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

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 03000 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.