Sediment waves on the Rio Grande Rise

Abstract

Rio Grande Rise (RGR) is one of the largest topographic features in the Atlantic Ocean. Recent interest in polymetallic crust exploitation in this region has amplified the need for knowledge on benthic habitats and sediment dynamics. The Geological Survey of Brazil (CPRM) has conducted a MBES acquisition (30 KHz) covering a 45,000 km2 area, with depths between 550 m on the plateau and 2775 m in the canyon incising the central area. These data offer a unique opportunity to examine distribution patterns of small-scale features (bedforms), which are often found parallel to the regional contours and have frequently been attributed to thermohaline and contour currents. Sediments are predominantly clay, consisted of foraminiferal and nannofossil oozes, with high carbonate content and low sedimentation rate (∼0.5 cm/ka). Five sites on the RGR presented bedforms classified as sediment waves, comprising 530 km2. These features were identified in depths from 600 to 1600 m, they average 1.2–2.9 m in height and 250–550 m in wavelength, with most crests oriented approximately in the east-west direction, parallel to the canyon. Some sediment waves are continuous for more than 30 km. Terrain slopes range from 0.01 degrees on the north plateau, which represents the most extensive and shallowest portion, to 0.3 on the south plateau. Breaking of internal waves may be the mechanism generating high-turbulence events near the bottom and sediment transport, resulting in these bedforms. During another CPRM cruise held on the RGR, flow measurements using an ADCP presented currents peaking at 0.5 m/s northward, whereas the residual flow after spectral analysis revealed values of 0.1 m/s, indicating the presence of internal waves. Bottom-current velocities associated with sediment waves typically observed in the deep ocean are in the interval of 0.05–0.2 m/s. Wavelengths of internal waves measured in the RGR (2–3 km) are not a match to the wavelengths of sediment waves in any of the areas. Further investigation is required to quantify the sediment transport. High-resolution seismic reflection profiles and new current measurements could also help to understand the origin and evolution of these bedforms.