The Amazon River: behaviour of metals (Fe, Al, Mn) and dissolved organic matter in the initial mixing at the Rio Negro/Solimُes confluence

We studied the changes in major elements and organic carbon concentrations during the initial stage of the mixing of the black (Rio Negro) and the White (Rio Solimões) waters in the Amazon River basin to understand the geochemical processes that could control the redistribution between particulate and dissolved fractions. Water samples were collected at six stations including the Rio Negro and the Rio Solimões and four stations downstream from the confluence. The relative contributions of the two tributaries were determined using a triple tracer approach (δ18O, δD, Cl−). Particulate (>0.2 μm) and dissolved (<0.2 μm) concentrations of major elements (Ca, Mg, Fe, Al, Si) and organic carbon (POC and DOC) were measured. Major elements in the particulate fraction were found to have a nonconservative behaviour in the initial stage of the mixing due to mineral removal. In the dissolved fraction, only the DOC, Fe, and Mn behaved nonconservatively. The Fe losses could be due to preferential removal of Fe bound to N-rich organic matter (OM) and/or to preferential removal of Fe oxyhydroxides. The increasing dissolved manganese content in the dissolved phase is explained by a reductive dissolution of manganese oxides due to massive inputs of phenolic-rich OM from the Rio Negro. The amount of DOC removed from the water column in the initial stage of the mixing would represent 4% of the total annual DOC flux of the Amazon River at the reference gauging station of Obidos.