The Geochemistry of the Amazon River System: Part I. The Factors that Control the Salinity and the Composition and Concentration of the Suspended Solids
On the basis of wet- and dry-season sampling along the Amazon River, at its mouth, and from its 16 large tributaries, the factors that control the salinity and the composition and concentration of the suspended solids of the Amazon River were determined. The multiple regression analysis of the analytical results of 74 river-water samples and numerous in situ measurements revealed: (1) of the 92 percent of the variability of the salinity accounted for, 85 percent was accounted for by relief—86 percent of the total dissolved salts discharged by the Amazon was supplied from the 12 percent of the total area of the Basin comprising the mountainous-environment type; (2) of the 62 percent of the variability of the concentration of the suspended solids accounted for, 43 percent was accounted for by relief—82 percent of the total suspended solids discharged by the Amazon was supplied from the mountainous environment-type. The multiple regression analysis of the analytical results obtained by X-ray diffraction from 50 suspended solids samples revealed: (1) increased-relief physical weathering controlled the percentages of quartz, plagioclase, K-feldspar, mica, chlorite, and amphibole; (2) the percentage of “calcic” rocks in the upper portions of tributary basins controlled montmorillonite concentrations; and (3) a combination of low-relief chemical weathering and the percentage of igneous and metamorphic rocks controlled the percentages kaolinite and gibbsite. The increased-relief physical weathering dominant in the Andean mountainous environment controls the geochemistry of the Amazon River, indicated by the following observations: (1) the over-all composition of the suspended solids discharged by the Amazon differs only slightly from that of the suspended solids eroded from the mountainous environment; and (2) 84 percent of the total amount of dissolved salts and suspended solids discharged is eroded from the 12 percent of the total area of the Amazon Basin comprising the mountainous Andean environment-type.