CO 2 drawdown from weathering is maximized at moderate erosion rates.

Uplift and erosion modulate the carbon cycle over geologic timescales by exposing minerals to chemical weathering. However, the erosion sensitivity of mineral weathering remains difficult to quantify. Solute-chemistry datasets from mountain streams in different orogens isolate the impact of erosion on silicate weathering-a carbon dioxide (CO 2 ) sink-and coupled sulfide and carbonate weathering-a CO 2 source. Contrasting erosion sensitivities of these reactions produce a CO 2 -drawdown maximum at erosion rates of ~0.07 millimeters per year. Thus, landscapes with moderate uplift rates bolster Earth's inorganic CO 2 sink, whereas more rapid uplift decreases or even reverses CO 2 sequestration. This concept of an "erosion optimum" for CO 2 drawdown reconciles conflicting views on the impact of mountain building on the carbon cycle and permits estimates of geologic CO 2 fluxes dependent upon tectonic changes.