Sensitivity of carbonate weathering to soil CO2 production by biological activity along a temperate climate transect
Résumé
We investigated the controls on carbonate weathering in a well-drained pure carbonate area subject to strong environmental gradients, the Jura Mountains, Western Europe. The water chemistry of sampled springs and resurgences is dominated by Ca2 + (87 to 96 Eq% of the cationic charge) and HCO3- (90 to 97 Eq% of the anionic charge), reflecting the overwhelming imprint of calcium carbonate dissolution by atmospheric/biogenic CO2. Ca2 + concentration, which directly gives access to the amount of calcium carbonate dissolved per unit of water runoff, shows a gradual two-fold decrease (from 3000 to 1400 μmol/L) along the elevation gradient (from 300 to 1200 m). After discussing the possible influence of each environmental parameter on the observed water chemistry gradient, a decreasing soil pCO2 (the main source of acidity) with increasing altitude appears as the most likely explanation. As no spatial and temporal record of soil pCO2 are available for the Jura Mountains, we performed soil pCO2 modeling using the ecological and hydrological ASPECTS model that allows reconstructing carbon and water exchange fluxes between the vegetation, soil and atmosphere. Modeling results suggest that soil pCO2 decreases with altitude in response to both the change in vegetation species from deciduous-dominated forest in the lowlands to evergreen-dominated forest above 800 m (responsible for 65% of the variation) and the change in climate and soil properties (responsible for 35% of the variation). Carbonate weathering would thus be strongly sensitive to the type of vegetation, which drives both temporal and spatial variations of soil carbon and water budgets. Based on field observations, we show that carbonate weathering rates are 20-30% higher under deciduous vegetation cover than under conifers (at a given water runoff value), in agreement with modeling results. Chemical denudation rates of carbonate in the Jura Mountains vary from 152 to 375 t/km2/yr, corresponding to 60-150 mm/ka of carbonate being removed. Carbonate weathering within the 10,000 km2 of the study area accounts for an atmospheric CO2 consumption of 0.3 TgC/yr, showing that carbonate rocks have an enhanced capacity of atmospheric CO2 neutralization at least transiently. This study demonstrates that carbonate weathering is sensitive to the ecosystem dynamics, a conclusion that might be much more general, and suggests that carbonate weathering and associated CO2 consumption fluxes quickly react to any global change or land use modification.