A theoretical model coupling chemical weathering rates with denudation rates

摘要

Uplift of the Himalayas has been proposed to have locally accelerated chemical weathering, thus leading to enhanced CO₂ sequestration and global cooling. This hypothesis assumes that rapid erosion exposes fresh, highly reactive minerals at Earth's surface. Empirical studies quantifying the relationship between erosion and weathering have produced apparently conflicting results, where the nature of the relationship is dependent on the weathering regime of the sampled landscapes. We derive a quantitative model that defines this relationship across the range of weathering regimes, from supply-limited to kinetically limited conditions. The model matches trends in field data collected by others and reconciles apparently conflicting results. The model also demonstrates that, as erosion rates increase, potential increases in weathering rate from the exposure of fresher materials are offset by the decrease in the total volume of minerals exposed due to thinner regolith. We conclude that the relationship between weathering and erosion is one of diminishing returns, in which increases in erosion rate lead to progressively smaller increases in weathering rate; indeed, at the highest erosion rates, weathering rates may decline. The ability, therefore, of accelerated uplift and erosion to stimulate greater CO₂ sequestration may be significant in landscapes eroding at rates of 100–102 t km–2 yr–1. However, where erosion rates are greater than 102 t km–2 yr–1, increases in denudation may not be matched by increases in chemical weathering. Finally, our results suggest that watersheds with regolith thicknesses of ~0.5 m will yield the greatest solute fluxes.