Study reveals complex behavior and environmental impact of unpredictable deep-sea currents

A comprehensive investigation headed by Lewis P. Bailey of the National Oceanography Centre and involving global collaborations revealed the complicated and unpredictable behavior of deep-sea currents. This four-year study offshore North Mozambique investigates how these currents interact with the seafloor, challenging existing models and bringing new insights into marine ecosystems.

• To comprehend the complicated behavior of deep-sea currents and their impact on sediment transport and marine ecosystems, it is revealed that these currents are very dynamic and can change direction in response to seafloor features.
• The study spanned four years and utilized Acoustic Doppler Current Profilers (ADCPs) and water column velocity models, with findings published in mid-2024.

A team of oceanographers and marine experts from numerous worldwide research organizations, including the University of Manchester and the National Oceanography Center (NOC), led by Lewis P. Bailey of the National Oceanography Centre undertook a thorough investigation of deep-sea currents. The study included collaborations with academics from the United Kingdom, Canada, Germany, and Italy, bringing together experts from many domains to investigate the intricate interplays of oceanic currents.

The study discovered previously unknown and complex behaviors of deep-sea currents, demonstrating how they interact with the bottom to influence sediment resuspension and transport. Unlike prior models that assumed deep-sea currents were continuous and steady, the study discovered that these currents can change direction, speed, and intensity, sometimes completely reversing, depending on the seafloor’s uneven and dynamic features. These discoveries have important implications for understanding the transport of nutrients and contaminants in the deep sea, as well as the geological processes that sculpt the ocean floor.

The study centred on deep-sea locations along continental margins, namely offshore North Mozambique, where the North Equatorial Current (NEMC) diverges along the African coast. This area has complex oceanic phenomena, including mesoscale eddies and seasonal fluctuations in ocean circulation, which have a considerable impact on deep-sea currents. The study’s findings, while specific to this location, provide information that can be extended to other deep-sea ecosystems worldwide.

“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 the current direction, and a strong steering and funneling influence by local seabed morphology,” the researchers stated.

The investigation was conducted over four years, with the latest results published in Nature Geoscience in mid-2024. Long-term monitoring of deep-sea currents and sediment transport processes enabled scientists to see both seasonal fluctuations and long-term trends, providing a clear picture of the active nature of deep-sea currents.

Understanding the activities of deep-sea currents is important for various reasons. These currents are critical for altering the ocean floor, influencing sediment distribution, and affecting marine ecosystems. Additionally, the study’s findings are essential to understanding the larger ramifications of climate change, as ocean circulation patterns are inextricably tied to global climate systems. The discovery also sheds light on the deep-sea channels of contaminants like microplastics and nutrients that support deep-sea organisms, allowing for a more accurate interpretation of seafloor deposits that record past climatic shifts.

The researchers used a mix of Acoustic Doppler Current Profilers (ADCPs) and water column velocity models to analyze deep-sea currents in detail. ADCPs measured sediment re-suspension by evaluating acoustic backscatter data, correlating with suspended sediment concentrations.

The study also used the GLORYS12V1 global ocean eddy-resolving model to monitor seawater velocities at various depths, connecting observed current patterns to mesoscale features such as eddies and seasonal ocean circulation changes. Thirty-four deep-sea moorings were set at depths of up to 2.5 km (1.5 miles), each fitted with high-frequency ADCPs that functioned as underwater speed cameras to detect variations in seabed currents.

The study revealed that deep-sea currents are more unpredictable than previously thought, with intensity varying by season and occasionally reversing over short periods. This active behavior calls for long-term monitoring of the deep sea, which will provide vital information for understanding the complicated interactions between bottom currents and the seabed.

“The wide variability in current velocity observed across 34 mooring locations underlines the challenges that remain in characterizing seafloor currents, even when direct measurements are available. Near-bed processes are complex and sedimentation across the study area is probably controlled by the combination and interaction of different processes. Our new observations highlight the value of sustained near-bed current measurements at multiple locations in understanding deep-sea transport systems and the resultant dynamic distribution and accumulation of sediment, carbon and pollutants,” the researchers concluded.

References:

¹ Highly variable deep-sea currents over tidal and seasonal timescales – Bailey, L.P., Clare, M.A., Hunt, J.E. et al. – Nat. Geosci. 17, 787–794 (2024) – July 25, 2024 – https://doi.org/10.1038/s41561-024-01494-2 – OPEN ACCESS