Paleo-sea levels, Late-Holocene evolution, and a new interpretation of the boulders at the Rocas Atoll, southwestern Equatorial Atlantic

Abstract

Rocas Atoll is a remote place and the only atoll in the southwestern Atlantic Ocean. This study presents new paleo-sea level reconstructions and a new interpretation on former ones. The new data indicate that sea level was higher at the Late-Holocene than it is at present and that the hydrodynamic factors overlap with sea-level changes, precluding more precise paleo-sea level reconstructions. According to previous works, the Rocas Atoll presents one of the most conspicuous paleo-sea level indicators, represented by reef remains. But, in this paper, these supposedly reef remains are reinterpreted as algal-reef boulders detached from the atoll reef-front by high-wave energy events and deposited over the reef rim, where they remain detached or are cemented and, sometimes, mushroom shaped. It is considered that the bigger boulders may have been deposited in their current position by the cumulative effects of past energetic wave events. Furthermore, the use of beachrocks as paleo-sea level indicators in former studies is discussed, leading to the conclusion that they actually indicate beach progradation and lateral accretion resulting from a positive sand budget. Finally, a schematic morphological evolution of the atoll during the Late-Holocene is proposed. At the atoll, leveling with RTK and imagery with laser scanner were collected, and eleven radiocarbon ages were measured by accelerator mass spectrometry (AMS).