Chemical Weathering of Steady-State Landscapes

The rate of movement of rock through a zone of chemical weathering may greatly influence fluxes of the weathered products, and hence CO2 drawdown in actively uplifting areas. We developed a coupled tectonic-erosional-chemical model that considers the effects of the vertical advection of rock through the weathering zone when rock uplift is balanced by erosion. Dimensional analysis of this model demonstrates that two end-member behaviors of chemical weathering are possible: When the residence time of rock or soil in the weathering zone is long compared to the time-scale of the chemical reaction, the flux of the weathering product is limited by the translation of rock through the weathering zone; we refer to this condition as supply-limited. In this case, the flux of the weathering product depends only on the initial abundance of minerals entering the weathering zone from the underlying bedrock and the rock uplift rate, and not on the kinetics of the weathering reactions. Conversely, when the residence time of rock or soil in the weathering zone is short compared to the reaction time-scale, much of the rock advected through the soil column does not undergo significant weathering, and so we refer to this condition as reaction-limited. Unlike supply-limited conditions, in this case the flux of material instead depends on the reaction kinetics, the thickness of the weathering zone, and the initial mineral abundance in the underlying bedrock and not the rock uplift rate. In general, reaction-limited conditions dominate when residence times of rock and soil in the weathering zone are on the order of thousands of years or less, while supply-limited conditions dominate when residence times are on the order of hundreds of thousands of years or greater. This model was calibrated using field data from the Skykomish watershed in the Central Cascades, Washington, USA and a global compilation of Si-flux, uplift, and soil thickness data. This model suggests that areas of active, rapid rock uplift (> 1 mm/yr) may be reaction-limited, and thus increasing rock uplift rates may not necessarily lead to accelerated chemical weathering in these environments. Instead, weathering zone thickness may be the dominant control on weathering product fluxes (and hence CO2 drawdown) in such areas. However, transient weathering phenomena such as the storage and weathering of material as it is transported from the actively uplifting areas to its ultimate location of burial may allow supply-limited conditions to exist even in rapidly uplifting environments.