Studies near the mouth of the Tapajos river, a tributary of the Amazon River, pointed to the presence of thick muddy deposits from the mouth entering upstream, which could be related to a significant influence of the Amazon River as a regulator in the dynamics of its tributary. This study aimed to investigate the geochemical distribution and Pb-Sr-Nd isotopic signatures of bottom sediments as indicators of mixing and provenance, as well as to help in understanding the hydrodynamics of the lower course of the river Tapajos and its interaction with the Amazon river. Samples of bottom sediments were collected in lower course of the Tapajós River, comprising a zone of confluence with the Amazon River, near the city of Santarém. Data collection was performed with Petersen grab sampler, which allows a sampling of about 10 cm depth, corresponding to a few years of deposition and enabling disregard seasonal variations. Textural and mineralogical patterns as well as geochemical and Pb-Sr-Nd isotopic compositions were determined on whole sample. Particle size and mineralogical analyses were performed by laser diffraction and X-ray diffraction, respectively. The chemical composition was determined by dissolution with an combination of multi acid HF-HNO3-HCl and HClO4. Major and trace element contents were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) in a commercial laboratory. Pb, Sr, Sm and Nd Isotopic analyses followed the analytical procedure of the Laboratório de Geologia Isotópica (Pará-Iso) at UFPA, which includes total dissolution with HNO3 + HF mixture followed by HCl. The chemical separation and element purification was performed by ion exchange chromatography. The isotopic compositions of Pb, Sr, Sm and Nd were determined on an Inductively coupled plasma mass spectrometer Thermo-Finnigan Neptune™. The textural data of sediment showed predominant fine sand pattern somewhat homogeneous throughout the course of the Tapajós river, with a significant presence of siltclay samples. Kaolinite and mica occur in the sediments from the upstream sectors (Tapajós Channel and Alter do Chão) and smectite and feldspar were observed in the sediments of lower sectors (Amazon and Santarem sectors), besides the presence of these former minerals. Major and trace elements showed constant concentration along the Tapajós channel sector and variables and higher contents downstream at the the confluence with the Amazon River. A linear and decreasing pattern in Zr and Hf contents was observed from the enlargement of the canal (ria) of the Tapajos river toward the confluence area, which may be related to detrital zircon abundance in the sediments. Samples from the Tapajós Channel and Alter do Chão sectors displayed Pb isotopic ratios (19,67 <206Pb/204Pb < 20,02; 15,87 <207Pb/204Pb < 15,91) distinct from those of Amazon River samples (18,84 <206Pb/204Pb < 18,94; 207Pb/204Pb 15,67). Intermediate values were obtained for samples of Santarém sector (19,02<206Pb/204Pb < 19,52; 15,68 <207Pb/204Pb <15,83), indicating that the contribution of the Amazon River is limited the confluence zone. The bottom sediments of the Tapajós River channel showed ƐNd values more negative (-21 < ƐNd < -19) and more radiogenic Sr isotopic signature (87Sr/86Sr 0,792) compared to sediments of the Santarém - Amazon river sectors (ƐNd -9 and 0,712 <87Sr/86Sr < 0,716). Nd and Sr isotopic data also indicated that the influence of the Amazon River on the Tapajós River sediments is restricted to the confluence zone. Geochemical signatures showed a pattern of predominantly felsic sources compatible with the composition of the main geological units that constitute the Proterozoic basement of the Tapajós drainage basin. ƐNd and TDM values and 87Sr/86Sr ratios of the sediments from the Tapajós channel sector indicate a provenance by erosion of Paleoproterozoic units of the Tapajós province (1.88 to 2.03 Ga) with a contribution from slightly younger Proterozoic sources coming from the headwaters of the Tapajós River and tributaries (Rondônia- Juruena Province, 1.54 to 1.82 Ga). On the other hand, ƐNd values and87Sr/86Sr ratios of the sediments of the Amazon River in the confluence area indicate a strong Andean contribution, in good agreement with isotopic signature of suspended material of the Solimões River. The accumulation of large amounts of muddy sediments in the Tapajós channel results from the influence of the Amazon River that retains its tributary discharge generating favorable conditions for deposition of sediments strictly derived from the Tapajós river. The results of this work suggest that the contribution of the Tapajós river to the bottom sediments of the Amazon River is insignificant. The study showed the potential of Pb, Sr and Nd isotopic signatures of bottom sediments to establish the influence of the Amazon River on its tributary and to investigate mixing processes between different river systems.
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