Matthieu J. B. Cartigny

Submarine landslides threaten seabed infrastructure and trigger the transport of globally important amounts of sediment, carbon, and pollutants to deep-sea environments. However, fully submarine landslides have never been directly monitored in nature. Past hypotheses for their failure mechanism and dynamics therefore remain unvalidated for natural settings, including that prolonged upslope-migrating failures (breaching) occur in sandy seafloor environments. In this work, we present direct underwater monitoring of fully submarine landslides at Homathko Delta in western Canada that retrogress slowly upslope, indicating breaching-type failure. Their prolonged rather than instantaneous nature has important implications for the stress that seabed structures must be designed to withstand. Prolonged landslides also initiate sustained sediment flows (turbidity currents) that efficiently transport sediment, carbon, and pollutants further offshore.

Wave‐induced seabed liquefaction can enhance coastal sediment suspension, but the wave‐cycle‐scale influence of interfacial‐wave motion remains poorly understood. Wave‐flume experiments were conducted on a silty bed under two regular‐wave conditions to compare near‐bed hydrodynamics and sediment suspension between a non‐liquefied condition and a liquefied condition. Under the non‐liquefied condition, suspended sediment concentration (SSC) exhibited a double‐peaked intra‐wave pattern and generally varied with turbulent kinetic energy and Reynolds shear stress, suggesting predominantly shear‐related suspension. After liquefaction, sustained interfacial waves developed at the mudline, with a root‐mean‐square amplitude of approximately 1.4 cm. Around the onset of sustained interfacial‐wave motion, near‐bed SSC increased from approximately 1.3 to 2.6 g L ⁻¹ within 4–5 wave cycles, the intra‐wave SSC distribution shifted to a single‐peaked pattern, and its relationship with Reynolds shear stress weakened. During rapid mudline descent, upward turbulent sediment transport was strongest and accounted for approximately 93% of the positive cycle‐integrated turbulent sediment flux. Under the experimental conditions examined, these observations suggest that interfacial‐wave motion may provide an additional phase‐dependent mechanism modulating sediment suspension over a liquefied seabed, offering a physical basis for improving the representation of sediment transport over liquefied beds.

Self-accelerating turbidity currents (SATCs) are hypothesized to be the primary mechanism for transporting vast amounts of sediment to the deep ocean. However, theoretical predictions of SATCs have preceded field observations by decades, leaving a critical gap in our understanding of this long-distance delivery process. Here, we present results from a four-year field survey of turbidity currents and bathymetric evolution in the Xiaolangdi Reservoir on the Yellow River (China), the world’s most sediment-laden river. We provide definitive in situ evidence of SATCs, characterized by synchronous down-channel increases in sediment mass and current momentum, alongside massive channel incision at the event scale. Notably, these SATCs occur in a low-gradient lacustrine setting, challenging the prevailing hypothesis that such phenomena are restricted to steep submarine channels or high velocity conditions. We identify a dimensionless threshold, incorporating current velocity, channel slope, and sediment settling velocity, that governs SATC formation across sublacustrine and marine environments. This threshold provides a robust framework for predicting SATC occurrence and informs engineering strategies to sustain reservoir capacity and restore sediment connectivity in dammed river systems.

Two sediment cores were collected from a submarine sediment wave field on the flank of Macauley volcano. Combined with seismic reflection surveys and numerical modeling, we present an integrated model of the formation of large‐scale, eruption‐fed, submarine sediment waves. The sediment waves are large (>100 m high, 1.5 km wide, tens of km long) and composed of juvenile ash and pumice lapilli of rhyolitic to dacitic composition. The sediment waves are interpreted to be related to the 6.3 ka caldera‐forming eruption and Sandy Bay Tephra deposit on nearby Macauley Island. The sediment waves were deposited as cyclic steps and antidunes under supercritical, syn‐eruptive density currents. Facies analysis indicates that the bedforms developed as amalgamated sets of m‐sized bedforms. Numerical modeling and seismic reflection data show that early‐stage density currents were rapid and erosive, whereas later currents persisted over days to weeks. Our data from Macauley volcano is compared to the submarine sediment waves from the 2022 eruption of Hunga volcano, and other known eruption‐fed sediment waves. We discuss implications for the modeling of volcanic eruptions from their submarine deposits, as bedforms associated with supercritical density flows may be confused with volcanic turbidites that have critically different deposition and distribution patterns. Further, the self‐erosive nature of supercritical density currents produces deposits composed of volcanic, syn‐eruptive material that has been continuously mixed through deposition and may obscure any stratigraphic trends associated with eruption dynamics. We discuss the implications for tsunami prediction and modeling associated with the unpredictable nature of these events.

