Experimental and Numerical Assessment of Basalt Weathering Rates under Different Environmental Conditions.

Weathering of parent materials is an important aspect of soil formation that is tightly coupled to the progressive colonization of grain surfaces by microorganisms and plants.  As part of a larger experimental study being designed for Biosphere 2, we seek to determine how different environmental conditions, including particle size, pH, presence of dissolved organic matter, Eh, and water flow rate affect the incongruent dissolution rates of basaltic tuff.  In order to achieve this objective, column experiments are being conducted with variable pore water velocities using simulated rainwater with and without plant-derived soluble organic matter in the influent solution.  Column effluents are analyzed for major and trace cations, anions, silica and organic solutes.  Dissolution rates of primary minerals and precipitation rates of secondary phases will be estimated by fitting the data to a numerical reactive transport model. At the end of the fluid flow experiment, column materials will be analyzed for biogeochemical composition to detect preferential dissolution of specific phases, the precipitation of new ones, and to monitor the associated formation of biofilms.  The influence on weathering patterns of key experimental parameters – including column length, fluid residence time and aqueous geochemistry – will be discussed.