Global Biogeochemical Cycle of Silicon And Silicate Weathering during Soil Development

This dissertation contributes to the understanding of the biogeochemical cycle of silicon (Si), one of the important nutrient elements, and its coupling with other nutrient cycles, natural and human perturbations. A Silicon Land Ocean Ecosystem Model (SLOEM) is developed to simulate the exogenic physical transport, biological and abiotic transformation of Si on Land, Coastal Ocean and Open Ocean Domains. Si isotope subroutine is also incorporated in SLOEM to quantify Si productivities. The simulated trend is then compared against available Si isotopic records.;SLOEM.LGM, demonstrating Si cycle during Last Glacial Cycle, supports the temperature and nutrient (dust transport) controls of paleo-ocean productivity, particularly between opal formation by marine diatom and radiolarian abundances. The simulated results are consistent with sedimentary records in terms of opal vs radiolarian content, and opal delta30Si excursion.;SLOEM.MOD runs from the year 1700 to 2100 to model the Si cycle during Anthropocene. A possible cause of the increase of riverine discharge of dissolved Si (DSi) from 1700-1950 is the slash-and-burn deforestation which increases the release of plant phytoliths into the groundwater. Later, the use of fertilizers and other agricultural means for increasing crop yield and land primary production lead to a decrease of riverine DSi. After 1900, the loss of DSi is accelerated by the damming of major rivers. This effect alone accounts for 4 Tmol Si/yr decrease in DSi discharge from land to the ocean, which is more than half of the total DSi discharge. In addition, the increase in Coastal Ocean primary production by excessive nitrogen and phosphorus, warming, and scarcity of Si lowers the Si/N values. The ecosystem becomes vulnerable to environmental deterioration and shift from diatom bloom to, e.g. toxic algal bloom.;Chapter 5 presents a reactive-transport model used to examine tectonic and climatic controls on such process by characterizing regolith development. The model supports that the maximum contribution should occur for medium uplift rate regions of silicate bedrock consolidated in warm, wet climates, because the combination of medium tectonic uplift, high temperatures, and rapid seepage velocities accelerates reaction front propagation, facilitates calcite depletion, and sustains deep regolith development over long timescales.