New insights into turbidity current structure and behaviour from direct measurements in the deep-sea

Abstract

Submarine canyons are important conduits for transporting sediment, organic carbon, nutrients, and pollutants to the deep sea. Oceanic sediment density flows called turbidity currents are the main mechanism responsible for redistributing this material on a global scale. Turbidity currents can be very powerful, and carry exceptionally large sediment volumes at high speeds (<19 m/s). Consequently, turbidity currents damage seabed telecommunications cables that carry > 95% of global data traffic, as well as oil and gas pipelines and other infrastructure for energy transport. Thus, it is crucial to understand how turbidity currents evolve over time and space. Yet, knowledge of how these seafloor flows behave is mostly derived from indirect studies, as the powerful nature of turbidity currents ensures that only a handful of direct measurements exist.

This thesis presents unusually high resolution (sub-minute) direct monitoring datasets for turbidity currents, from two submarine canyons in different locations. The first location is Monterey Canyon, offshore California, where turbidity current evolution is analysed through observations of flow behaviour and mixing processes. In particular, turbidity currents are shown to reverse in buoyancy (termed lofting), and rise vertically from the seafloor, after sufficient sediment had been deposited. The water column then took < 2 days to recover to pre-event conditions. These first deep-sea observations of lofting have important implications for the structure of deposits that can form valuable hydrocarbon reserves. Additionally, the amount of sediment and seawater entrained into turbidity currents is quantified at multiple locations along Monterey Canyon. It is shown that turbidity currents at this site grew in water volume by up to a factor of four, which is significantly smaller than predicted by previous numerical and lab-based models, and this discrepancy has important implications for improving seafloor hazard mitigation strategies. The second field dataset comes from the Congo Canyon off West Africa, and it is used to document how the front structure of turbidity currents evolves. At this site, turbidity currents travelling along the same flow path developed one of two different flow-front shapes. Faster turbidity currents developed a distinct head, while slower flows did not. For the first time, it is demonstrated that a thin lens of seawater can be pushed in front of faster flows, similar to air blasts observed during snow avalanches. Overall, this thesis delivers unique insight into how turbidity flows develop over time and space. Such analysis is timely, given that the spatial extent of seafloor infrastructure is predicted to increase by between 50-70% by 2028.