Influence of hydrodynamic patterns on foraminifera in reef sediment sat the Praia do Forte beach and Itacimirim, Bahia, Brazil
The present work analyses the influence of hydrodynamic conditions on the proportion of foraminifera in the sediment of two coastal reefs from the northern Bahia State, northeastern Brazil. The Itacimirim reef is 1,114 m long and 471 m wide, while the Praia do Forte beach reef is 686 m long and 128m...
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Veröffentlicht in: | Anuário do Instituto de Geociências 2006-01, Vol.29 (1), p.563-564 |
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Zusammenfassung: | The present work analyses the influence of hydrodynamic conditions on the proportion of foraminifera in the sediment of two coastal reefs from the northern Bahia State, northeastern Brazil. The Itacimirim reef is 1,114 m long and 471 m wide, while the Praia do Forte beach reef is 686 m long and 128m wide. Both of them occur in the fore reef zone in waters less than 10 m deep, while the back reef zone is inclined towards the siliciclastic sand beach, so it is surrounded by sand. The reef tops stay exposed during the low tide, which reveals great colonies of coral truncated by erosion, and between them, little pools and a channel inhabited by living corals and algae. The collecting was carried out in the winter (May and July) and summer (November and December) periods of 1999. Fourteen samples were collected along a delimited transect across the length of the reefs. The surficial sediment samples were taken from the top and the bottom of the recifal pools. At the laboratory, the samples were washed under a stream of water, dried, weighed and submitted to the pattern technique of grain size analysis. The first 300 grains of the fraction over 0.062mm in size were identified as one of the following groups: Halimeda, red algae, bryozoans, corals, crustaceans, echinoderms, sponges, foraminifera, molluscs, polychaetes and quartz. The quartz grains were predominant, varying in the winter vs. summer period, with 58.23% and 70.36%, respectively, from the Praia do Forte beach, and 44.90% to 45.39% in Itacimirim. Concerning carbonate grains, only red algae and molluscs in the winter and red algae in the summer had average percentages over 5% on the Praia do Forte beach. In Itacimirim, of the red algae, molluscs, and Halimeda in the winter, only the two first groups in the summer had percentages over this value. The foraminifera showed average percentages of 2.56% and 1.88% on the Praia do Forte beach and 2.48% and 3.31% in Itacimirim; the lowest percentages (Praia do Forte beach) and highest percentages (Itacimirim) of these organisms were both obtained in the summer period. Considering grain size, the percentages of carbonate grains decrease when the grain size of the sediment decreases, so carbonate grains predominate in the gravel and coarse sand fractions on the Praia do Forte beach reef, and the gravel and coarse to medium sand fractions in Itacimirim. The foraminifera are the most abundant of all groups, in the gravel fraction (4.5% in the winter and 3.0% in the |
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ISSN: | 0101-9759 1982-3908 |
DOI: | 10.11137/2006_1_563-564 |