Earth and Planetary Science Letters 394 (2014) 168–178 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Contrasting iron isotopic compositions in river suspended particulate matter: the Negro and the Amazon annual river cycles Giana Márcia dos Santos Pinheiro a,b,∗ , Franck Poitrasson a,b , Francis Sondag a,b , Gérard Cochonneau b,c , Lucieth Cruz Vieira a a Instituto de Geociências, Universidade de Brasília, 70910-900, Brasília, Brazil b Laboratoire Géosciences Environnement Toulouse, Institut de Recherche pour le Développement – Centre National de la Recherche Scientifique – Université de Toulouse III, 14-16, Avenue Edouard Belin, 31400 Toulouse, France c Agência Nacional das Águas – ANA, Brasília, DF, Brazil article info abstract Article history: Received 16 February 2013 Received in revised form 23 February 2014 Accepted 6 March 2014 Available online 2 April 2014 Editor: G.M. Henderson Keywords: iron isotopes Amazon and Negro equatorial rivers temporal series annual river cycle water suspended particulate matter water dissolved matter Iron isotopic compositions (δ 56 Fe IRMM-14 ) 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. δ 56 Fe IRMM-14 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.  2014 Elsevier B.V. All rights reserved. 1. Introduction On a global scale, the Amazon River Basin supplies the great- est amount of suspended particulate matter (SPM) to oceans (Gaillardet et al., 1997), figuring as a very important carrier of iron (in dissolved, colloidal and particulate phases – see definitions in Methods section). It is, therefore, an interesting area to study vari- ations on the iron isotopic signature in the different river fractions. A variety of physical and chemical characteristics recognized in the Amazon Basin rivers lead to the classification of the waters in three different types (Fittkau, 1971; Gibbs, 1972; Sioli, 1984; Lewis et al., 1995): clear and black water rivers (e.g., the Tapajós and the Negro, respectively) are mostly related to chemical erosion, whereas white water rivers (e.g., Amazon and Solimões) are asso- ciated with mechanical erosion and, consequently, contain much more important sediment loads (Allard et al., 2004). * Corresponding author at: Instituto de Geociências, Universidade de Brasília, 70910-900, Brasília, Brazil. E-mail address: giana@unb.br (G.M. dos Santos Pinheiro). Iron (Fe) concentration in river SPM is normally at the percent level in Amazonian rivers (Allard et al., 2004) and organic com- plexation of Fe 2+ is minor when compared to that of Fe 3+ (e.g., Weber et al., 2006; Allard et al., 2011). Iron is transported primar- ily in the particulate form in the Amazon mainstream, whereas in the Negro River half of the iron is carried in the dissolved and col- loidal load (Bergquist and Boyle, 2006). A reconnaissance study made in the Amazon region reported distinct δ 56 Fe IRMM-14 values for the Negro River dissolved and par- ticulate phases (Bergquist and Boyle, 2006). The study demon- strated that the dissolved load is isotopically heavy (δ 56 Fe IRMM-14 = +0.3❤), whereas the suspended load is light (−0.9❤). The light iron isotopic composition for the Negro River SPM was recently confirmed (at the Paricatuba Station), however, to a lesser ex- tent (δ 56 Fe IRMM-14 ∼−0.3❤, dos Santos Pinheiro et al., 2013). A similar relationship between river dissolved and particulate fractions was reported by Escoube et al. (2009) for the North River, Massachusetts, though in a smaller magnitude. Nonetheless, δ 56 Fe IRMM-14 variations exist among suspended phases from differ- ent types of tributaries of the Amazon River (Bergquist and Boyle, 2006; dos Santos Pinheiro et al., 2013). http://dx.doi.org/10.1016/j.epsl.2014.03.006 0012-821X/ 2014 Elsevier B.V. All rights reserved.