A pluma do rio Amazonas: fonte: dinâmica e transporte de sedimentos para estuários e manguezais do litoral leste amazônico.

Abstract

The Southeastern Amazon Coastal Zone (SACZ) includes the largest mangroves belt in the world and more than 20 macrotidal estuaries. In this area, a large volume of fine sediments has been deposited during the Holocene, however, it is known that the local rivers are black water rivers, carrying small amounts of suspended sediment. Therefore, since decades the sources and transport mechanisms of fine sediments to estuaries and mangroves have been speculated and investigated. It was recently demonstrated the existence of an offshore source of muddy sediments to the mangroves. Due to its proximity, the Amazon River has been seen as the probable main source of sediments, where an average flow of 170x103 m3 .s-1 and suspended sediment concentrations in the order of 80 mg.L-1 would represent a nearly inexhaustible source of mud to both estuaries and mangroves. However, the provenance and transport processes by which the mud of the Amazon would reach the SACZ, providing mangrove progradation, are not yet understood, since the plume is mostly drifted to northwest. In this context, the sedimentary constitution of the inner continental shelf and its reworking must also be evaluated, as well as the possibility of mud supply to the east of the study region. The present study aimed to integrate and complement efforts on sediment dynamics in the estuaries (Mocajuba, Caeté and Gurupi) and mangroves of the SACZ, combining studies on the inner continental shelf. Therefore, tools and approaches from hydrodynamics, sedimentology and biogeochemistry were used to identify and understand the sources and transport mechanisms of muddy sediments for the sector, composing a distance gradient from the mouth of the Amazon River, as well as a gradient of the drainage basins size. The Mocajuba estuary presents quite peculiar aspects in terms of morphology and hydrodynamics. The morphology of the estuary is strongly influenced by structural evolution and faults caused by neotectonic events since the Neogene, resulting in deep and straight areas in the lower and middle portions of the estuary. The tide propagates into the estuary without significant deformations, as a result of the combination between inherited morphology and hydrodynamic aspects. Salinity showed higher values during dry seasons, while the concentration of suspended sediment was higher during rainy seasons. Estuarine circulation in Mocajuba is similar to fjord because of the deep, however, without salinity stratification. The estuarine turbidity maxima zone did not occur due to the high depth areas, low current velocities, and low suspended sediments concentration. However, a “high turbidity wedge” was observed near the mouth of the estuary, evidencing the Amazon River plume influence. The Caeté estuary is classified as a coastal plain, tide-dominated, with a semidiurnal regime and considerable variations during the spring and neap phases. From a scientific point of view, this is one of the most studied estuaries in the SACZ, including geology, geomorphology, hydrodynamics, sedimentology, geochemistry, biogeochemistry, ecology in general, among other studies areas. For this Thesis, the Caeté estuary was considered a “model estuary” due to its geographic location (regarding to the Amazon River mouth, Mocajuba and Gurupi estuaries distances), the hydrographic basin size and especially because of the data available in scientific articles that prove the influence of an offshore source of fine sediments. Gurupi is a typical coastal plain estuary, shallow, funnel-shaped, tide-dominated, but partially mixed. Salinity levels and suspended sediment concentrations were higher at the mouth and decreased upstream. The estuarine turbidity maxima zone was observed in both seasons, however, the increase in river discharge, during the rainy season, attenuated and displaced this zone seaward. The tide propagated asymmetrically, with hypersynchronous behavior near the mouth, being attenuated upstream. Regarding sedimentary organic matter, the data showed that estuarine samples present more negative δ13C values, as a result of stronger terrestrial and mangroves influences. The δ13C values of samples collected on the inner shelf showed less negative values, indicating a mixture between marine and mangrove carbon sources. Furthermore, the results showed that the size of the drainage basin of local rivers is also a relevant factor in the dynamics of organic matter. The Gurupi River, for example, is large enough to contribute organic and terrigenous sediments to the estuary and internal shelf. In summary, the SACZ estuaries are characterized by the macrotidal regime, subject to singular aspects of geology, geomorphology and the local river. In the Mocajuba estuary, inherited morphology had a strong influence on hydrodynamic and sedimentary processes. In the Gurupi estuary, the high river flow played a fundamental role in the estuarine dynamics.