Biologically active, fine‐grained sediment forms abundant sedimentary deposits on Earth's surface, and mixed mud‐sand dominates many coasts, deltas, and estuaries. Our predictions of sediment transport and bed roughness in these environments presently rely on empirically based bed form predictors that are based exclusively on biologically inactive cohesionless silt, sand, and gravel. This approach underpins many paleoenvironmental reconstructions of sedimentary successions, which rely on analysis of cross‐stratification and bounding surfaces produced by migrating bed forms. Here we present controlled laboratory experiments that identify and quantify the influence of physical and biological cohesion on equilibrium bed form morphology. The results show the profound influence of biological cohesion on bed form size and identify how cohesive bonding mechanisms in different sediment mixtures govern the relationships. The findings highlight that existing bed form predictors require reformulation for combined biophysical cohesive effects in order to improve morphodynamic model predictions and to enhance the interpretations of these environments in the geological record.

Here we show how major rivers can efficiently connect to the deep-sea, by analysing the longest runout sediment flows (of any type) yet measured in action on Earth. These seafloor turbidity currents originated from the Congo River-mouth, with one flow travelling >1,130 km whilst accelerating from 5.2 to 8.0 m/s. In one year, these turbidity currents eroded 1,338-2,675 [>535-1,070] Mt of sediment from one submarine canyon, equivalent to 19–37 [>7–15] % of annual suspended sediment flux from present-day rivers. It was known earthquakes trigger canyon-flushing flows. We show river-floods also generate canyon-flushing flows, primed by rapid sediment-accumulation at the river-mouth, and sometimes triggered by spring tides weeks to months post-flood. It is demonstrated that strongly erosional turbidity currents self-accelerate, thereby travelling much further, validating a long-proposed theory. These observations explain highly-efficient organic carbon transfer, and have important implications for hazards to seabed cables, or deep-sea impacts of terrestrial climate change.

This work was funded by the UK Natural Environment Research Council (NERC) under the COHBED project (NE/1027223/1). Paterson was funded by the Marine Alliance for Science and Technology for Scotland (MASTS).The dimensions and dynamics of subaqueous bedforms are well known for cohesionless sediments. However, the effect of physical cohesion imparted by cohesive clay within mixed sand-mud substrates has not been examined, despite its recognized influence on sediment stability. Here we present a series of controlled laboratory experiments to establish the influence of substrate clay content on subaqueous bedform dynamics within mixtures of sand and clay exposed to unidirectional flow. The results show that bedform dimensions and steepness decrease linearly with clay content, and comparison with existing predictors of bedform dimensions, established within cohesionless sediments, reveals significant over-prediction of bedform size for all but the lowermost clay contents examined. The profound effect substrate clay content has on bedform dimensions has a number of important implications for interpretation in a range of modern and ancient environments, including reduced roughness and bedform heights in estuarine systems and the often cited lack of large dune cross-sets in turbidites. The results therefore offer a step change in our understanding of bedform formation and dynamics in these, and many other, sedimentary environments.Publisher PDFPeer reviewe

Turbidity currents transport prodigious volumes of sediment to the deep sea. But there are very few direct measurements from oceanic turbidity currents, ensuring they are poorly understood. Recent studies have used acoustic Doppler current profilers (ADCPs) to measure velocity profiles of turbidity currents. However, there were no detailed measurements of sediment concentration, which is a critical parameter because it provides the driving force and debate centers on whether flows are dilute or dense. Here we provide the most detailed measurements yet of sediment concentration in turbidity currents via a new method using dual‐frequency acoustic backscatter ADCP data. Backscatter intensity depends on size and concentration of sediment, and we disentangle these effects. This approach is used to document the internal structure of turbidity currents in Congo Canyon. Flow duration is bimodal, and some flows last for 5–10 days. All flows are mainly dilute (<10 g/L), although faster flows contain a short‐lived initial period of coarser‐grained or higher‐concentration flow within a few meters of the bed. The body of these flows tends toward a maximum speed of 0.8–1 m/s, which may indicate an equilibrium in which flow speeds suspend available sediment. Average sediment concentration and flow thickness determine the gravitational driving force, which we then compared to average velocities. This comparison suggests surprisingly low friction values, comparable to or less than those of major rivers. This new approach therefore provides fundamental insights into one of the major sediment transport processes on Earth.

Until recently, despite being one of the most important sediment transport phenomena on Earth, few direct measurements of turbidity currents existed. Consequently, their structure and evolution were poorly understood, particularly whether they are dense or dilute. Here, we analyze the largest number of turbidity currents monitored to date from source to sink. We show sediment transport and internal flow characteristic evolution as they runout. Observed frontal regions (heads) are fast (>1.5 m/s), thin (<10 m), dense (depth averaged concentrations up to 38% _vol ), strongly stratified, and dominated by grain-to-grain interactions, or slower (<1 m/s), dilute (<0.01% _vol ), and well mixed with turbulence supporting sediment. Between these end-members, a transitional flow head exists. Flow bodies are typically thick, slow, dilute, and well mixed. Flows with dense heads stretch and bulk up with dense heads transporting up to 1000 times more sediment than the dilute body. Dense heads can therefore control turbidity current sediment transport and runout into the deep sea.