Modern sedimentation and morphology of the Rocas Atoll shelf, and the role of Quaternary sea-level changes

The Rocas Atoll is in the Fernando de Noronha Fracture Zone South Atlantic Ocean and was developed on top of an ancient oceanic volcano, mainly during the Holocene. Quaternary sea-level fluctuations, especially during the last transgression, left geomorphic marks on the atoll and its surrounding shelf. This study investigated the shelf of Rocas Atoll 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, predominantly flat with a water depth of 25 m. The shelf break occurs between 45 and 50 m water depth, reaching up to 80 m locally on northwestern. Inner and outer shelf zones are marked by step-terrace features. The deepest break in relief (T1) occurs at ∼70 m water depth, a terrace (T2) at 40 m, and another terrace (T3) occurs at 30 m. 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 toward the outer shelf, the reefs occur isolated. The geomorphologic patterns of this shelf are analogous to the northeast Brazilian shelf, reflecting the influence of Pleistocene/Holocene sea-level oscillations. Furthermore, multi-temporal satellite images between 2001 and 2019 reveal that modern processes of reef erosion, as a response to decadal climate phenomena, intensify the shelf sedimentation, and the South Equatorial Currents shape the modern shelf and affect seasonally the sediment transport and sand body dimensions.