Geochemistry of the Amazon: 3. Weathering chemistry and limits to dissolved inputs

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Abstract
Chemical mass balance models of river solution chemistry, constrained by geologic data and thermodynamic models, provide a consistent description of the weathering processes that occur in the Amazon Basin. In areas with high weathering rates, such as the Andes, calcium, magnesium, sulfate, and alkalinity come from the weathering of evaporite minerals, sulfides, and carbonates. The inputs of calcium, magnesium, and alkalinity from exclusively carbonate terrains are limited by calcite, and perhaps by dolomite, saturation. When evaporites are present, only the alkalinity inputs are so limited because of additional weathering sources of calcium and magnesium. Dissolved silica, potassium, and sodium can be used to define mass balance relationships that in turn establish the nature of the clay mineral suites which are consistent with solute compositions. Use of thermodynamic constraints, specifically kaolinite and quartz stability, assists in data interpretation. At lowest weathering rates, all common primary minerals exposed to weathering are broken down to release silica and cations; quartz, kaolinite and (Fe,Al)‐sesquioxides all appear to be unstable. At higher weathering rates, enough silica is available to stabilize kaolinite, then quartz. The stabilization of Si‐bearing phases is reflected by a drop in the ratio of silica to other dissolved components. Finally, at the highest weathering rates, such as in the Andean catchments, high cation levels result in the formation of the 2:1 clays, which sequester even more silica, and any clear relationship between silica and other solutes degenerates.