Pinheiro G. M. D., Poitrasson F., Sondag Francis, Cochonneau Gérard, Vieira L. C. (2014). Contrasting iron isotopic compositions in river suspended particulate matter : the Negro and the Amazon annual river cycles. Earth and Planetary Science Letters, 394, p. 168-178. ISSN 0012-821X.
Titre du document
Contrasting iron isotopic compositions in river suspended particulate matter : the Negro and the Amazon annual river cycles
Pinheiro G. M. D., Poitrasson F., Sondag Francis, Cochonneau Gérard, Vieira L. C.
Source
Earth and Planetary Science Letters, 2014,
394, p. 168-178 ISSN 0012-821X
Iron isotopic compositions (delta Fe-56(IRMM-14)) of suspended particulate matter (SPM) from two major rivers in the Amazon Basin, the Amazon River itself (at the Obidos 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. delta Fe-56(IRMM-14) values for the Amazon River at Obidos range between 0.00 and +0.15 parts per thousand, 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 parts per thousand, 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.
