Deep-sea sediment transport research has been revolutionized by precise, long-term monitoring of near-seabed currents (and, in part, Teledyne Acoustic Doppler Current Profiler (ADCP) improvements). Almost everything in the deep ocean is linked to the movement of sediment, from the state of the world’s oceans to the health of vital infrastructure to how we adjust to an evolving climate. In this article, we talk about Teledyne ADCP technology’s possible role in sediment transport data collection.
This value of sediment transport in mixed turbidity-contour current networks was discovered by new surveys conducted on the Mozambique continental slope and is discussed in a recent paper in the prestigious journal Nature, Bailey, L.P., Clare, M.A., Hunt, J.E. et al. Highly variable deep-sea currents over tidal and seasonal timescales.
Nat. Geosci. 17, 787–794 (2024). https://doi.org/10.1038/s41561-024-01494-2.
Sediment Transport in Deep-Sea Environments
Two kinds of currents dominate sediment transport in deep-sea systems:
Gravity flows: Turbidity currents are gravity-fed currents that can flow downslope rapidly, creating subterranean landmasses and pumping in massive quantities of sediment.
Contour Currents: Produced by the global ocean thermohaline circulation and flowing parallel to the ocean floor, these currents generate a calmer but slower flow that moves sediment.
These were formerly ‘stand-alone’ processes, but recent studies have increasingly demonstrated that they occur together and interlock in multi-layered dynamics, creating depositional systems of mixed turbidite-contourite form. This alters typical sediment-transport chains and can change where carbon- and pollution-laden granules are distributed along the seafloor.
Teledyne ADCP Technology Recording Mixed Current Systems
Teledyne ADCPs operate worldwide, recording high-resolution velocity and direction of current at various levels in the water column. The most extensive application of ADCP technology for sediment transport is tracing fine-scale currents at meter-scale distances from the ocean or lakeshore where most sediment flows occur.
In the Mozambique experiment, 34 ADCP moorings extended four years on the seafloor tracked changing behavior as low as three meters above the seabed. They gave hourly and daily reports of near-bed currents’ speed and direction and their relationship to topography.
Spatial and Temporal Disturbance
ADCPs showed that near-bed currents were highly time-dependent, and current speeds vary markedly from hours or days to seasons. Currents, for instance, are typically turned over from downstream to upstream within days or weeks of detection. This means that near-bed currents are space- and time-dependent over small distances, and some features can only be described using ADCPs’ long-term data. But let’s suppose these time-switch phenomena (which occur on days, weeks, or months) are not captured with ADCP surveys. In that case, they risk being overlooked, which, in turn, can result in flawed assumptions regarding current behavior.
Observations of near-bed currents offshore North Mozambique. Rose diagrams showing current direction as a percentage of total measurements from closest to seabed bin at each mooring. Inset: Bathymetry (GEBCO, 2022) offshore East Africa showing location of study site and regional ocean circulation patterns.
Geomorph-Driven Currents
These also depend on how seafloor topography – submarine canyons and gullies – ensconces and directs them. The canyon walls clogged and diverted contour flows from their standard slope-in patterns, so they interacted with turbidity flows unexpectedly. ADCPs charted how currents carried sediment up and down canyon walls and into the notches in remarkably local sediment transport pathways unimaginable to scientists.
Vertical Profiling and Seabed Relationship
Teledyne ADCPs are renowned for vertical profiling, so these sensors are very good at showing the relationship between current velocity and direction and seafloor height. The ADCPs in Mozambique recorded currents at a range of locations between three meters and 85 meters above the seafloor so researchers could track changes in the current with increasing depth.
Near-Bed Transport
ADCP data are particularly informative about near-bed currents because high velocities there could re-suspend fine-grained sediments, which then get pushed around the slope. This re-suspension and transport are particularly important where contour currents are forced into submarine canyons – and where older slope deposits are found.
Resuspension of Sediments
ADCPs, too, display high values of acoustic backscatter, an acoustic indicator of suspended sediments; high levels of suspended sediment indicate that near-bed currents transport not just new sediments but also re-suspended sediments from deposited sediments, obscuring interpretations of laminations and palaeoceanographic reconstructions.
Advancing Numerical Models and Predictions
The same abundant information from ADCP technology is also used to refine and calibrate numerical models, forecasting sediment transport on reduced assumptions when the data was scarcer and acquired much higher up in the sea. The vast majority of models modeled sediment as a generic black lump. Digital records from many years and many sites have changed all that: measurements from the field can now be added to the models to make the predictions much more accurate.
Palaeoceanographic Reconstructions
Reconstructing past sea conditions from sediment layers is one of the most challenging tasks in modern marine research. And ADCP can assist us in doing that by letting us track how sediments are deposited, redistributed, or eroded. Suppose we know how currents have behaved with the sea floor in the past. That way, we’ll be better equipped to understand what we’re reading from sediment cores and reimagine past oceanography.
Protecting Marine Infrastructure
Also required is precise, high-resolution current information for transfers to and from coastal structures. For instance, pipelines and cables on the seabed or man-made structures like offshore wind farms are subject to scouring, in which the currents whip the surrounding sediment on the seabed away from beneath the structure. With a look at the flows, engineers can see the likelihood and the most likely sites of scouring and intervene to shore up the buildings.
For example, ADCP profile information from the Mozambique study indicated near-bed currents as high as 0.6 meters per second in some instances, suggesting flows could release sediment and create scour around marine structures. Equipped with such predictive power, engineers might act in advance to save their systems.
Bottom Line-The Long Term of Deep-Sea Science with Teledyne ADCP Technology
As the practicality of data buoy arrays rose with Teledyne Marine’s ADCP technology, so too can investigating the transport of deep-sea sediments in the complex mixed-current systems most places have. This is another impressive byproduct of the increasing piles of high-resolution, long-term, and near-seabed recent data. It allows scientists to revise models, save infrastructure, and unearth new deep-sea processes. As humans and an evolving climate alter oceans in the decades ahead, Teledyne ADCPs will be needed as we try to track and manage these intricate systems, building predictive tools to mitigate those environments’ effects on marine life and human infrastructure.
