The passage of turbidity currents within Monterey Canyon was measured with unprecedented precision (Fig. 2), enabling new insights into flow triggering and their internal structure. Fourteen events originated in Monterey Canyon in less than 290 mwd. The event which was first detected in 1290 mwd may have come from Soquel Canyon tributary (Fig. 2). Previous work mainly suggests that major events, such as river floods37, earthquakes38, or anomalously-large wave heights39 trigger turbidity currents28,29,40. None of the flows documented here are linked to earthquakes (>M w 2.0) and only the last event occurred when there was any significant discharge occurring in the Salinas River (Fig. 3c), which has been engineered to enter the ocean directly at the head of Monterey Canyon under low flow conditions. Fourteen of the fifteen flows occurred in the winter months (Fig. 3). These events typically coincide with large storm wave heights (Fig. 3b), which may have triggered seabed failure in the upper canyon. However, one of the most powerful flows (1 September 2016), which ran out at speeds of up to 5 m s−1 (Fig. 2) through the whole sensor array (Fig. 3), occurred in a period without large wave heights, floods or earthquakes. This event suggests that turbidity currents do not always require major external triggers. Small perturbations (e.g., normal wave heights) may cause seafloor failure that produces powerful and long runout flows (Figs. 2, 3).

Flow-front transit velocities between moorings and BEDS reached up to 7.2 m s−1, and typically exceeded the highest velocities measured by ADCPs (Figs. 2, 4). This key observation indicates that the fastest part of the flow is located within ≤2 m above the seabed, where ADCP measurements are compromised41, or within underlying remobilized seafloor.

Although sediment concentrations were not measured directly, our observations support the existence of a dense (i.e., ≫10%26,42 volume) remobilized layer for the following reasons. First, rafting and entombment within a dense layer of flowing sediment explains the successive exhumation, movement and burial of heavy objects (Fig. 4d). Similar successive down-canyon movements and burial were observed previously with prototypes of the BEDs31. It seems less likely that an entirely dilute flow, perhaps with a thin bedload layer, could transport these sometimes exceptionally heavy objects. If the 800 kg AMT-frame was entombed in a dense layer, then the thickness of that dense layer is at least comparable to its diameter (2 m). Second, temperature sensors on the heaviest object (800 kg AMT frame) ceased recoding tidal fluctuations, suggesting the AMT was most likely entombed within the remobilized bed, although it could have been exhumed for short periods between these measurements (Fig. 4c). Third, objects with very different size, shape and densities moved at broadly similar speeds down-canyon behind the flow front (Fig. 4b). This includes an irregularly shaped 800 kg AMT-frame, and much smaller and less dense BEDs (Fig. 4b). This is more consistent with rafting than being dragged beneath a dilute flow, where such objects with different size, shape and densities would be expected to travel at different speeds. Finally, flows that moved heavy objects are often <15 m thick, as documented by ADCP data (Fig. 4a). If these flows are entirely dilute, they are unlikely to displace entire >80 m high moorings with 450 kg anchors (Figs. 1, 2). Their motion is better explained by the anchors being rafted in a dense layer, rather than by drag on the mooring cable from a relatively-thin, dilute flow.

We lack detailed in situ seabed measurements of how dense remobilized layers originate. However, the floor of Monterey Canyon often comprises loose-packed sand that is susceptible to failure and liquefaction22,43. Indeed, liquefaction of canyon floor sand has been observed to be induced by vibration during coring operations (see ref. 44 supplementary video), or by divers45. Detailed measurements from partially water-saturated sediment below terrestrial debris flows with similar (4–15 m s−1) speeds are also informative46,47. They emphasize how contractive shear displacement of loose-packed substrates, and liquefaction, have a key role in substrate remobilization, as well as reducing basal friction46,47. Sudden undrained loading produces high pore pressures beneath the front of these large-scale subaerial debris flows, which erode the partly-water-saturated substrate at their front, such that the debris flow accelerates and is self-sustaining. The substrate on the floor of Monterey Canyon is fully water-saturated, and for reasonable values of sand permeability and basal shear rates, high pore pressures are likely to develop during flows47. We thus infer that liquefaction of loose-packed sand may have an important role in producing the fast-moving dense remobilized layer at the base of the turbidity current.

Models of submarine flows with a dense remobilized layer (Fig. 6a) must be consistent with the existence of crescent-shaped bedforms, which are ubiquitous along the floor of Monterey Canyon (Fig. 5)32,48. These bedforms have heights of 1 to 3 m, and wavelengths of 20 to 80 m (Fig. 5). Similar bedforms occur in many other sandy submarine canyons and channels worldwide32,48,49,50. They have been attributed to flow instabilities (termed cyclic steps) that develop within supercritical flows, which lead to hydraulic jumps and trains of up-slope migrating bedforms30,35,49,50,51,52,53. What is unknown is whether the bedforms are preserved during the dense sediment current processes observed here, which appear to remobilize the bed potentially through liquefaction.

