay among biological fractionation, the biological pump and global thermohaline circulation governs the distribution of Si isotopes in the sea. The response of the isotopic composition of dissolved and particulate silica production in the surface water will be examined by comparing time series of silica exported to depth. The samples from surface waters and sediment traps for the proposed comparisons have been, or will be, collected through existing research efforts allowing us to propose an extensive study at low cost. Success will provide a powerful new tool for examining diatom activity in the modern ocean and the ability to use variations in Si isotopes in sediment cores to examine changes in silica production on geologic time scales.