LAGOAS COSTEIRAS INTERDUNARES E BRANQUEAMENTO DE DUNAS NO NORDESTE BRASILEIRO (coastal interdune ponds and whitenning of dunes in North-east Brazil)
Abstract
As mudanças na cor das areias de dunas, de branca para amarela ou vermelha, é freqüentemente interpretada como significando uma maior idade dos materiais de origem. Neste estudo, demonstramos o efeito sazonal que as lagoas interdunares de água doce têm no branqueamento da cor da areia das dunas através da redução de ferro nas condições anaeróbicas que elas criam. Combinando medidas espectrais de amostras de campo da areia dunar simultaneamente com a análise de imagens de satélite que cobrem três campos de dunas no Nordeste do Brasil (Lençóis Maranhenses, Jericoacoara e Canoa Quebrada), demonstramos tal situação no campo. Em áreas onde as lagoas cobrem 41% do campo de dunas (como nos Lençóis Maranhenses), um branqueamento quase total na cor da areia acontece, após o qual as dunas permanecem brancas, sem relação com a distância do litoral. Em áreas com menos lagoas (por exemplo, Jericoacoara ou Canoa Quebrada), o branqueamento das dunas é menos ativo, e há áreas onde avermelhamento de dunas acontece. Como este processo pode ter ocorrido em outros campos de dunas durante condições climáticas passadas que podem ter sido diferentes das atuais, as interpretações da idade das dunas baseadas na sua cor devem ser tratadas com cuidado.
Palavras chave: dunas litorâneas; lagoas interdunares; óxidos de ferro; avermelhamento; redução; sensoriamento remoto
Abstract
The changes in color of sand dunes from white to yellow or red is often interpreted to signify their age or their source materials. In this study we demonstrate the effect seasonal inter-dune freshwater ponds have on the bleaching of the color of sand dunes by iron reduction in the anaerobic conditions they create. By combining spectral measurements of field samples of dune sand together with the analysis of satellite images covering three dune fields in NE Brazil (Lençóis Maranhenses, Jericoacoara and Canoa Quebrada) we demonstrate its existence in the field. In areas where ponds cover 41% of the dune field (as in Lençóis Maranhenses) an almost total bleaching in the color of the sand occurs after which the dunes remain white with no relation to distance from the coastline. In areas with less ponds (e.g. Jericoacoara or Canoa Quebrada) dune whitening is less active, and there are areas where dune rubification occurs. As this process may have occurred in other dune fields during past climate conditions that may have been different from current ones, interpretations of dunes age based on their color should be handled with care.
Key words: Coastal dunes; Ponds; Iron oxides; Rubification; Reduction; Remote sensing
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