An international team of researchers led by GEOMAR Helmholtz Centre for Ocean Research Kiel and Durham University has successfully captured the internal structure of the longest-runout sediment flow ever recorded on Earth.
Using seismic measurements, the researchers have for the first time been able to analyse in detail the internal structure of these tens to hundreds of kilometres long turbidity currents – an oceanographic phenomenon that has been studied for almost a century, but never directly observed.
The team deployed seismometers in October 2019 in the Congo Canyon and Channel off the west coast of Africa – one of the largest and deepest submarine canyons in the world. The instruments were placed several kilometres outside the canyon-channel axis, beyond the destructive reach of the currents, allowing them to record the seismic signals generated by flow turbulence and associated sediment transport.
The researchers tracked two turbidity currents moving at speeds of 5 to 8 metres per second (m/s) over a distance of 1,100 kilometres – from the mouth of the Congo River through the Congo deep-sea fan and canyon system. These are the longest-runout sediment flows ever recorded. The flows also damaged several submarine cables in January and March 2020, disrupting internet and data communications in West Africa during a particularly critical phase of the early COVID-19 pandemic.
The results show that the dense front of these canyon-flushing turbidity currents is not a single continuous flow, but consists of many pulses, each lasting between five and 30 minutes. The fastest pulses occur up to 20 kilometres behind the front. These surges eventually overtake the leading edge, suppling sediments and the momentum needed to sustain the flow over long distances.
This finding challenges previous assumptions that the highest velocities occur at the flow front. Instead, the new data suggest that turbulent mixing with seawater or other retarding forces significantly influence the behaviour of these flows over long distances.
The new insights into the dynamics of these powerful currents will help improve risk assessments for underwater infrastructure, such as submarine cables, and refine models of sediment and carbon transport in the ocean.
The findings are published in the journal Nature Communications Earth and Environment.