Iron isotope composition of the bulk waters and sediments from the Amazon River Basin Franck Poitrasson a,b, ⁎, Lucieth Cruz Vieira b , Patrick Seyler a,b , Giana Márcia dos Santos Pinheiro a,b , Daniel Santos Mulholland a,b , Marie-Paule Bonnet a,b , Jean-Michel Martinez a,b , Barbara Alcantara Lima b , Geraldo Resende Boaventura b , Jérôme Chmeleff a , Elton Luiz Dantas b , Jean-Loup Guyot a,b , Luiz Mancini b , Marcio Martins Pimentel b , Roberto Ventura Santos b , Francis Sondag a,b , Philippe Vauchel a a Laboratoire Géosciences Environnement Toulouse, Institut de Recherche pour le Développement, Centre National de la Recherche Scientifique, Université de Toulouse, 14-16, avenue Edouard Belin, 31400 Toulouse, France b Instituto de Geociências, Universidade de Brasília, Campus Darcy Ribeiro, 70910-900, Brasília, Brazil abstract article info Article history: Received 25 July 2013 Received in revised form 28 March 2014 Accepted 29 March 2014 Available online 12 April 2014 Editor: Michael E. Böttcher Keywords: Amazon River Fe biogeochemical cycling Iron isotopes Freshwater and continental sediments The present study provides iron concentrations and isotopic compositions determined by multi-collector induc- tively coupled plasma mass spectrometry (MC-ICP-MS), along with key chemical, mineralogical and physical properties of 35 representative bulk (unfiltered) waters and bulk sediments from the Amazon River Basin. These samples from the Amazon River, five of its main tributaries (the Solimões, Negro, Madeira, Tapajós and Trombetas rivers) and four sub-tributaries (the Purus, Jaú, Ucayali and Napo rivers) were essentially collected during seven field missions conducted for over two years. These encompassed the centennial flood of May 2009 and the exceptional low water stage of September–October 2010, thereby providing the most extreme hy- drological situations that have been recorded over the last hundred years. While the data confirmed massive losses of iron (up to ~19000 tons/day, ca. 50% of the Amazon River bulk water budget) in the Solimões and Negro rivers mixing zone, the Fe isotope signatures of these bulk waters behaved conservatively. This property allows the use of bulk water Fe isotope signature to track iron sources and explain such isotopic signature in terms of simple mixing. Unfiltered samples from the organic-rich black water rivers present light δ 57 Fe relative to the average continental crust composition. This contrasts with the composition of the bulk white waters car- rying a high mineral suspended load that have δ 57 Fe values undistinguishable from the crustal isotopic signature (~0.1‰ relative to IRMM-14). This observation indicates that the Fe isotopic composition represents a reliable di- rect tracer of the iron speciation and, therefore, of the host phases of iron in its sources. Specifically, the white water δ 57 Fe most likely trace the signatures of igneous and sedimentary sources, as well as of their lateritic soil minerals, while the bulk black water δ 57 Fe track a preferential release of Fe that has gone through a reduction step in the organic-rich horizons of tropical podzols as a result of the biological activity. This study shows that the total iron transferred by the Amazon River represents between 5 and 30% of the world's ocean Fe input by rivers, and this Amazon bulk water iron displays an isotopic composition indistinguishable from that of the aver- age continental crust. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Iron is the fourth most abundant element in the Earth's crust and as such, it plays a key role in many biogeochemical processes at the Earth's surface, notably through its changes in redox states. Iron oxyhydroxide particles are an important carrier for other metals in aquatic systems, and Fe is key for plant and animal metabolism (Langmuir, 1996; Crichton, 2001). Its oxyhydroxide minerals also constitute hill-forming lateritic ferruginous crusts that affect continental water flows in inter- tropical zones. Despite extensive studies for over half a century, some important questions pertaining to the iron cycling on continental surfaces remain unanswered. For instance, the role of the vegetation in iron transfer from soils to rivers is still poorly quantified (Pokrovsky et al., 2006). The Amazon River Basin, which is the largest watershed in the world, delivers ~17% of riverine freshwater to the oceans (Callède et al., 2010) and therefore a large fraction of the metals coming from rivers. Despite the significance of the Amazon River Basin, the iron cycling re- mains little known in this watershed. This is partly because the sources Chemical Geology 377 (2014) 1–11 ⁎ Corresponding author at: Laboratoire Géosciences Environnement Toulouse, Centre National de la Recherche Scientifique, UPS, IRD, 14-16, avenue Edouard Belin, 31400 Toulouse France. Tel.: +33 5 61 33 26 19; fax: +33 5 61 33 25 60. E-mail address: Franck.Poitrasson@get.obs-mip.fr (F. Poitrasson). http://dx.doi.org/10.1016/j.chemgeo.2014.03.019 0009-2541/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo