The geochemistry of river particulates from the continental USA: major elements.
River particulates have been collected from twenty-three rivers from throughout the continental USA. The rivers drain mostly large basins (basin areas range from 2.1 x 10(3) to 2970 x 10(3) km2) composed of mixed lithologies, and spanning a wide range of climatic conditions as evidenced by a large variation in air and ground water temperatures, precipitation, and runoff. Suspended particulates have been analyzed for organic and inorganic carbon, as well as the major elements Al, Fe, Mn, K, Si, Ca, Mg, and Na. Also determined were suspended sediment loads at the time of sampling, particle surface areas, and grain size distributions. Five-year average river water chemistry and suspended sediment concentrations, obtained from USGS water supply reports, are also included as supporting information. Particle chemistry systematically varies with rates of runoff, with high runoff rivers transporting the most heavily altered particulates and low runoff rivers carrying the least altered. Degrees of alteration are indicated by the extent to which the refractory, nonmobile elements Al and Fe are concentrated into the particulates and the extent to which the most easily weathered elements Na, Ca, and Mg have been leached. Overall, the susceptibility of elements towards leaching is consistent with numerous previous descriptions of element mobility on weathering. A simple predictive model has been developed to explore and explain the observed trends between particle composition and river runoff. Thus, the elemental composition of riverine particulates may be given by the expression PCOi = [(Propi)TM-DCOi]/SS, where PCOi is the concentration of element i (as the oxide) in a riverine particulate, Propi is the proportion of i (as the oxide) in unweathered rock, DCOi is the dissolved concentration of i (as the equivalent oxide), TM is the total rock derived mass dissolved in river water, and SS is suspended sediment concentration. To utilize this equation, a model was first developed to predict the concentration of river solutes with the input of temperature, precipitation, and a limited number of weathering parameters. Combining this solute model with the above predictive equation for particle composition, it was discovered that the observed trends between particle composition and runoff require the special circumstance of decreasing SS with increasing runoff. Hence, the composition of river particulates depends both on the climate parameters of runoff and temperature (as they control dissolved river chemistry) and the nonclimate parameters including elevation, relief, tectonics, and basin area that control the SS load of rivers.