Impactos ENSO en cuencas atlánticas de Sudamérica

Interannual changes in precipitation cause significant effects on the Pacific watersheds of South America. Large rivers flowing to the Atlantic Ocean as the Paraná and Uruguay are also influenced by ENSO-triggered floods causing significant impacts on the economies of Argentina, Brazil and Uruguay. At the same time, the floodplains of small rivers are also sensitive to strong ENSOs. Floods occur approximately simultaneously in subtropical watersheds of Brazil and high-latitude watersheds of North Patagonia. ENSO-triggered floods were recorded during the years 1941-42, 1982-83 and 1997- 98. Several floods impacted along the Itajaí River affecting the higher (Blumenau city) and lower estuary (Itajaí city). The Guaiba River, at the headlands of Lagoa dos Patos, has repeatedly flooded Porto Alegre. At the Río de la Plata basin, significant increases in the discharges were assigned to the Paraná and the Uruguay rivers. The small basin of the Quequén Grande River without extended floodplains is subject to periodic failure of its margins. The Colorado River, the northern limit of Patagonia, is allochthonous and its watershed was artificial and progressively modified; the ENSO event of 1983 caused an increase in its salinity due to the sudden reactivation of the DesaguaderoCuracó system.Los cambios interanuales en las precipitaciones causan significativos efectos en las cuencas pacíficas de Sudamérica. Los grandes ríos que fluyen hacia el Océano Atlántico como el Paraná y el Uruguay también son influenciados por estas inundaciones originadas por estos eventos ENSO causando significativos impactos en las economías de Argentina, Brasil y Uruguay. Asimismo, las planicies aluviales de los pequeños ríos también son sensibles a los ENSOs fuertes. Las crecidas ocurren casi simultáneamente en las cuencas subtropicales de Brasil como en las cuencas de altas latitudes de Patagonia. Inundaciones gavilladas por eventos ENSO fueron registradas en los años 1941-42, 1982-83 y 1997-98. Varias inundaciones afectaron al río Itajaí afectando las porciones más altas (Blumenau) y bajas del estuario (Itajaí). El río Guaíba, en las cabeceras de la Lagoa dos Patos, ha repetidamente inundado Porto Alegre. En la cuenca del Río de la Plata, incrementos significativos de su descarga fueron originados por los ríos Paraná y Uruguay. La pequeña cuenca del río Quequén Grande sin grandes planicies aluviales está sujeta a periódicos derrumbes de sus albardones. El río Colorado, límite norte de la Patagonia, es alóctono y su cuenca fue progresiva y artificialmente modificada; el ENSO de 1983 causó un aumento de su salinidad debido a la súbita reactivación del sistema DesaguaderoCuracó.Fil: Isla, Federico Ignacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencia Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; ArgentinaFil: Toldo Junior, Elirio Ernestino. Universidade Federal do Rio Grande do Sul; Brasi

ENSO impacts on Atlantic watersheds of South America

Interannual changes in precipitation cause significant effects on the Pacific watersheds of South America. Large rivers flowing to the Atlantic Ocean as the Paraná and Uruguay are also influenced by ENSO-triggered floods causing significant impacts on the economies of Argentina, Brazil and Uruguay. At the same time, the floodplains of small rivers are also sensitive to strong ENSOs. Floods occur approximately simultaneously in subtropical watersheds of Brazil and high-latitude watersheds of North Patagonia. ENSO-triggered floods were recorded during the years 1941-42, 1982-83 and 1997-98. Several floods impacted along the Itajaí River affecting the higher (Blumenau city) and lower estuary (Itajaí city). The Guaiba River, at the headlands of Lagoa dos Patos, has repeatedly flooded Porto Alegre. At the Río de la Plata basin, significant increases in the discharges were assigned to the Paraná and the Uruguay rivers.  The small basin of the Quequén Grande River without extended floodplains is subject to periodic failure of its margins. The Colorado River, the northern limit of Patagonia, is allochthonous and its watershed was artificial and progressively modified; the ENSO event of 1983 caused an increase in its salinity due to the sudden reactivation of the Desaguadero-Curacó system

