Chemical weathering and CO2 consumption in the Lower Mekong River

- TDS and major elements are inversely related to river runoff modeled using power law.
- About 70% of the annual solute load is Ca2 + and HCO3−.
- CO2 consumed by chemical weathering equates to HCO3− export.
- High runoff and relief contribute to intense chemical weathering.
- CO2-consumed rate by the Mekong (like other Himalayan rivers) exceeds global average.

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 μmolc/L, and the mean (1572 μmolc/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 CO2 consumption 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.