Pb-Sr-Nd isotopic tracing of the influence of the Amazon River on the bottom sediments in the lower Tapajos River

The isotopic signatures of Pb-Sr-Nd in recent bottom sediments were used to investigate the hydrodynamics of the lower stream of the Tapajos River and its interaction with the Amazon River. Samples from the Tapajos River have Pb isotopic ratios (19.67 < Pb-206/Pb-204 < 20.02; 15.87 < (207)pb/(204)pb < 15.91) different from those of the bottom sediments found downstream in the Amazon River (18.84 < (206)pb/(204)pb < 18.94; Pb-207/Pb-204 approximate to 15.67). In the confluence zone, the ratios have intermediate values (18.69 < (206)pb/(204)pb < 19.53; 15.65 < (207)pb/Pb-204 < 15.83). The sediments in the Tapajos River have lower epsilon(Nd(0)) (-21 < epsilon(Nd(0)) < 19) values and. more radiogenic isotopic Sr ratios (Sr-87/Sr-86 approximate to 0.792) than those of the sediments from the Amazon River (epsilon(Nd(0)) approximate to -9 and 0.712 < Sr-87/Sr-86 < 0.716). The isotopic data suggest that the Amazon River influences the sediments in the Tapajos River, but this influence is restricted to the confluence zone. Additionally, the concentrations of major and trace elements and the mineralogy of the sediments are in agreement with the isotopic data. We conclude that the accumulation of muddy sediments in the lower stream of the Tapajos River is a result of the influence of the Amazon River, which retains this discharge from its affluent thus generating favorable conditions for depositing the finer sediments coming from the Tapajos River without any significant contribution of sediments from the Amazon River itself. The values of epsilon(Nd(0)) and Tom and of Sr-87/Sr-86 ratio of the Tapajos River bottom sediments indicate that the source of the sediments is essentially the erosion of the Paleoproterozoic felsic units from the Tapajos (2.03-1.88 Ga) and Juruena (1.82-1.54 Ga) geotectonic provinces. (C) 2016 Elsevier Ltd. All rights reserved.