Fig. 6 Conceptual drawings of sediment density flow events in Monterey Canyon. a The highest velocities (V 1 ) occur in a dense basal layer near the flow front. This dense basal layer forms via liquefaction or mechanical erosion of underlying loose-packed sand, and helps to generate trains of crescentic bedforms. b Increased turbidity in the water column is coincident with slowing of the remobilized layer. c The evolution of a flow as it progresses down canyon. (Stage 1) A failure in the canyon floor results in the liquefaction of the seafloor at the front of the flow, (Stage 2) it propagates down-canyon as a dense remobilized layer, (Stage 3) the fast flow progressively generates an expanding dilute turbulent cloud, (Stage 4) which continues as a dilute turbidity current Full size image

The motion data recorded by the BEDs, as they were carried down canyon, provide important information about when bedforms are present (Figs. 4, 7). The movements of individual BEDs probably reflect conditions a short distance behind the flow front (Fig. 6). Pressure records indicate BEDs often experienced vertical oscillations with amplitudes of 1–3 m, even for the BED attached to the 800 kg AMT-frame (Fig. 7). The high density of this AMT-frame (>6 g cm−3) suggests that it would move along the base of the flow. The amplitude and wavelengths of these vertical oscillations (Fig. 7b) are broadly similar to crescent-shaped bedforms (Fig. 5). Thus, these oscillations suggest that the AMT-frame traveled over bedforms, which were thus not wiped-clean by frontal plowing or other erosional processes (Fig. 6).

Fig. 7 Movement of massive 800 kg frame down canyon during the 24 November 2016 event. a Black line shows depth (converted from pressure) vs. time from BED 11 which was attached to the 800 kg frame (AMT/BED11; Fig. 4) during the 24 November 2016 event. Red line segments are polynomial fits to three sections of these data. BED 11 traveled at an average speed of 4 m s−1. b Deviations (blue line) of BED 11 trajectory (a: black line) from the fitted polynomials (a: red line) show vertical oscillations between 1–3 m Full size image

Suitable in situ physical properties measurements (e.g., pore pressure44,45) to determine exact processes of erosion and bedform generation near the flow front were not collected. However, field-observations and detailed laboratory experiments show that cyclic steps and up-slope migrating bedforms can form beneath supercritical flows with very high (20–40% volume) sediment concentrations54,55, as well as beneath dilute supercritical flows48,49,50,51,52. Previous work notes that bedform migration below dense near-bed layers can be accompanied by local bed liquefaction54, and bedform dimensions may be controlled by properties of this dense near-bed layer55. We therefore propose that the frontal part of the flow liquefies (and possibly also mechanically erodes) the sandy canyon-floor, helping to sustain a dense near-bed layer below which bedforms persist and develop. Our time-lapse surveys are also too infrequent to distinguish between models in which the flow-front wipes out pre-existing bedforms, and new bedforms are created; or flow simply modifies these pre-existing bedforms (Fig. 6). Bedforms may be sculpted further by the dilute trailing body of the event, which itself may be supercritical (Fig. 6).

We conclude with a model (Fig. 6) for the evolution and anatomy of turbidity currents, based on these novel field data. Turbidity currents are initiated in the upper canyon mainly by failure within the loosely packed sand in the canyon axis or within sediment draping the flanks of the canyon (Fig. 6c). The failure and liquefaction creates a dense fast-moving layer (dense remobilized layer) that accelerates downslope (Fig. 6b; Fig. 6c Stages 1 and 2). Erosion and liquefaction of canyon-floor sand behind the flow front produces a self-sustaining, fast and dense basal layer, which drives the overall flow-event. Migration of the crescent-shaped bedforms underneath the dense remobilized layer, as a consequence of substrate erosion on the lee side and deposition on the down-canyon stoss side, explains the ±3 m amplitude bathymetric change observed between repeat AUV surveys (Fig. 5).

Shear between the dense remobilized layer and overlying water causes mixing (Fig. 6b; Fig. 6c Stages 2 and 3) that generates an overlying dilute, turbulent sediment-suspension (Fig. 6a; V 2 and V 3 ). A few minutes (~2–5 min) after arrival of the flow front, the velocity of the dense remobilized layer declines (Fig. 6b; V 4 ). This is demonstrated by relaxation of the mooring cable after its initial abrupt pull down. While the initial powerful, fast, dense, remobilized layer dies out, the dilute turbulent sediment flow that it spawns can last for hours (Figs. 2 and 6b; Fig. 6c Stage 4).

Turbidity currents have previously been compared to rivers. However, our work suggests that this comparison is not always justified, as their basic structure can be fundamentally different56,57. Rivers are almost always entirely dilute sediment suspensions, with dense bedload layers that are only a few grains thick58. Rivers lack the dense remobilized layers that are several meters thick, which we document in these turbidity currents (Fig. 6). These dense basal layers can carry exceptionally heavy (800 kg) objects, at speeds of >4 m s−1 approaching that of the flow front, for kilometres. This study also shows that powerful turbidity currents do not need major external triggers. It thus documents a new view of submarine flows that dominate sediment transfer via canyons into the deep-sea.