Competition between erosion and reaction kinetics in controlling silicate-weathering rates

Abstract

Weathering rates of minerals may be limited by the reaction kinetics of the constitutive minerals or the supply of fresh minerals delivered to the near surface by denudation. We use a model of denudation and reaction kinetics to analyze the relative importance of each in moderating silicate-weathering fluxes in different erosional environments for different, commonly occurring silicate minerals. We find that minerals that reside in the near-surface weathering zone for far longer than the reaction takes to complete, as is the case with slow denudation, produce weathering fluxes that respond most strongly to changes in denudation rate. Conversely, when transit through the weathering zone is rapid relative to a given mineral's weathering timescale, the weathering responds most vigorously to changes in the reaction kinetics. Based on field- and laboratory-measured values of silicate-weathering kinetic constants, observed weathering zone thicknesses in relation to denudation rates, and denudation rates inferred from topography, it appears that fresh mineral supply may play a sub-equal or dominant role in moderating silicate-weathering fluxes for four mineral phases considered. If correct, this suggests that the concentration of atmospheric CO 2, which is regulated by silicate weathering over geologic timescales, may depend on those factors that control long-term erosion rates across Earth's surface.