Highly variable deep-sea currents over tidal and seasonal timescales

Abstract

Deep-sea transport of sediment and associated matter, such as organic carbon, nutrients and pollutants, is controlled by near-bed currents. On the continental slope, these currents include episodic down-slope gravity-driven turbidity currents and more sustained thermohaline-driven along-slope contour currents. Recent advancements in deep-sea monitoring have catalysed a step change in our understanding of turbidity currents and contour currents individually. However, these processes rarely operate in isolation and the near-bed current regime is still to be quantified in a mixed system. Such measurements are crucial for understanding deep-sea particulate transport, calibrating numerical models and reconstructing palaeoflow. Here we use 4 years of observations from 34 instrument moorings in a mixed system offshore of Mozambique to show that near-bed currents are highly dynamic. We observe spatial variability in velocity over tidal and seasonal timescales, including reversals in current direction, and a strong steering and funnelling influence by local seabed morphology. The observed near-bed currents are capable of mobilizing and distributing sediments across the seabed, therefore complicating deep-sea particulate transport and reconstruction of palaeoceanographic conditions. Mooring observations suggest that deep-sea currents exhibit substantial variability over tidal and seasonal timescales, driving a complex pattern of sediment transport.