Iron isotopes in the Amazon River system: Weathering and transport signatures

Trace metal clean plankton tows, river samples, aerosol leachates, and porewater samples from the Amazon River system were measured for their iron isotopic composition. The overall isotopic range observed was 1.5 parts per thousand with Fe isotopic variability observed between different types of tributaries. Dissolved Fe and suspended load Fe from two main channel sites were isotopically similar (approximate to -0.1 to -0.3 parts per thousand). In contrast, the dissolved Fe and suspended load Fe from an organic rich tributary, the Negro River, were isotopically distinct with the dissolved slightly isotopically heavy (+0.3 parts per thousand) and the suspended load isotopically light (-0.9 parts per thousand). The isotopically lightest sample from the Amazon region was shelf porewater (-1.3 parts per thousand). In river water-seawater mixing experiments, the Fe isotopic signal of dissolved Fe of river water was modified by flocculation of isotopically heavy Fe. The variability of the Fe isotopic composition of natural samples including biological and aqueous samples demonstrates that significant fractionation is associated with Fe biogeochemistry in the environment. (c) 2006 Elsevier B.V. All rights reserved.