Insights into iron sources and pathways in the Amazon River provided by isotopic and spectroscopic studies
Résumé
The present study investigated the weathering and transport mechanisms of Fe in the Amazon River. A particular emphasis was placed on Fe partitioning, speciation, and isotopic fractionation in the contrasting waters of the Solimões and Negro rivers and their mixing zone at the beginning of the Amazon River. Samples collected in the end-member rivers and thirteen sites distributed throughout the mixing zone were processed through frontal vacuum filtration and tangential-flow ultrafiltration to separate the different suspended solid fractions, i.e., particulate (P\textgreater0.45μm and P\textgreater0.22μm), colloidal (0.22μm\textgreaterC\textgreater5kDa) and truly dissolved elements (TD\textless5kDa). The Fe isotopic composition and electron paramagnetic resonance (EPR) species were measured on these different pore-sized fractions. The acidic and organo-Fe-rich waters of the Negro River displayed dissolved and colloidal fractions enriched in heavy isotopes (~1.2\textperthousand, in δ57Fe values relative to IRMM-14), while the particulate fractions yielded light isotopic compositions of -0.344\textperthousand for P\textgreater0.22μm and -0.104\textperthousand for P\textgreater0.45μm fractions). The mineral particulate-rich waters of the Solimões River had dissolved and colloidal fractions with light isotopic composition (-0.532\textperthousand and -0.176\textperthousand, respectively), whereas the particulate fractions yielded δ57Fe values close to those of the continental crust (i.e., -0.029\textperthousand for P\textgreater0.22μm and 0.028\textperthousand for P\textgreater0.45μm). Ten kilometers downstream from the Negro and Solimões junction, the concentrations of colloidal and dissolved Fe species deviate markedly from conservative mixing. A maximum Fe loss of 43μg/L (i.e., 50% of the dissolved and colloidal Fe) is observed 110km downstream from the rivers junction. The contrasting Negro and Solimões Rivers isotopic compositions along the pore-sized water fractions is attributable to the biogeochemical processes involving different types of upland soils and parental materials. For instance, the isotopic composition of colloidal and dissolved Fe from the Negro River are consistent with Fe oxidation and complexation mechanisms at the interface between waterlogged podzols and river networks, as supported by strong organo-Fe complexes signals observed by EPR. Conversely, the particulate and colloidal fractions from the Solimões River have δ57Fe consistent with strong mechanical erosion in the Andean Cordillera and upland soils, as evidenced by high concentrations of Fe3+-oxides sensu lato measured by EPR. The massive dissolved and colloidal Fe removal is associated with the evolution of the physical and chemical composition of the waters (i.e., ionic strength) during mixing, which influences organo-Fe3+ and Fe3+-oxyhydroxides stability. Several models are discussed to explain Fe non-conservative behavior, including dissociation of organo-Fe complexes and the subsequent formation of solid Fe3+-oxyhydroxides and semiquinone free radicals, as evidenced by EPR spectra demonstrating that organo-Fe signals decrease as Fe3+-oxyhydroxides and free radicals signals increase. As in estuarine regions, the mechanisms involving Fe transfer and loss in the mixing zone has a negligible effect on the bulk water Fe isotopic composition. This result suggests that a tropical basin similar to the Amazon River Basin delivers to the ocean waters with an Fe isotopic composition similar to that of the Earth\textquoterights continental crust.