Modeling of bottom currents for estimating their erosional-depositional potential in the Southwest Atlantic

Abstract

Contourites are sediments deposited or substantially reworked by the persistent action of bottom currents which are capable of forming a wide range of erosional and depositional features on the seafloor. The study of contourites is a multidisciplinary topic linking physical oceanography and marine geology. While near-bottom circulation is an important subject itself due to different dynamic aspects and climatic estimates of abyssal water transports, it becomes especially important for studies of lateral sedimentation. The precise direct data on bottom currents on the South American continental rise and adjacent abyssal plains remain extremely rare. Numerical modeling can appear helpful for filling the gaps in our knowledge on bottom circulation in the region. This work describes an approach for mapping erosion- and deposition-dominated areas based on the modeled circulation in the bottom layers. The model domain covers the regions with different seabed topography and velocities of bottom currents: the Vema Channel, the Santa Catarina Plateau, the Rio Grande Rise, and parts of the Argentine and Brazil basins. The high vertical resolution used in the model allowed us to study the near-bottom circulation in the deep ocean, while high horizontal resolution enabled modeling currents in narrow underwater passages such as the Vema Channel. The equations that encompass the bottom current velocities and data on surface sediment grain-size are well-known; in this study, we estimate the erosional-depositional potential of bottom currents using these equations. This approach allowed identification of the regions of dominant erosion (to the northeast of the northern exit from the Vema Channel), deposition (northern part of the Santa Catarina Plateau), and non-deposition under the influence of bottom currents. The results were validated by the analysis of echo-facies distribution in the study area and variations in the surface sediment grain-size. This study embraces the transition zone between the Argentine and Brazil basins, which is crucial for understanding the deep and bottom circulation in the western South Atlantic under modern conditions and in the geological past. The presented technique can improve our knowledge on how bottom currents shape the seafloor and control sediment distribution in the Southwest Atlantic.