University of Hull scientists define mysterious, high-energy currents that trigger movements of ocean floor

An international team of scientists including experts from the University of Hull has investigated one of the least understood phenomena on our planet in one of the most intensive monitoring research campaigns of high-energy currents in underwater canyons that has provided insights into previously-unknown large-scale movements of the seafloor itself.

Underwater flows on the seafloor, known as turbidity currents, which sweep down canyons on the seafloor carrying sand and mud into the deep sea, have been the focus of an 18-month research study using highly-specialised technology at Monterey Canyon in California. 

The discovery relating to how these flows destabilise the ocean floor, which has been published in the prestigious Nature Communications, will lead to greater understanding of:

• Earth’s geological history by improved interpretation of the flows and the ‘barcode-like’ deposits left behind 
• the risk these currents pose to global data communications between countries resulting from cable breaks, as well as gas and oil pipeline infrastructure 
• the impact these currents have on the carbon cycle by transporting and burying globally significant amounts of organic carbon into the deep ocean, effectively locking it away.

Dan Parsons, Professor in Sedimentology and Director of the Energy and Environment Institute at the University of Hull, said:

“More than 70% of the world’s surface is covered by ocean but much of what happens deep below the surface remains a mystery. This is the first time we have really been able to measure and begin to understand these enigmatic flows that ultimately shape the seafloor and sustain life on the abyssal plains, at the unfathomable depths of the ocean.

“The dynamics of these submarine flows essentially control the way in which the evolution of the Earth is recorded in the sediments laid down in the deep sea. These deposits are like a barcode of earth's history and if we want to understand what the deposits are telling us then we need to understand the flows that carried the sediments to the deep sea floor.”

The international collaboration, led by Prof Charlie Paull of the Monterey Bay Aquarium Research Institute (MBARI), has intensively monitored the canyon examining how these underwater, high-energy currents form and develop as they tear down the underwater canyon off the coast of California. 

Professor Parsons said:

“This is the biggest and most complex research programme I have been involved with. The combined expertise of earth scientists, engineers and technical capabilities across a range of international partners have come together to produce a step-change in our understanding of these submarine systems, which are the biggest channels on Earth.”

Professor Parsons and Dr Steve Simmons, Research Associate in the Faculty of Science and Engineering at the University of Hull, together with colleagues at MBARI, the United States Geological Survey, the Ocean University of China, the Universities of Durham and Southampton, and the National Oceanography Centre, have used an array of highly-specialised instruments including ‘smart boulders’ and acoustic flow meters deployed over a distance of 50 km to a depth of 1,850 metres. 

Turbidity currents are responsible for transporting huge volumes of sediment to the deep ocean on an annual basis, often through channels and canyons that have been carved out by the actions of the flows over centuries. The Monterey Canyon stretches out for more than 100 km from the coast of California and its steep walls reach the same height as the sides of the Grand Canyon. Similar underwater channels are found all over the world.

Professor Parsons said: 

“The fact that the sediment within these underwater currents often contain organic carbon that has been delivered to the oceans by the rivers and from organisms living in the water is also fascinating. 

“How much of this carbon is locked away within the deposits of these flows and how much is recycled as nutrients that support life in the deep ocean will become clearer now that we have a greater understanding of the magnitude of these flows.

“This knowledge may also help ocean engineers to avoid damage to deep water pipelines and data communications, going some way to alleviating the economic risk of large-scale projects.”

Dr Steve Simmons said:“It has been a privilege to work on such an exciting project. The 18month monitoring programme combined expertise and technological innovation from project partners across the world. The extensive data we collected is continuing to give us new insights into how these poorly understood flows develop and transport sediment to the deep ocean.

“The highly energetic nature of the flows, combined with their episodic nature, has until recently thwarted attempts to monitor turbidity currents. Special instruments were developed for the Monterey project that could withstand the forces generated as the flows raced past the instrument frames and moorings, but that could also capture essential information about the behaviour of these flows. Previous conceptual models have been developed from laboratory experiments which often focused on the plumes of sediment-laden water above the sea floor. “

The findings of the current research have revealed that a critical component of the structure of the flows is actually a dense layer of water-saturated sediment that moves rapidly over the sea floor and mobilises more sediment into suspension from the bed.”

One innovation that facilitated this discovery is the Benthic Event Detector (BED), or 'smart boulder'. The BED is a 44cm sphere and has an electronic package encased within in it. The BEDs were left on the canyon floor and recorded pressure and motion as they moved down the canyon under the influence of the flows. The most energetic flow event, observed on January 15th 2016, had a speed of greater than 7 metres per second. It dragged the ‘smart boulders’ and sensors, some with anchors weighing more than a tonne, a distance of 7km down the canyon. 

The information provided by these instruments during the larger flows enabled the scientists to infer they consist of fast and dense near-bed layers, caused by remobilisation of the seafloor, overlain by dilute clouds of suspended sediment. The seabed itself becomes mobile in the upper reaches of the canyon, probably due to a process known as liquefaction, with the heavy instruments rafted within the fast-moving dense layer of sediment. 

The University of Hull researchers are also taking part in another international collaboration monitoring similar flows in a tidal inlet fed by snow melt in British Columbia. They also have funding from the Natural Environment Research Council to participate in a four year collaboration monitoring the Congo Canyon offshore West Africa.