Investigation of sedimentary structures associated with the Deep Western Boundary Current in French Guiana (Demerara Plateau)

Abstract

The Demerara Plateau in the Equatorial Atlantic is a transform marginal plateau (TMP) where the Deep Western Boundary Current (DWBC) transports North Atlantic Deep Water (NADW) to the South Atlantic. This current, circulating in the depth between 1300 and 3500 m, represents the deep part of the thermohaline circulation. It also forms hundreds of sedimentary structures along its path, looking like giant flute-casts and called 'comets’. Those comets can reach 3 km in length and theses field of 'comets' more than ten kilometers large. Nautile dives, AUV acquisitions (equipped with Multibeam Sounder SMF, Doppler current meter ADCP, Sediment Sounder) and a mooring, implemented during the DIADEM campaign (N/O Pourquoi Pas?, January-February 2023, DOI : 10.17600/18000672), first allow to document the dynamics of the current DWBC in this equatorial domain, its spatial and temporal variability. Furthermore, investigated the numerous associated sedimentary systems associated with this current, as the "comets" forming giant erosion structures. Two AUV bathymetric surveys and four Nautile dives have helped to better understand the location of these hydrodynamic structures. They are located along outcrops of intensely tilted and fractured carbonated rock, probably associated with an ancient sliding mass. Nautile data coupled with photogrammetry are also used to reconstruct the outcrops of these carbonate blocks and to characterize their deformation. AUV ADCP data (hydrodynamics) acquired in parallel highlight the difference in current intensity between the comet head, where the current has a much greater magnitude than in the comet tail, which appears more sedimented. Measurements of currents and turbidity recorded at the mooring deployed upstream of the comet over 17 days of recordings clearly demonstrate the effect of the semidiurnal tide in the high-frequency variability of currents and sediment resuspension. Taken together, these geomorphological, oceanographic and sedimentary parameters provide a clearer picture of those complex seafloor sedimentary structures that seem to result from the interaction of the DWBC with remobilized carbonated outcrops. Our observations also suggest that those comets initially formed under higher hydrodynamic conditions than those recorded today.