Chemical weathering and CO2 consumption in the Lower Mekong River

Data on river water quality from 42 monitoring stations in the Lower Mekong Basin obtained during the period 1972–1996 was used to relate solute fluxes with controlling factors such as chemical weathering processes. The total dissolved solid (TDS) concentration of the Lower Mekong varied from 53 mg/L to 198 mg/L, and the median (114 mg/L) was compared to the world spatial median value (127 mg/L). Total cationic exchange capacity (Tz⁺) ranged from 729 to 2607 μmol_c/L, and the mean (1572 μmol_c/L) was 1.4 times higher than the world discharge-weighted average. Calcium and bicarbonate dominated the annual ionic composition, accounting for ~ 70% of the solute load that equalled 41.2 × 10⁹ kg/y. TDS and major elements varied seasonally and in a predictable way with river runoff. The chemical weathering rate of 37.7 t/(km² y), with respective carbonate and silicate weathering rates of 27.5 t/(km² y) (13.8 mm/ky) and 10.2 t/(km² y) (3.8 mm/ky), was 1.5 times higher than the global average. The CO₂consumption rate was estimated at 191 × 10³ mol CO₂/(km² y) for silicate weathering, and 286 × 10³ mol CO₂/(km² y) by carbonate weathering. In total, the Mekong basin consumed 228 × 10⁹ mol CO₂/y and 152 × 10⁹ mol CO₂/y by the combined weathering of carbonate and silicate, constituting 1.85% of the global CO₂ consumption by carbonate weathering and 1.75% by silicates. This is marginally higher than its contribution to global water discharge ~ 1.3% and much higher than (more than three-fold) its contribution to world land surface area. Remarkable CO₂ consumed by chemical weathering (380 × 10⁹ mol/y) was similar in magnitude to dissolved inorganic carbon as HCO₃⁻(370 × 10⁹ mol/y) exported by the Mekong to the South China Sea. In this landscape, atmospheric CO₂ consumption by rock chemical weathering represents an important carbon sink with runoff and physical erosion controlling chemical erosion.