Phosphorus in sediments of the Amazon River and estuary: Implications for the global flux of phosphorus to the sea

Abstract

The distribution of various forms of P in suspended and depositional bank sediments of the Amazon River and bottom sediments of the mixing and freshwater portions of the Amazon estuary and shelf have been determined using a modified sequential extraction method. Forms of P distinguished are: P associated with ferric oxides/hydroxides (Fe-P), organic P (org-P), P extracted with acetate buffer at pH 4 (acet-P), and detrital apatite (detr-P). Results indicate that there is a distinct drop in Fe-P, org-P, and total P after deposition of suspended sediment on the banks of the river. Similar results are found for sediment deposited on the freshwater portion (river channel bottom) of the continental shelf. Sediment deposited on estuarine portions of the shelf also shows a significant drop in total P, due mostly to a drop in org-P. No appreciable change in acet-P was found between river suspended sediments and those of either the river banks or of the estuarine shelf. An inverse correlation is found for all sediments between concentrations of Fe-P and detr-P because of the association of detrital apatite with the coarser fractions of the sediments and Fe-P with the finer fractions. These results suggest that considerable amounts of P are added to the oceans by the bacterial decomposition of river transported organic matter and by the desorption of P from iron oxides/hydroxides (some desorption from Al silicates may also occur.) The organic P is released to interstitial solution during early diagenesis and this P may be adsorbed by Fe oxides in the sediment. During frequent resuspension events, the Fe oxides are carried to the overlying water where the P is desorbed and thereby released to solution. Based on our Amazon results, the total global P flux to the oceans by rivers may be more than three times that carried only in solution by the rivers. Further work is needed on other river systems to check these preliminary results.