Basalt weathering laws and the impact of basalt weathering on the global carbon cycle

This study attempts to characterise the chemical weathering of basalts and to quantify the flux of carbon transferred from the atmosphere to the ocean during this major process at the surface of the Earth. To this aim, we have compiled different published chemical compositions of small rivers draining basalts. Basaltic river waters are characterised by relatively high Na-normalized molar ratios (Ca/Na: 0.2–3.9; HCO3/Na: 1–10; Mg/Na: 0.15–6) in comparison with those usually observed for river draining silicates. The data also show the climatic influence on basalt weathering and associated CO2 consumption. Runoff and temperature are the main parameters controlling the chemical weathering rate and derived CO2 consumption during basaltic weathering. From these relationships and digital maps, we are able to define the contribution of basalts to the global silicate flux. Taking account of this result, we estimate that the CO2 flux consumed by chemical weathering of basalts is about 4.08×1012 mol/year. The fluxes from the islands of Indonesia and regions of central America represent around 40% of this flux. The global flux of CO2 consumed by chemical weathering of basalts represents between 30% and 35% of the flux derived from continental silicate determined by Gaillardet et al. [Chem. Geol. 159 (1999) 3]. Finally, it appears that volcanic activity not only acts as a major atmospheric CO2 source, but also creates strong CO2 sinks that cannot be neglected to better understand the geochemical and climatic evolution of the Earth.