Relationship between mechanical erosion and atmospheric CO₂ consumption in the New Zealand Southern Alps

To examine the influence of mountain uplift on the long-term carbon cycle, we used geochemical, hydrologic, and suspended-load data for 12 streams draining the New Zealand Southern Alps to quantify rates of erosion, weathering, and atmospheric CO₂ consumption. Rapid uplift in the western Southern Alps elevates mechanical erosion rates by a factor of ∼13 relative to those on the tectonically stable eastern side [125 × 10⁸ vs. 9.4 × 10⁸ g/(km²·yr), respectively]. Similarly, the average chemical weathering rate is ∼5 times higher on the western compared to eastern side of the mountain range [9.8 × 10⁷ vs. 2.0 × 10⁷ g/(km²·yr), respectively]. However, because the proportion of stream-water Ca²⁺ and Mg²⁺ from carbonate weathering increases as the rate of mechanical erosion increases, the long-term atmospheric CO₂ consumption rate on the western side is ∼2 times higher than that on the eastern side [14 × 10⁴ vs. 6.9 × 10⁴ mol/(km²·yr), respectively] and only ∼1.5 times higher than the global mean value [∼9 × 10⁴ mol/(km²·yr)]. Data for major world rivers (including Himalayan rivers) provide a consistent interpretation regarding the relationship between mechanical erosion intensity and the ratio of silicate to carbonate weathering. Thus, we conclude that mountain building increases atmospheric CO₂ consumption rates by only a factor of ∼2, which is much lower than previous estimates.