The role of tectonics, climate, and biology in chemical weathering and carbon dioxide consumption

Chemical weathering is an important driver of geochemical cycles over geologic time however, the relationships and feedbacks between climatic, tectonic, and biologic processes and weathering are poorly understood. This thesis examines the controls of chemical weathering from the microbe-mineral interface to the mountain-belt scale. In the Washington Cascades, chemical fluxes from twelve small catchments within the Skykomish basin show that chemical weathering rates are correlated with long-term exhumation rates and only weakly related to climatic parameters such as temperature or precipitation. Weathering flux data from these catchments are then used to calibrate a chemical weathering model that emphasizes the role of tectonics or erosion as a means of regulating the rate of rock supply to the weathering environment. This calibrated model is used to predict regimes for which the rate of supply of weatherable material limits the overall chemical weathering rate (supply-limited) or regimes in which reaction kinetics (reaction-limited) serve as the rate-limiting factor. Measurement of chemical weathering rates across the northern Himalaya, southern Tibetan plateau and the eastern syntaxis of the Himalaya shows that chemical weathering rates are highest in regions of the highest physical erosion and generally low in areas of low precipitation and low rates of physical weathering. In total, silicate weathering in the eastern syntaxis of the Himalaya, the most tectonically active region of the globe, accounts for greater than 15-20% of the total CO2 consumption by weathering in the Brahmaputra basin from less than 4% of the total land area. In the final related study, microbial biofilms of Caulobacter Cr. and Anabena are grown on apatite mineral surfaces in solutions in which the apatite is the only source of phosphorus. Through scanning electron microscopy, laser confocal imaging, and chemical analyses of solutions, this work shows that in an open system in which apatite is the only source of P available for microbial growth, these two organisms develop biofilms on apatite mineral surfaces but seem to show no control over bulk mineral dissolution rates.