Bioturbation in carbonate sediments from the Rocas Atoll (Equatorial South Atlantic)

Abstract

The carbonate sediments of oceanic islands are often characterized by the great number of bioturbation structures  produced by species that modify their geomorphology, but there  few studies related to islands of the Tropical Southwest  Atlantic.  The structures present in the main sedimentary deposition systems in Rocas Atoll were observed: sand cays,  lagoon and in the intertidal sand flat. The results show a series of bioturbations originated by the action of land, semi-land  and sea taxons. Birds, turtles and crustaceans are the main bioturbators in the sand cays (islands) in the supralitoral  and mesolitoral zones, while infauna organisms (nematodes, polychaetes, oligochaetes and crustaceans) and ichthyofauna  is the main responsible for the intensive bioturbation in the pools, lagoon and intertidal sand flat. The grain size of the  sediments varied from very fine sand to pebbles, fine and coarse sand being the most common in structures in emersed  zones. The island of biogenic carbonate sand, with greater area extension, high biological diversity and lower erosion  rates, had the greatest quantity of bioturbations. The bioturbation structures are fundamental for understanding the  sedimentary geology and evolution of the only atoll in the Equatorial South Atlantic.