Instability of deep-sea currents over tidal and seasonal timescales

Abstract Seafloor currents, including episodic sediment-laden turbidity flows and more sustained thermohaline-driven bottom currents, control the ultimate fate of sediment, carbon and pollutants in the deep-sea. Collectively, these currents form the largest sediment accumulations on Earth, from which past climate records are reconstructed. A long-standing view has been that thermohaline-driven bottom currents are persistent and stable; however, a paucity of direct near-seabed monitoring means this remains untested at field-scale. Here, using the most spatially-extensive and long-duration (4-yrs) monitoring of bottom currents at 34 deep-sea instrumented moorings, we show that such currents are far from steady and continuous. Bottom currents instead show dynamic, spatial variability in velocity and direction over tidal and seasonal timescales, being strongly influenced by local seabed morphology. Prior lower resolution monitoring has underestimated this spatio-temporal variability of bottom currents; hence our results highlight a need for near-bed measurements to more robustly quantify deep-sea sediment, carbon and pollutant fluxes.