Large wetlands of South America: a model for Quaternary humid environments

摘要

Large wetlands are important landscapes in South America, where today they cover one million square kilometers. Such environments probably were as important in the Quaternary of other continents. Tensional tectonic processes in the Andean foreland and other lowlands result in the sinking of blocks several thousand square kilometers in surface. Under humid climates, such depressions become occupied by shallow waters and a dense paludal vegetation. These features are a special case of wetlands, which owing to their areal extension, complexity, internal fluxes of sediments and salts, must be considered as macrosystems (Neiff et al., Large tropical South American wetlands: a review UNESCO Ecotones Workshop, Seattle, UNESCO, Paris, 1994, 15 pp.) and are a major depositional environment in South America. Such areas are plains, characterized by more or less periodic flooding and complex ecosystems adapted to large water fluctuations. Such water bodies have frequent anaerobic conditions and accumulate organic matter in various degrees of decomposition. South America contains 14 wetlands with areas larger than 10000 km~2. Other somewhat smaller wetlands appear in abandoned fluvial belts inside megafans. Examples of this type can be found in the Pilcomayo river system in Argentina, where an old belt is at present a swamp 250 km long and 7-12 km wide. Large inland wetlands in South America can be grouped in two types: Pantanales, or sandy swamps; and Mud Wetlands. Both are clearly different in geomorphology, sediments, types of salts, and ecology. From the point of view of Quaternary Science, the degree of knowledge of the large wetlands is really poor. In general, it is restricted to the survey of the present conditions, which is itself important because similar environments have generally been reclaimed for agriculture since hundreds of years in other continents. An example of a large wetland of Late Pleistocene age (the Tapebicua Fm, Argentina) is presented in this article. It is also clear that tropical swamps did exist throughout the Quaternary, and they very probably were "hot spots" for evolution of plants and animals and refuges for particular ecosystems.