Influência de controles autogênicos na morfologia de leques submarinos : uma abordagem com base na modelagem física de correntes de turbidez 3D

Autogenic controls have significant influence on deep-water fans and depositional lobes morphology. In this work, we aim to investigate autogenic controls on the topography and geometry of deep-water fans. The influence of the sediment concentration of turbidity currents on deep-water fans morphology was also investigated. From the repeatability of 3D physical modeling of turbidity currents, two series of ten experiments were made, one of high-density turbidity currents (HDTC) and another of low-density turbidity currents (LDTC). All other input parameters (discharge, sediment volumetric concentration and grain size median) were kept constant. Each deposit was analyzed from qualitative and quantitative approaches and statistical analysis. In each experimental series, the variability of the morphological parameters (length, width, L/W ratio, centroid, area, topography) of the simulated deep-water fans was observed. Depositional evolution of the HDTC fans was more complex, showing four evolutionary steps and characterized by the self-channelizing of the turbidity current, while LDTC fans neither present self-channelizing, nor evolutionary steps. High disparities on the geometrical parameters of the fans, as characterized by the elevated relative standard deviation, suggest that autogenic controls induced a stochastic morphological behaviour on the simulated fans of the two experimental series.Controles autogênicos influenciam significativamente a morfologia dos leques de águas profundas e seus lobos deposicionais. Neste trabalho, objetivamos investigar a ação dos controles autogênicos sobre a topografia e a geometria de leques de águas profundas, bem como a influência da concentração de sedimentos gerados por correntes de turbidez sobre a morfologia destes. Por meio da repetibilidade de experimentos de modelagem física 3D de correntes de turbidez, foram realizadas duas séries de experimentos: correntes de turbidez de alta densidade (HDTC) e correntes de turbidez de baixa densidade (LDTC). Todos os demais parâmetros de entrada (vazão, concentração volumétrica de sedimentos e tamanho de grão) foram mantidos constantes. Cada depósito foi analisado mediante abordagens qualitativas e quantitativas e análise estatística. Em cada série experimental, foi observada a variabilidade dos parâmetros morfológicos dos depósitos gerados (comprimento, largura, razão comprimento/largura, área, topografia). Os leques de HDTC apresentaram evolução deposicional complexa, com quatro etapas evolutivas bem marcadas ao longo do tempo que resultaram no processo de autocanalização das correntes de turbidez, enquanto que nos leques de LDTC não foi identificado autocanalização ou quaisquer etapas evolutivas distintas. Altas disparidades na geometria dos leques, caracterizadas pelos elevados valores de desvio padrão relativo, sugerem que os controles autogênicos induziram a um comportamento morfológico estocástico nos leques

Taphonomic analysis of bioclastic accumulations generated by physical modeling of a wave-dominated shallow water system

