The intellectual merit of this proposal lies in the development of a more complete understanding of the processes that cause fractionation of 30Si and 28Si during soil development. Herein, measurements are presented that demonstrate interpretable patterns of change as unweathered minerals derived from basalt and granite are transformed during weathering. During the early stages of weathering d30Si values of soil solids decrease whereas the d30Si values of soil solution increase relative to the parent material. Eventually, secondary minerals begin to weather releasing Si that has lower d30Si that parent material and therefore the d30Si signatures of soil solution and leaching water decreases. Globally, the results are in agreement with predictions based on the d30Si values of igneous rocks, large rivers and ocean water, but they raise fundamental questions regarding the fate of Si isotopes as minerals weather, secondary minerals are formed, and dissolved Si is leached. The d30Si signatures of the reactants and products of weathering should be the most effective tracer of Si, but more needs to be understood about the behavior of Si isotopes in different aspects of this complex system. The proposed research will develop a more comprehensive and quantitative understanding of the processes that produce Si isotopic fractionations during weathering and soil formation. It will combine systematic field- and laboratory-based experiments and measurement of the changes in Si-isotope composition in solid and aqueous phases in order to isolate the reactions that are most strongly responsible for driving changes in d30Si values. The experiments target the following processes in which Si participates: 1) dissolution of primary and secondary minerals, 2) precipitation of secondary minerals, 3) the combined impact of near-simultaneous dissolution and precipitation reactions, and 4) the integrated aspects of weathering, leaching and clay synthesis. The first 3 of these experiments will be in-lab benchtop experiments; and the fourth will utilize two ongoing experiments at the US Geological Survey, one utilizing leaching columns, the other a well-monitored soil chronosequence. The first 3 experiments simplify soil processes to insure maximum interpretability whereas the column and field experiments measure the integrated results of complex soil forming processes. The chronosequence is in arkosic parent material, and thus will add a useful dimension to an existing database on silicon isotopic changes associated with progressive weathering in the Hawaiian basaltic chronosequence. Chemical weathering unlocks elements from the lithosphere allowing them to participate in dynamic biogeochemical reactions in terrestrial and ultimately marine systems. Silicon is the second most abundant element in the Earth's crust and the dominant solute in rivers that drain continents supplying at least 80% of the dissolved Si entering oceans. The broader impact of this proposal lies in the importance of capturing the power of Si isotopes as a tracer for silicate weathering, and the subsequent participation of Si in Earth surface and ecosystem processes. Silicate weathering is critical for buffering acid rain, for long-term regulation of atmospheric CO2, and is the ultimate source of dissolved Si in natural waters. Silicon's abundance, chemical reactivity, and the variation in the structural stability of its host minerals ensure that it persists in soils even as its loss supports downstream ecosystems and as it accumulates in sedimentary deposits. The proposed research will augment understanding of the global Si cycle, and specifically the weathering-controlled transfer of Si from continents to oceans.
