Geoquímica elemental e isotópica Pb-Sr-Nd dos sedimentos de fundo do sistema estuarino de Belém e do litoral paraense

Abstract

In recent years, urban growth has occurred in a disorga¬nized manner in the metropolitan area of Belém, State of Pará, and this is reflected directly in the quality of water and sediments of the Guajará Bay, the main hydrological element of the estuarine system of Belém. This study aimed to conduct an elemental and isotopic geochemical research on bottom sediments from the western margin of Guajará Bay and Carnapijó river to understand the geographical and historical variations of heavy metals (Cu, Cr, Ni, Pb and Zn) contents and isotopic signature of Pb, and to evaluate the gradual environmental degradation in the hydrographic system of Belém. In addition, the provenance of the bottom sediments in three areas of the Amazon Coastal Zone (mouth of the Amazon River, Marajoara Gulf and northeastern coast of Pará) was investigated by elemental and Sr-Nd-Pb isotopic geochemistry. Sediments from the western margin of the Guajará Bay and from the Carnapijó River display mineralogical homogeneity and their sand to silty sand textural composition reflects very high hydrodynamic conditions. 210Pb dating furnished a rate around 0.7 cm.year-1 for the sedimentation at the western margin of Guajará bay. The trace metal contents in the fine fraction indicate that there are no significant anthropogenic contribution for Cu, Cr, Ni and Zn in the sediments from these sectors of the hydrographic system of Belém. However, the Pb contents suggest a weak anthropic action for this metal. The concentrations of Cu, Cr, Ni, Pb and Zn in the exchangeable fraction below the Threshold Effect Level (TEL) indicate that the metals do not offer risk to the local biota. Comparison between the studied sediments and those of the riverside of Belém point to a higher contribution in the latter from domestic sewage and industrial waste for Pb and Ni, followed by Cr and virtually nonexistent for Cu and Zn. The study indicates that the sediments of the Carnapijó River are suitable for use as background of the estuarine system of Belém in future geochemical studies. Isotopic signatures of sediments from the western margin of Guajará Bay confirm an anthropogenic contribu¬tion of Pb throughout the entire bay. The Pb accumulation process has become more efficient over the last 10 years, and this can be attributed to the rapid population growth of Belém city. Sediments in Carnapijó River are not affected by human activities, and the average concentration values (Pb = 19.6 ± 3.7 mg kg-1) and isotopic signatures (206Pb/207Pb = 1.196 ± 0.004) confirm the background Pb values previously proposed for the river system in the Belém region. The isotopic signatures of suspended matter on the eastern (206Pb/207Pb = 1.188) and western (206Pb/207Pb = 1.174) margins of Guajará Bay show that suspended matter is an efficient Pb trans¬portation mechanism of domestic and industrial wastewater from Belém to the western margin of the bay due to tidal effects at the confluence with Guamá River. The Pb isotopic compositions along the sediment cores show a decrease of the 206Pb/207Pb ratio to a value of 1,180 in the shallower 20cm of the cores from the western margin of Guajará Bay, not observed in the cores of Carnapijó River. This decrease indicates a probable anthropogenic contribution over the past 15 years in the western margin of the bay. The provenance of the bottom sediments in three areas of the Amazon Coastal Zone (mouth of the Amazon River, Golfão Marajoara and northeastern coast of Pará) was investigated by elemental and Sr-Nd-Pb isotopic geochemistry. The geochemical signatures of the bottom sediments from the three sectors indicate a provenance from felsic geological units of the upper continental crust. Sediments of the Southern Channel of the Amazon River, north of the Marajó island (mouth of the Amazon River) had lower contents of Na2O and K2O and a higher CIA, indicating a higher degree of weathering. The distinct isotopic signatures of Sr, Pb and, principally, Nd of bottom sediments of the three studied sectors indicate different sources in nature and age. The sediments of the mouth of the Amazon River derived predominantly from the Andean and sub-Andean regions, as already shown in previous works. The more radiogenic Sr signatures and more negative ƐNd values of the sediments from Guajará Bay and Carnapijó and Guamá rivers, in Belém area, Marajoara Gulf (0,7267< 87Sr/86Sr <0,7316; -17,97< ƐNd < -13,58) and from the estuaries of Caeté and Maracanã rivers, northeastern coast of Pará (0,7220< 87Sr/86Sr < 0,7264; -24,05< ƐNd<-17,58) indicate a greater contribution of Precambrian rocks in their sources. Nd-TDM model ages (1.62-1.99 Ga) of the sediments from the Golfão Marajoara suggest a predominant contribution from the metasedimentary units of the Araguaia Belt and a subordinated contribution from the magmatic and metamorphic basement of Tocantins Province. Nd-TDM model ages (1.70-2.83 Ga) of sediment in the estuaries of Maracanã and Caeté rivers portray a strong contribution of Precambrian basement rocks (Craton Fragments of San Luis and Gurupi Belt) that outcrop in coastal region of northeastern Pará.