Turbidity currents, and other types of submarine sediment density flow, redistribute more sediment across the surface of the Earth than any other sediment flow process, yet their sediment concentration has never been measured directly in the deep ocean. The deposits of these flows are of societal importance as imperfect records of past earthquakes and tsunamogenic landslides and as the reservoir rocks for many deep-water petroleum accumulations. Key future research directions on these flows and their deposits were identified at an informal workshop in September 2013. This contribution summarizes conclusions from that workshop, and engages the wider community in this debate. International efforts are needed for an initiative to monitor and understand a series of test sites where flows occur frequently, which needs coordination to optimize sharing of equipment and interpretation of data. Direct monitoring observations should be combined with cores and seismic data to link flow and deposit character, whilst experimental and numerical models play a key role in understanding field observations. Such an initiative may be timely and feasible, due to recent technological advances in monitoring sensors, moorings, and autonomous data recovery. This is illustrated here by recently collected data from the Squamish River delta, Monterey Canyon, Congo Canyon, and offshore SE Taiwan. A series of other key topics are then highlighted. Theoretical considerations suggest that supercritical flows may often occur on gradients of greater than , 0.6u. Trains of up-slope-migrating bedforms have recently been mapped in a wide range of marine and freshwater settings. They may result from repeated hydraulic jumps in supercritical flows, and dense (greater than approximately 10% volume) near-bed layers may need to be invoked to explain transport of heavy (25 to 1,000 kg) blocks. Future work needs to understand how sediment is transported in these bedforms, the internal structure and preservation potential of their deposits, and their use in facies prediction. Turbulence damping may be widespread and commonplace in submarine sediment density flows, particularly as flows decelerate, because it can occur at low (, 0.1%) volume concentrations. This could have important implications for flow evolution and deposit geometries. Better quantitative constraints are needed on what controls flow capacity and competence, together with improved constraints on bed erosion and sediment resuspension. Recent advances in understanding dilute or mainly saline flows in submarine channels should be extended to explore how flow behavior changes as sediment concentrations increase. The petroleum industry requires predictive models of longer-term channel system behavior and resulting deposit architecture, and for these purposes it is important to distinguish between geomorphic and stratigraphic surfaces

Rivers (on land) and turbidity currents (in the ocean) are the most important sediment transport processes on Earth. Yet how rivers generate turbidity currents as they enter the coastal ocean remains poorly understood. The current paradigm, based on laboratory experiments, is that turbidity currents are triggered when river plumes exceed a threshold sediment concentration of ~1 kg/m3. Here we present direct observations of an exceptionally dilute river plume, with sediment concentrations 1 order of magnitude below this threshold (0.07 kg/m3), which generated a fast (1.5 m/s), erosive, short‐lived (6 min) turbidity current. However, no turbidity current occurred during subsequent river plumes. We infer that turbidity currents are generated when fine sediment, accumulating in a tidal turbidity maximum, is released during spring tide. This means that very dilute river plumes can generate turbidity currents more frequently and in a wider range of locations than previously thought.

8Submarine canyons are one of the most important pathways for sediment transport into ocean 9 basins. For this reason, understanding canyon architecture and sedimentary processes has 10 importance for sediment budgets, carbon cycling, and geohazard assessment. Despite increasing 11 knowledge of turbidity current triggers, the down-canyon variability in turbidity current frequency 12 within most canyon systems is not well constrained. New AMS radiocarbon chronologies from 13 canyon sediment cores illustrate significant variability in turbidity current frequency within Nazaré 14 Canyon through time. Generalised linear models and Cox proportional hazards models indicate a 15 strong influence of global sea level on the frequency of turbidity currents within the canyon. 16Radiocarbon chronologies from basin sediment cores indicate that larger, canyon-flushing turbidity 17 currents reaching the Iberian Abyssal Plain have a significantly longer average recurrence interval 18 than turbidity currents that fill the canyon. The recurrence intervals of these larger turbidity currents 19 also appear to be unaffected by long-term changes in global sea level. This indicates that the factors 20 triggering, and thus controlling, the frequency of canyon-flushing and canyon-filling events are very 21 different. Canyon-filling appears to be predominantly controlled by sediment instability during sea 22 level lowstand and by storm and nepheloid transport during the present day highstand. Canyon-23 flushing exhibits time-independent behaviour. This indicates that a temporally random process, or 24 summation of non-random processes that cannot be discerned from a random signal, are triggering 25 canyon flushing events. 26