The Demerara Plateau, located in the equatorial Atlantic, is particularly well-suited for recording the activity of the Deep Western Boundary Current (DWBC), which transports North Atlantic Deep Water (NADW) southward into the Atlantic basin. This current, active between 1500 and 3500 m depth, constitutes the deep part of the global thermohaline circulation and plays a crucial role in climate regulation. The Demerara Plateau is remarkable for the abundance and wide distribution of comet mark-type sedimentary structures, which can reach several kilometres in length. These hydrodynamic bedforms, interpreted as erosional features associated with strong bottom currents, are currently used as proxies for deep currents velocities, with minimum formation thresholds estimated between 0.60 and 0.75 m/s according to the literature (Rebesco et al., 2014; Werner et al., 1980). The DIADEM (Dive At DEMerara) oceanographic cruise (Basile and Loncke, 2023) enabled detailed investigation of one such structure using a combination of complementary tools with the aim of better understanding their functioning and evolution with in-situ observation, physical records, and sampling. Those new data were also combined with formerly acquired high-resolution seismic data, allowing a new vision of these bedforms their formation, and their evolution over time. The main findings are the following: (1) comet-marks localize on carbonate mass transported blocs outcropping on the seafloor; (2) Clearly show a polyphase evolution of the bedforms of comet mark presenting alternations of erosion and sedimentation phases. The bedforms therefore record long-term variations in bottom current activity with an alternation of intense hydrodynamic events (erosion) and quieter hydrodynamic periods (deposition) through time, reflecting a complex hydrodynamic history; (3) At present, comet-marks recorded a significant decrease in current velocity within the erosional zones located inside the comet tails associated with fined-grained sediments infill in this area, highlighting the need for cautious interpretation of such features as direct indicators of present-day current intensity; (4) Finally, a result that was not necessarily anticipated but documented by exploring those bedforms with the Nautile submersible, comet-marks host quite important benthic and epibenthic biodiversity with a wide variety of associated species.
Blin et al. (Thu,) studied this question.