Este estudo procurou detalhar os efeitos deposicionais das dinâmicas de concentração de conchas (retrabalhamento mecânico, winnowing e bypass dinâmico) governadas pelas ondulações. Parâmetros tafonômicos, incluindo orientação e posicionamento das conchas, empacotamento e o conteúdo percentual de matriz (em peso), foram analisados a partir de amostras rígidas e inconsolidadas dos depósitos modelados, bem como pelo imageamento do modelo físico. As orientações topo-base das conchas foram predominantemente concordantes em todo perfil simulado (>70%), porém com um percentual maior de bioclastos oblíquos e verticais nas amostras mais distais do modelo, associadas ao winnowing e ao bypass dinâmico. As orientações em planta mostraram tendências bimodais ou unimodais nos depósitos intermediários do perfil, associadas a dinâmica de retrabalhamento mecânico (na rebentação) e winnowing, e polimodais nas amostragens mais proximais e distais, associados ao retrabalhamento mecânico no espraiamento e bypass dinâmico, respectivamente. O posicionamento da concavidade dos bivalves foi majoritariamente para baixo ao longo da seção analisada. O empacotamento e percentual de matriz foi, respectivamente, denso e mais baixo (30%) no depósito gerado pelo winnowing, sendo os demais com empacotamento moderado ou disperso. A análise tafonômica empregada em concentrações bioclásticas (coquinas) geradas por modelagem física de ambiente costeiro dominado por onda pode ser utilizada para previsão de determinadas dinâmicas sedimentológicas de acumulação de conchas.This analysis aims to detail the depositional effects of the shell concentration dynamics (mechanical reworking, winnowing and dynamic bypass) controlled by the waves. Taphonomic parameters such as the shell orientation and positioning, the packing and the percentage content of matrix (by weight), were analyzed from rigid and inconsolidated samples of the simulated deposits, as well as by the image survey of the physical modeling surface. The top-base orientations of the shells were most concordant (> 70%) along the entire simulated profile, but with a higher percentage of oblique and vertical bioclasts in samples from more distal sector of the model, associated to the winnowing and dynamic bypass. The fabric orientation showed bimodal and unimodal tendencies at the intermediate deposits of the profile, associated with the dynamics of mechanical reworking (from breaking zone) and winnowing, and polymodal in the more proximal and distal deposits, associated to the mechanical reworking (from swash zone) and dynamic bypass, respectively. The bivalve shells were mostly concave-down in all simulated deposits. The packing and the percentage of matrix were, respectively, dense and lower (30%) in the deposit generated by the winnowing, being the others with moderate or disperse packing. The taphonomic analysis employed on bioclastic concentrations (coquina) physically modelled as a wave-dominated coastal environment can be used to predict specific sedimentological dynamics of shell accumulation

Perfil praial de equilíbrio da praia de Serinhaém, PE

Apresentamos neste trabalho os resultados da aplicação do conceito do perfil praial de equilíbrio proposto por Dean (1977), para a Praia de Serinhaém, PE. Este conceito foi inicialmente empregado em praias com características de tamanhos uniformes de grão, e foi usado satisfatoriamente na Praia de Serinhaém, que apresenta diferentes tamanhos de grão sobre a zona de surfe e a antepraia. O desequilíbrio observado entre o perfil praial medido e o perfil previsto indicam um déficit de sedimentos no sistema praial. Esta interpretação é concordante com o fenômeno erosivo registrado na região costeira em estudos que descrevem o recuo da linha de praia (Manso, 1997). A presença de um banco de arenito de praia além da profundidade de 5 m, sobre a antepraia, pode constituir-se no mecanismo que atua na proteção natural do pós-praia das significativas taxas de erosão, mas sem modificar o resultado do perfil de equilíbrio aplicado à Praia de Serinhaém.Dean’s equilibrium beach profile concept, established for beaches with uniform grain sizes, was used, with some adjustments, atthe Serinhaém beach, Pernambuco State, Brazil. As beach and shoreface presented different grain sizes, the equilibrium profile was calculated including the grain size of the shoreface beyond the surf zone. A better profile adjustment was obtained with the value of ½ to exponent of the Dean’s equation. Comparison between the adjusted and measured profiles indicated a sediment deficit agreeing with observed beach recession (Manso, 1997). Beach rock outcrops at depths beyond the 5 m isobath, acting as asubmerged breakwater protecting the beach from large erosion rates, did not influence the result of application of the equilibrium profile

Threshold of motion and orientation of bivalve shells under current flow

Disarticulated shells of three bivalve mollusk species (Anomalocardia brasiliana, Codakia orbicularis, and Divaricella quadrisulcata) were experimentally tested in laboratory flumes to determine the threshold of motion and final orientation of the valves. A total of 150 current flow experiments were conducted on single shells resting on a fixed sand bed. This study demonstrated that shells in the convex-up position are more resistant to flow when the umbo is pointing downstream rather than upstream. Moreover, species with higher frontal areas were more likely to be entrained at lower flow velocities. Results of dimensionless shear stress exhibited values far below the threshold of grains movement for beds of uniform roughness (Shields curve). It was observed that circular shells in convex-up positions were mostly orientated with the umbo pointing downstream. Conversely, elliptical shells in convex-up position tended to align their longer axis parallel to the flow with the posterior side of the valve pointing downstream. These results are not only directly applicable in interpretations of incipient shell motions and in paleocurrent analyses from field and sample data, but also support construction of accurate geological models