Long-term isovolumetric leaching of aluminum from rocks during weathering: Implications for the genesis of saprolite

Isovolumetric geochemical analyses of 26 weathering profiles (saprolites) on crystalline rocks in Brazil and South Carolina indicate that, on average, Si and Al are leached from rocks in a long-terms molar ratio of 8.3. Hewever, with the exception of the Rio Negro River, the molar ratio of Si to Al in rivers draining these regions is about 100. If saprolites are still forming, the large discrepancy in Al mobilization between the saprolite record and modern river chemistry may be due to degassing of dissolved CO₂ from soil water entering streams with attendant increase in pH and Al precipitation. If saprolites are relic, the Rio Negro basin may be a model for the environmental conditions under which they form. These conditions are 1) low relief, 2) abundant rainfall and intense leaching and 3) organic rich soils which produce large concentrations of CO₂ and dissolved organic acids in the soil. Abundant rainfall maintains dilute soil water; large CO₂ pressure lowers pH; dissolved organic matter complexes Al (and Fe) and thus enhances the solubility of secondary clays. All of these effects hinder the precipitation of secondary weathering products and thus promote rock dissolution that is more nearly congruent. The problem with the Rio Negro model is to reconcile the low relief of the basin (which inhibits rapid groundwater movement) with the short contact time indicated by the diluteness of its chemistry.