Abstract

The ocean floor comprises two thirds of our planet, and it hosts spectacular networks of channels and canyons formed by often powerful episodic sediment-laden flows, called turbidity currents. These submarine channels can extend for thousands of kilometres into the deep ocean, and are fed by submarine canyons that are as big as the Grand Canyon. The turbidity currents that created these channels remain poorly understood, as measurements of their velocities and sediment concentrations are only available in seven locations worldwide. This lack of observations reflects the relatively inaccessible and powerful nature of the flows, some of which powerful enough to drag 2,000 kg anchors for kilometres along the ocean floor. Fortunately, new technology now allow us to monitor turbidity currents in unprecedented detail.

These new field observations are important as turbidity currents are of societal and economic relevance. These flows are the main supplier sediment, organic carbon and nutrients to much of the deep-sea, as turbidity currents rival rivers in their global capacity to transport sediment across our planet. These fluxes make turbidity currents an important part of the carbon cycle that affects long term climate change, and they sustain rare ecological communities on the deep sea bed. Turbidity currents pose a hazard to submarine infrastructure, and have forced pipeline operators to invest millions of dollars in re-routing pipelines. Furthermore, these flows create the largest sedimentary bodies on our planet (e.g. the Bengal submarine fan holding tens of million km3 of sediment), and these sedimentary body host a significant part of our oil and gas reservoirs.

Here I will present observations of three turbidity current monitoring sides: submarine channels in Canadian fjords, Monterey Canyon and the Congo Canyon. The observations show that turbidity currents can substantially deviate from the textbook models. The dynamics of the turbidity currents are controlled by a fast-moving and dense frontal cell that set-up the more dilute cloud that is seen in most models. Additionally, the observations show a bifurcation in the behaviour of the flows, where a flow either develops as fast and dense or as slow and dilute. Furthermore, the observations provide new insights into the mixing processes that occur between these flows and the ambient seawater. Overall, the new seafloor data will hopefully play a key role in validating and modifying existing turbidity current models.