Processes drIving Submarine Canyon fluxES

The shallow shelf seas that surround all the continents are connected to the deep open ocean by a steep continental slope, which rises from thousands of metres beneath the ocean surface to just a few hundred. Phytoplankton (microscopic plants) in shelf seas are reliant on flows of deep ocean water up the continental slope to provide them with the nutrients they require to grow and reproduce. Photosynthesis by phytoplankton is the foundation of the shelf sea food web, so all organisms, including commercially important fish species, are influenced by up-slope currents. Flow down the continental slope is equally critical. Drawdown of atmospheric carbon dioxide into shelf waters by the extra photosynthesis allowed by deep ocean nutrients is only an effective long-term buffer to anthropogenic emissions if the carbon is then transported off the shelf and locked away in the deep ocean.

The mechanisms that allow currents to flow up and down the continental slope are poorly understood and are not accurately simulated by the numerical models used to predict future climate. Where the continental slope is straight and smooth, there is limited cross-slope transport because currents generally flow along the slope rather than across it. However, where continental shelves are incised by submarine canyons, along-slope currents are blocked by the canyons' steep walls. This results in enhanced cross-slope transport through the canyons by two key physical processes. The first is upwelling and downwelling - caused by along-slope currents becoming unstable over the steep walls and being turned onto, or off, the shelf. The second process is internal tides and turbulent mixing - caused by subsurface waves breaking against the steep walls. This creates turbulence and mixes deep ocean water with shallow water above the canyon rim. Both processes are challenging to observe and decipher due to their small scales and the complexity of canyon geometry. Submarine canyons are common along continental margins worldwide - created by high-density, sediment-laden currents rather than rivers - so they have the potential to make a major contribution to the total transport of nutrients onto continental shelves globally. This project aims to observe, understand and predict these two crucial canyon processes so that they can be accurately simulated, or realistically approximated, by the next generation of global models.

To achieve these aims, we will intensively study both physical processes in Whittard Canyon, a large, branching system that incises the Celtic Sea continental shelf. Within the different limbs of the canyon, the two processes are expected to play greater or lesser roles in cross-slope nutrient transport: some limbs are expected to be dominated by upwelling and downwelling; other limbs by internal tides and turbulent mixing. We will use a broad range of technologies, including cutting-edge autonomous vehicles, a wide variety of ship-board and moored instruments, and state-of-the-art high-resolution ocean models, to measure critical ocean properties and help us understand the dominant processes. Specifically, we will use autonomous ocean gliders equipped with bespoke sensors measuring current velocity, dissolved nutrient concentration, and turbulent mixing, to determine nutrient transport through the canyon. These gliders are driven by buoyancy instead of a propeller, so they can monitor the canyon environment for months on a single battery. The observations and high-resolution model simulations will be complementary so that we can: (1) investigate how canyon geometry controls the two processes; (2) determine which processes dominate in Whittard Canyon and along the whole European northwest shelf break; (3) assess how cross-slope nutrient transport is affected by along-slope current speed, tidal energy, and changes in stratification (layering of the ocean); and (4) improve the simulation of these processes by global ocean and climate models.