Collaborative Research: Ge/Si as a Tracer of Terrestrial Silica Cycling

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ABSTRACTWeathering of silicate minerals on continents is a major source of silicon (Si) for formation of secondary silicate minerals in soil, opal phytoliths in plants and ultimately for diatoms in the oceans. Even though Si is a significant element in the biogeochemistry of Earth, it does not have a well developed tracer to provide a clear understanding of how organic and inorganic processes affect Si behavior and the pathways that Si follows as it moves from rock to clay or water or organism. Germanium (Ge) and Si behave chemically in a similar fashion and have been treated as a pseudo-isotopic tracer of the behavior of Si in the surficial environment. We have conducted preliminary research that demonstrates clear and logical changes in the Ge/Si ratio in response to soil and plant processes. Here we propose a systematic analysis of Ge fractionation from Si during: 1) incongruent weathering and the formation of secondary alumino-silicate clay minerals, 2) formation of iron oxides in soil, and 3) plant uptake and opal biosynthesis leading to formation of phytoliths.We propose a field-based project where we quantify the Ge/Si ratios of rock minerals, soil minerals, soil water and plant phytoliths in soils forming on extrusive volcanic rock and on intrusive plutonic rocks in a humid tropical weathering environment. Ge/Si ratios will be determined on fresh parent materials and soils from a series of well-studied tropical soil sites in Hawaii (basaltic substrate) and Luquillo, Puerto Rico (granitoid substrate). Pore waters and mineral separates (where applicable) from the same horizons as the soil samples will be analyzed. We will also analyze plant phytoliths from dominant vegetation. We will use the data to test specific hypotheses about processes that control Ge/Si fractionation in the weathering environment. The proposed research will compare and contrast the behavior of the Ge/Si system in aphanitic basaltic environments with plutonic granitoid environments, where we expect that the mechanisms of fractionation will differ because of the differences in chemical composition, mineralogy, and crystallinity. It will also specifically investigate the role of plant uptake of Si and the biogenic silica cycle in soil systems. The results of this study will provide the basic data needed to develop Ge/Si ratios as a quantitative tracer of silica during weathering, plant cycling, and in surface waters.