Morfologia e sedimentação da Plataforma do Atol das Rocas

Abstract

The Rocas Atoll is in the Fernando de Noronha Fracture Zone, South Atlantic Ocean, developed on top of an ancient oceanic volcano mainly during the Holocene. Quaternary sea level fluctuations left geomorphic marks on the atoll and its surrounding shelf. This study investigated the geomorphology and sedimentation of Rocas Atoll shelf based on single-beam, sonography, grabbed bottom sediments, and multi-temporal satellite imagery. The Rocas Atoll has a surrounding shelf with general E-W orientation, mostly flat with water depth of 25 m. The shelf break occurs between 45 and 50 m water depth, reaching up to 80 m locally on northwestern. The shelf is compartmentalized into inner and outer zones that are limited by step-terraces features. The first terrace level (T1) occurs at ~70 m depth, the second terrace (T2) at 40 m depth and the third terrace level (T3) occurs at 30 m depth. Most of the hard-bottoms are reefs. Close to the atoll, in shallow depths, reefs occur aggregated, forming a tridimensional structure of low relief mounds and paths. As the water depth increases towards the outer shelf, the reefs occur isolated. ENE-WSW longitudinal and WNW-ESE transversal dunes migrate along the shelf. The geomorphologic patterns of this shelf are analogous to the northeast Brazilian shelf, reflecting the influence of Pleistocene/Holocene sea-level oscillations. Furthermore, the modern processes of reef erosion, as a response to decadal climate phenomena as El Niño, intensify the shelf sedimentation and the South Equatorial Currents shape the modern shelf and affect seasonally the sediment transport and sand body dimensions.