Ocean process-driven sediment transport in submarine canyons along the northern Cascadia margin: Morphological control of triggers

“Ocean process-driven sediment transport in submarine canyons along the northern Cascadia margin: Morphological control of triggers”

Submarine canyons act as a major highway between the coastal and deep ocean, transporting enough water, nutrients, and sediment to have a global impact on geology, biology, and climate. Just like canyons on land, submarine canyons are large, eroded valleys, but they are not carved by rivers. Instead, submarine canyons are carved by downslope-flowing currents of sediment and water akin to an avalanche.

Scientists are challenged in predicting the onset and distance traveled by these flows. This project aims to identify the physical processes in these flows off the coast of Washington and learn how differences in canyon structure can affect the flows generally. A set of seafloor instruments in two submarine canyons will be installed to record sediment gravity flows as they pass through them.

These data are quite rare and greatly aid understanding of flow mechanics and enable development of numerical models that predict where and when flows occur. Six seafloor instrument packages will be maintained in Astoria Canyon and Quinault Canyon, both fed by the Columbia River, for a complete year. Sediment cores will be collected from both canyons, where event-layers from past sediment-gravity flows will be identified and analyzed for their composition and historical frequency.

The researchers predict that river floods and marine storm events (large waves, strong winds) can trigger major sediment gravity flows in these canyons today, but differences in canyon terrain trigger sediment-gravity flows under different conditions.

Submarine canyons are present along nearly every continental margin and are the sites of globally significant ocean mixing, sediment transport, and nutrient exchange between the coastal and abyssal environment. Earth’s submarine canyons are maintained by density-driven flows of water and sediment, but the mechanisms that trigger flows—and their downslope flow evolution—are still not well understood.

This project examines the pathways of sediment through two canyons along Cascadia Margin which are morphologically distinct yet share the same fluvial source, the Columbia River. This study’s experimental design includes a targeted instrument deployment, comprehensive seabed coring survey, and hydrodynamic modeling of sediment gravity flows. The project is intended to advance knowledge of canyon sedimentary processes under modern high stands in sea level and constrain the role of morphology and oceanographic disturbance in initiating seabed resuspension and density-driven flow.

Seafloor sediment loading from fluvial sources, triggering of flows by bathymetric focusing of wave and current energy, and synchronous downslope transport across distant canyon systems will be studied as functions of canyon morphology in Astoria and Quinault Canyons. The investigators predict that sedimentary deposits in relatively deep-water canyon environments may be created via oceanic triggering, and aim to place quantitative constraints on the downslope limit of such flows.

They also predict that processes in differing canyon morphologies result in deposits that are both regionally synchronously and asynchronously emplaced at depth.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.