Major and trace elements of stream sediments from the lowermost Amazon River

Geochemical results from river-bottom sediments taken from the lowermost Amazon River are presented. Bedload sediments were analyzed for 10 major and 38 minor elements. Mud bulk sediments exhibit a tendency to a slight enrichment in Cr, Mn, Rb, Sr, Zr, Cs, Ba and Hf, pointing to the importance of these elements in the silt fraction. Generally, Cu, Zn, Rb, Cs, Ba and Sn are clearly associated with clay minerals, while Y, Zr, Hf, Th and U are associated with heavy minerals. The location of this area in an intensive chemical weathering environment results in the strong modification of sediment composition. Chemical weathering processes strongly modify original sediment composition and can be recognized apart from provenance processes on the basis of their geochemical signature. Discriminant analysis makes the detection of less marked differences in sediment characteristics possible, thus, improving the accuracy of distinctions between groups. The Amazon Mouth and Xingu Mouth (XM) are discriminated principally by the textural maturity of their sediments. The Jari Mouth (JM) shows highest Co and Fe2O3 contents. Despite similarities between the Amazon North Branch (ANB) and Amazon South Branch (ASB), the ANB can be differentiated on the basis of its higher Sr, P2O5, and Na2O contents, the ASB on the basis of its higher Zn, MnO, and CaO contents, and the Estreitos de Breves (EST) based on its higher abundance of Ni and Cu. Higher contents in P2O5, Fe, and Mn are related to laterite covers.