Barramentos naturais no Vale do Rio Peruaçu: conseqüências ecológicas e ambientais de eventos pretéritos e futuros

Abstract

Fossil assemblage found in caves record important environmental information concerning the recent past of the surrounding area. In the karstic canyon of the Peruaçu River, Cavernas do Peruaçu National Park, Minas Gerais - Brazil, organic deposits suggests the occurrence of major floods resulting from natural barring in the river bed. This dissertation sought to investigate fossil assemblage present mainly in Carlúcio Cave, and outline the material importation and deposition events. The objective is to understand the ecological and environmental consequences of Peruaçu River natural barring based on analysis of past events and predicting the effect of future occurrences. For this, all the fossil assemblage found in Carlúcio Cave were inventoried and mapped. Representative occurrences were topographed in relation to the level of Peruaçu River, measured in July 2007. Three fossil remains were dated using carbon-14 and the calcite covers of two of these samples were dated by uranium series. Using geoprocessing programs, the maximum level of deposition found was represented in the current landform taking as the axis of the barring, the Brejal Cave, the Arco do André Cave and the Janelão Cave interpreted as environments more favorable to collapse. Land use, phytogeography and infrastructure georeferenced bases permitted the interpretation of the reach of future events in both natural and human interference areas. A total of 1,479 occurrences of subfossils in Carlúcio Cave were found, the vast majority being related to land mollusk shells of the Megalobulimidae and Bulimulidae families. The occurrences of 20 bones, 57 vegetable deposition sites, 4 charcoals, one coconut fruit and one ear of corn were catalogued. The ages found were from 1,630 +/- 50 years BP for a sample of wood, to 3,050 +/- 50 years BP for a Megalobulimus shell and 1,758,5 +/- 430.5 years BP for its calcite cover, and 9,380 +/- 40 years BP for another Megalobulimus shell and 8,108.5 +/- 49.0 years BP for its calcite cover. Those ages suggest at least two organic material importation events for Carlúcio Cave. The minimum flood reach of 19.6 meters above the drainage found for the maximum fossil occurrence in Carlúcio Cave was extrapolated to the landform located upstream from the Brejal Cave, the Arco do André Cave and the Janelão Cave. In case of these future natural barrings the loss of areas would affect essentially primary vegetation from the training plant Riparian Forest, Deciduous Mesophyle Forest, Close Savanna Woodland and Open Wooded Savanna. Infrastructure directed to public use, research, control and management of the conservation area, such as roads, trails and support centers, would also be affected. The results provide subsidies for prediction of natural barring impacts in the Peruaçu River, an area whose the natural tendency for collapse has been recently aggravated by land tremors.