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Distribution of benthic Foraminifera in organic rich mud layers of the Amazon shelf Distribuicao de Foraminiferos bentonicos em sedimentos argilosos ricos em materia organica da Plataforma Amazonica

Distribution of benthic Foraminifera in organic rich mud layers of the Amazon shelf Distribuicao de Foraminiferos bentonicos em sedimentos argilosos ricos em materia organica da Plataforma Amazonica

Anais da Academia Brasileira de Ciencias. Marco; 691: 77-82

The characteristics of sediment discharge and deposition in the Amazon continental shelf influence the benthic foraminifera microfauna distribution. Benthic foraminifera are very sensitive to environmental changes, both natural and anthropogenic. The present study showed microhabitat preferences in mud layers interbedded with organic matter laminations, in the Amazon continental shelf, northern Brazil. Characteristic species, adapted to high or low oxygen levels, are encountered in organic rich mud layers. Samples from two box-cores collected, respectively, in the middle and outer shelves, northwest of the river mouth, were analyzed. In the outer shelf there are rare, small, broken and/or abraded foraminiferal tests down to a depth of 3 cm below the sediment-water interface, which may be reworked. In the middle shelf there are numerous and diversified foraminiferal species down to a depth of 5 cm below the sediment-water interface. The epifaunal species Ammonia beccarii dominated in the first centimeter, and many of them were stained. The epifaunal/shallow infaunal Cassidulina crassa, and Globocassidulina subglobosa were dominant down to a depth of 5 cm, and there are no macrofauna bioturbations. Their morphologies are adapted to mobility within low oxygen and mud layers. The characteristics of the foraminiferal microfauna and its distribution suggest a quasi-anaerobic biofacies below the sediment-water interface.

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