Impact of atmospheric CO 2 levels on continental silicate weathering (original) (raw)

2010, Geochemistry, Geophysics, Geosystems

1] Anthropogenic sources are widely accepted as the dominant cause for the increase in atmospheric CO 2 concentrations since the beginning of the industrial revolution. Here we use the B-WITCH model to quantify the impact of increased CO 2 concentrations on CO 2 consumption by weathering of continental surfaces. B-WITCH couples a dynamic biogeochemistry model (LPJ) and a process-based numerical model of continental weathering (WITCH). It allows simultaneous calculations of the different components of continental weathering fluxes, terrestrial vegetation dynamics, and carbon and water fluxes. The CO 2 consumption rates are estimated at four different atmospheric CO 2 concentrations, from 280 up to 1120 ppmv, for 22 sites characterized by silicate lithologies (basalt, granite, or sandstones). The sensitivity to atmospheric CO 2 variations is explored, while temperature and rainfall are held constant. First, we show that under 355 ppmv of atmospheric CO 2 , B-WITCH is able to reproduce the global pattern of weathering rates as a function of annual runoff, mean annual temperature, or latitude for silicate lithologies. When atmospheric CO 2 increases, evapotranspiration generally decreases due to progressive stomatal closure, and the soil CO 2 pressure increases due to enhanced biospheric productivity. As a result, vertical drainage and soil acidity increase, promoting CO 2 consumption by mineral weathering. We calculate an increase of about 3% of the CO 2 consumption through silicate weathering (mol ha −1 yr −1 ) for 100 ppmv rise in CO 2 . Importantly, the sensitivity of the weathering system to the CO 2 rise is not uniform and heavily depends on the climatic, lithologic, pedologic, and biospheric settings.