Marcio Sousa da Silva

The dynamics of a frictionally-dominated Amazonian estuary

A hidrodinâmica, morfologia e sedimentologia do estuario do Taperacu foram investigadas. Este e um entre varios estuarios do litoral amazonico que integram a maior extensao continua de manguezais do mundo, apresentando uma descarga de agua doce muito reduzida, atipica para a regiao. Os resultados revelam grandes bancos arenosos que ocupam em grande parte a porcao central do estuario. Areias muito finas e bem selecionadas de origem marinha prevalecem. Fases de enchente mais curtas, com velocidades de corrente substancialmente mais altas, sao observadas na porcao superior do estuario, como esperado para um estuario raso e dominado por friccao. Por outro lado, uma vazante mais intensa poderia ocorrer como resultado de grandes areas de manguezais associadas e intenso preenchimento estuarino, sendo que ambas as condicoes sao observadas no Taperacu. Neste caso, a prevalencia da enchente parece estar associada a ausencia de uma descarga fluvial efetiva. Alem disso, alguns canais de mare conectam o Taperacu com seu vizinho estuario do Caete, o que contribuiria para o dominio de enchente. Como um todo, os resultados demonstram uma complexa interacao de aspectos de configuracao (friccao, drenagem fluvial, conexoes com estuarios vizinhos, preenchimento e grandes areas intermareais) na determinacao dos padroes hidrodinâmicos, contribuindo para o entendimento dos estuarios da regiao amazonica.

Modern pollen rain as a background for palaeoenvironmental studies in the Serra dos Carajás, southeastern Amazonia

Several alternative interpretations of the pollen records of lake sediments in the Amazonia may arise due to a lack of understanding of the modern environments, mainly related to pollen–vegetation relationship. Therefore, in this work, we studied the modern pollen rain in sediments of the Amendoim Lake, plateaus of the Serra Sul dos Carajás, southeastern Amazonia. This study indicates the predominance of pollen grains from forest formation (ombrophilous forests and capão florestal) over montane savanna in the lake sediments, despite the fact that montane savanna occupies ~90% of the drainage basin. Additionally, these sediments reflect stronger signal of pollen from ombrophilous forests that occur in the slopes of the plateau. Important components of montane savanna such as Vellozia and Cereus were not observed in the modern pollen rain, which may be related to their shorter flowering periods restricted to the beginning of wetter conditions. The absence of Sacoglottis, Trichilia, Clusia, Emmotum, Guapira and Simarouba in the pollen rain is likely associated with different pollination strategy. The occurrence of windblown pollen of Alchornea, Pseudolmedia, Ilex and Cecropia, which are found from low to highlands of the Carajás region, reinforces a regional vegetation signal in sediments. Several plants from the studied site have been improperly described, according to taxonomy and ecology. For example, the current occurrence of Ilex and Styrax in the study site clearly suggests that they cannot be used as indicators of colder palaeoclimate conditions, as previously described. In addition, Poaceae can be found in drier to wetter substrate conditions, and abundance of their pollen grains in lakes can significantly vary according to relief morphology and cannot indicate palaeovegetation openness. Therefore, this work is a good background for further palaeobotany studies for this region.

Holocene history of a lake filling and vegetation dynamics of the Serra Sul dos Carajás, southeast Amazonia

Down-core changes in sedimentary facies, elemental geochemistry, pollen, spore, δ13C, δ15N and radiocarbon records from a filled lake, named R4, of the Serra Sul dos Carajás were used to study the relationship between the paleomorphological and paleoecological processes and their significance for Holocene paleoclimatology of the southeast Amazonia. The sediment deposition of the R4 lake started around 9500 cal yr BP. Increase of detrital components from 9500 to 7000 cal yr BP suggests high weathering of surrounding catchment rocks and soils, and deposition into the lake basin under mudflows. At that time, montane savanna and forest formation were already established suggesting predominance of wet climate. However, from 7000 to 3000 cal yr BP, a decline of detrital input occurred. Also, forest formation and pteridophytes were declined, while palms and macrophytes were remained relatively stable, indicating that water levels of the lake is likely dropped allowing the development of plants adapted to subaerial condition under drier climate conditions. After 3000 cal yr BP, eutrophication and low accommodation space lead to high lake productivity and the final stage of the lake filling respectively, and forest formation may has acquired its current structure, which suggests return of wetter climate conditions.

Morphology and morphometry of upland lakes over lateritic crust, Serra dos Carajás, southeastern Amazon region

High-resolution satellite images, digital elevation models, bathymetric and sedimentological surveys coupled with statistical analysis were used to understand the physical environment and discuss their influence on water quality of the five upland lakes of Serra Sul dos Carajás, southeast Amazonia. The lakes have mid-altitude ranges (elevation), very small (catchment) and shallow to very shallow (central basins). Based on the length, area and volume, Violão and TI (Três Irmãs)-3 lakes may present large vertical movements of the water due to wind action and weakly stratified waters. Trophic conditions based on depth and shore development (Ld) parameters must be used with caution, since Amendoim Lake is relatively deep, but it is oligotrophic to ultra-oligotrophic. Ld values suggest that the lakes are circular to subcircular and are likely formed by solution process, as also suggested by volume development. TI-2 Lake is only presenting convex central basin and has highest dynamic ratio (DR), thus it may have high sedimentation and erosion rates. Based on the relationship between studied parameters, morphometric index and DR likely influence temperature and dissolved oxygen of waters of TI-2 Lake due to its depth profile and wind-induced surface mixing. Nevertheless, water quality parameters are controlled by catchment characteristics of the lakes.