Contrasting iron isotopic compositions in river suspended particulate matter: the Negro and the Amazon annual river cycles

Abstract

Iron isotopic compositions (δFeIRMM-1456) of suspended particulate matter (SPM) from two major rivers in the Amazon Basin, the Amazon River itself (at the Óbidos Station) and the Negro River (at the Serrinha Station), were investigated in the present study. The main objective was to search for temporal variations during their annual river cycles. δFeIRMM-1456 values for the Amazon River at Óbidos range between 0.00 and +0.15‰, indistinguishable from the average continental crust value. In contrast, the iron isotopic compositions of the Negro River (Serrinha Station) SPM vary between −0.34 and −0.82‰, whereas the dissolved matter is isotopically heavier in this river. The lack of significant isotopic variations in the Amazon River indicates that one individual SPM subsurface sample is representative of the river during the whole annual river cycle, in opposition to results obtained for the Negro River. The data suggest that in organic-poor white water rivers, such as the Amazon, iron isotopic signatures of the suspended fraction reflect a detrital crustal component with little isotopic fractionation. On the other hand, in the organic-rich Negro River, which has tropical podzols as the main iron source, the iron redox cycling at the water–soil interface influences the iron isotopic composition.