Geochemistry of the Amazon: 2. The influence of geology and weathering environment on the dissolved load

Abstract

In the Amazon Basin, substrate lithology and erosional regime (seen in terms of transport‐limited and weathering‐limited denudation) exert the most fundamental control on the chemistry of surface waters within a catchment. Secondary effects, such as the precipitation of salts within soils and in stream beds, biological uptake and release, and cyclic salt inputs, are more difficult to discern. Samples can be separated into four principal groupings based on relationships between total cation charge (TZ+) and geology. (1) Rivers with 0<TZ+<200 μeq/l drain the most intensely weathered materials (Upper Tertiary sediments, soils of the Negro Basin, and similarly weathered regions). These rivers show high levels of Fe, Al, H+, and coloration and are enriched in Si relative to other major species; they exhibit cation ratios similar to those of substrate rocks. (2) Rivers with 200<TZ+<450;μeq/l drain siliceous terrains. These rivers are also rich in silica relative to other species. Rivers draining weathering‐limited siliceous terrains exhibit the highest TZ+ and their cation load is typically preferentially enriched in Na over K and Ca over Mg when compared to the rocks in their catchments. (3) Rivers with 450<TZ+<3000 μeq/l drain marine sediments or red beds with high cation concentrations (resulting from the presence of carbonates and minor evaporites in the Peruvian Andes and reduced shales and minor carbonates in the Bolivian Andes). These rivers exhibit relatively high levels of Ca, Mg, alkalinity, and SO 4(in rivers draining reduced shales and minor evaporites). (4) Rivers with TZ+>3000 μeq/l drain massive evaporites. These rivers are rich in Na and Cl. In the third and fourth categories, rivers tend to have 1:1 (equivalent) ratios of Na:Cl and (Ca+Mg):(alkalinity+SO 4), caused primarily by the weathering of carbonates and evaporites. Supplement available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document C83‐002; $2.50. Payment must accompany order.