Solid phase associations, oceanic fluxes and the anthropogenic perturbation of transition metals in world river particulates

The solid phase associations of particulate Mn, Cu, Ni, Co, Cr and Zn in eight world rivers (representing 19% of the global sediment flux) have been determined in order to estimate the fluxes and sources of particulate transition metals in different phases on a global scale. A sequential extraction procedure measured the metals progressively dissolved by dithionite, concentrated HCl, and HF–HClO4–HNO3 reflecting decreasing availability in surface environments. The combined evaluation of phase associations, enrichment factors and theoretical and observed flux ratios highlight the differing sources for these elements in river particulates. Zn appears to be significantly affected by pollutant inputs on a global scale, with total fluxes that may have doubled relative to natural background concentrations. By contrast, riverine particulate Mn and Cu appear to be relatively unaffected by anthropogenic perturbations. The Cu contents of river particulates are high relative to the average Cu contents of surficial sediments, and in general, this probably arises due to geochemical fractionation in the soil column or differential weathering processes. However localised pollution may produce exceedingly high Cu levels associated with organic or sulphide phases. Cr and Ni show no clear evidence of significant pollutant contributions, but Co has somewhat enhanced enrichment factors and flux ratios which may be indicative of anthropogenic additions.