Glacial lake outburst floods can transport large volumes of sediment. Where these floods reach the coastline, much of the particulate matter is delivered directly to the marine environment. It has been suggested that offshore deposits, specifically in fjord settings, may provide a faithful record of past outburst flood events. However, a lack of observations means that the mechanics and the timing of sediment transport offshore following a glacial lake outburst event remain poorly constrained. Here, we document the changes in sea surface sediment dynamics following the 28 November 2020 Elliot Lake outburst flood in British Columbia, which transported ∼4.3 × 10⁶ m³ of sediment into an adjacent fjord (Bute Inlet) as a deep nepheloid layer directly following the event. However, analysis of sea surface turbidity using in situ measurements and satellite‐derived estimates reveals that changes in fjord‐head surface turbidity in the months following the major flood were surprisingly small. The highest measured sea surface turbidity instead occurred 5 months after the initial outburst flood. This delayed increase in seaward sediment flux coincided with the onset of the spring freshet, when the discharge of the rivers feeding Bute Inlet increases each year. We suggest that large quantities of sediment were temporarily stored within the river catchment and were only remobilized when river discharge exceeded a threshold level following seasonal snowmelt. Our results reveal a temporal disconnect, where onshore to offshore transfer of sediment is stepped following a glacial lake outburst flood, which could complicate the sedimentology of subsequent deposits.

Turbidity currents, which are stratified, sediment-laden bottom flows in the ocean or lakes, can run out for hundreds or thousands of kilometres in submarine channels without losing their stratified structure. Here, we derive a layer-averaged, two-layer model for turbidity currents, specifically designed to capture long-runout. A number of previous models have captured runout of only tens of kilometres, beyond which thickening of the flows becomes excessive, and the models without a lateral overspill mechanism fail. In our framework, a lower layer containing nearly all the sediment is a faster, gravity-driven flow that propels an upper layer, where sediment concentration is nearly zero. The thickness of the lower layer is controlled by competition between interfacial water entrainment due to turbulent mixing and water detrainment due to sediment settling at the interface. The detrainment mechanism, first identified in experiments, is the key feature that prevents excessive thickening of the lower layer and allows long-runout. Under normal flow conditions, we obtain an exact solution to the two-layer formulation, revealing a constant velocity and a constant thickening rate in each of the two layers. Numerical simulations applied to gradually varied flows on both constant and exponentially declining bed slopes, with boundary conditions mimicking field observations, show that the predicted lower layer thickness after 200 km flow propagation compares with observed submarine channel depths, whereas previous models overestimate this thickness three- to fourfold. This formulation opens new avenues for modelling the fluid mechanics and morphodynamics of long-runout turbidity currents in the submarine setting.

