Cation-exchange characteristics of Amazon River suspended sediment and its reaction with seawater

Abstract

The cation-exchange characteristics of Amazon River suspended sediment have been studied in order to determine the contribution of exchangeable cations to the geochemical fluxes from the river. Sediment samples were obtained throughout most of the Amazon Basin. The range of exchangeable cation compositions is very narrow in the river and in seawater as well. In river water, the exchangeable cation complement (equivalent basis, exclusive of H +) is 80% Ca 2+, 17% Mg 2+, 3% Na + plus K +. In seawater Na + and Mg 2+ are about equal (38%) while Ca 2+ ~ 15% and K + ~ 9%. On reaction with seawater, river suspended sediment took up an amount of Na + equal to nearly one-third of the dissolved river load, as well as amounts corresponding to 15–20% of the dissolved fluvial K + and Mg 2+. These estimates reflect an unusually high suspended-sediment:dissolved-solids ratio of 6.4 at the time of sampling. At a more representative world average ratio of four, the uptake of Na + would be 20% of the dissolved fluvial load, and that for K + and Mg 2+ about 10%. Over the annual cycle of the Amazon, it is estimated that ion exchange has a still smaller effect, as a consequence of the low average suspended-solids:dissolved-solids ratio of 1.7. Variations in the ratio X Ca X Mg , the equivalent fraction of exchangeable Ca 2+ and Mg 2+, throughout the river, can be described by a single isotherm. This same isotherm accurately describes the distribution of exchangeable Ca 2+ and Mg 2+ on sediment equilibrated with seawater, despite that a high proportion of exchange sites is occupied by Na + and K +.