Sediment waves are widely observed on the seafloor in a variety of marine environments (e.g., open slope, submarine channels, levees). They are important for understanding marine hazards because they can influence slope stability and be indicators of currents capable of damaging seafloor infrastructure (e.g., telecommunication cables). However, sediment wave dynamics may vary in different settings and several mechanisms have been invoked to explain their formation including gravity-driven (sediment failure, turbidity currents) and oceanographic (bottom currents, internal tides) processes. In this study, we investigate the generation of large unconfined sediment wave fields along the continental slope of the Northwest African margin using an integrated dataset acquired on the R/V Maria S. Merian cruise MSM113. Data collection included direct monitoring of ocean currents and water column properties over sediment wave fields by CTD casts, acoustic water column profiling, and deployment of short-term moorings equipped with velocity (ADCPs), temperature, salinity, and turbidity sensors. Additional datasets such as shallow and multichannel 2D seismic profiles, multibeam bathymetry, gravity cores, and box cores capture the geomorphic, subsurface, and sedimentary characteristics of the seafloor features. Sediment wave fields occur on the mid-lower slope between 600 – 1900 m water depths and are intersected by straight channels up to 2 km wide and 300 m deep. Individual waves have slope-parallel crests, wavelengths between 400 – 2000 m, and wave heights between 6 – 56 m. In subsurface seismic profiles, sediment waves are composed of upslope-stacking reflectors that indicate preferential deposition on their stoss slopes and upslope crest migration. Sediment cores from sediment waves are predominantly composed of bioturbated gradational sequences of mud, sandy mud, muddy sand, and sand that vary depending on location, suggesting a progressive process of differential sedimentation. Intermittent chaotic muddy deposits and sharp-based sand layers represent occasional punctuated flow events. Time series from moored instruments are dominated by strong semidiurnal tidal fluctuations with current velocities up to 0.3 m/s. Water column measurements and acoustic images reveal a stratified water column with wavy interfaces and small-scale fluctuations caused by the passage of internal waves. Collectively, these findings suggest that downslope gravity flows, along-slope currents, and internal tides contribute to sediment transport along the Northwest African margin; although, tide-topographic interactions are the most likely candidate for maintaining sediment waves. Our integrated analysis provides insight into oceanographic processes, which shape the seafloor and transport sediment along ocean margins.

Turbidity currents carve the deepest canyons on Earth, deposit its largest sediment accumulations, and break seabed telecommunication cables. Powerful canyon‐flushing turbidity currents break sensors placed in their path, making them notoriously challenging to measure, and thus poorly understood. This study provides the first remote measurements of canyon‐flushing flows, using ocean‐bottom seismographs located outside the flow's destructive path, revolutionizing flow monitoring. We recorded the internal dynamics of the longest sediment flows yet monitored on Earth, which traveled >1,000 km down the Congo Canyon‐Channel at 3.7–7.6 m s ⁻¹ and lasted >3 weeks. These observations allow us to test fundamental models for turbidity current behavior and reveal that flows contain dense and fast frontal‐zones up to ∼400 km in length. These frontal‐zones developed near‐uniform durations and speeds for hundreds of kilometres despite substantial seabed erosion, enabling flows to rapidly transport prodigious volumes of organic carbon, sediment, and warm water to the deep‐sea.

Glacial lake outburst floods can transport large volumes of sediment. Where these floods reach the coastline, much of that particulate load is delivered directly to the marine environment. It has been suggested that offshore deposits, specifically in fjord settings, may provide a faithful record of the frequency and timing of past outburst flood events. However, a paucity of observations means that the mechanics and the timing of offshore transport of sediment following a glacial lake outburst event remain poorly constrained. Here, we document the changes in sea surface sediment dynamics following the 28th November 2020 Elliot Lake outburst flood in British Columbia, which transported ~4.3x106 m3 of sediment into an adjacent fjord (Bute Inlet) as a deep nepheloid layer directly following the event. However, analysis of sea surface turbidity using in situ measurements and satellite-derived estimates reveals that changes in fjord-head surface turbidity immediately following the major flood were surprisingly small. The highest measured sea surface turbidity instead occurred five months after the initial outburst flood. This delayed increase in seaward sediment flux was coincident with the onset of the spring freshet, when discharge of the rivers feeding Bute Inlet increase each year. We suggest that large quantities of sediment were temporarily stored within the river catchment, and only remobilised when river discharge exceeded a threshold level following seasonal snowmelt. Our results reveal a temporal disconnect, where onshore to offshore transfer of sediment is stepped following a glacial lake outburst flood, which could complicate the architecture of subsequent deposits.