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.

Underwater communication cables are essential components of global infrastructure, carrying over 99% of international data traffic. On 14 March 2024, a significant disruption to this network occurred because of a cable break offshore Ivory Coast, leading to widespread internet outages in the West African region. To investigate the cause of this cable break, we analyze hydroacoustic data recorded between 6 and 22 March on the two hydrophone triads installed near Ascension Island by the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organization. We detect a low-frequency (<60 Hz) signal on three northern and two southern triad hydrophones on 12 March 2024. The signal had a duration of 85 s on the north triad compared to 45 s on the south triad. We used the generalized cross correlation with phase transform method to show that the detected signal originated at a bearing of 38.9° ± 4.6°, consistent with the location of the cable break off Ivory Coast, and with steep bathymetric slopes mapped in the Trou Sans Fond Canyon. We do not observe associated signals on the nearby land-based seismic stations in Ghana and Ivory Coast, confirming the marine origin of this event. In addition, template matching shows that the same signal was not recorded in the preceding and following eight days, implying that this event was an isolated case. Given the scarcity of natural earthquakes offshore Ivory Coast, this combination of evidence suggests that the hydroacoustic signals are likely caused by a submarine landslide in the Trou Sans Fond Canyon. Our results show that an investigation of the causative submarine landslide events is also needed to realize the potential of these hydroacoustic methods for hazard risk assessment and mitigation.

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 × 10 6 m 3 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.

Abstract Turbidity currents carve Earth’s deepest canyons, form Earth’s largest sediment deposits, and break seabed telecommunications cables. Directly measuring turbidity currents is notoriously challenging due to their destructive impact on instruments within their path. This is especially the case for canyon-flushing flows that can travel >1000 km at >5 m/s, whose dynamics are poorly understood. We deployed ocean-bottom seismometers safely outside turbidity currents, and used emitted seismic signals to remotely monitor canyon-flushing events. By analyzing seismic power variations with distance and signal polarization, we distinguish signals generated by turbulence and sediment transport and document the evolving internal speed and structure of flows. Flow-fronts have dense near-bed layers comprising multiple surges with 5-to-30-minute durations, continuing for many hours. Fastest surges occur 30–60 minutes behind the flow-front, providing momentum that sustains flow-fronts for >1000 km. Our results highlight surging within dense near-bed layers as a key driver of turbidity currents’ long-distance runout.

The Mid-Norwegian continental margin is home to multiple submarine landslides of prodigious volume (megaslides), thought to have occurred once each glacial cycle over the last -500 kyr. Here, we present new 2D seismic reflection data that image the headwall of a previously unknown mega-slide buried beneath Quaternary sediment in the North Sea Fan. The deposits linked to this mega-slide, named the Solsikke Slide, have a volume exceeding 15,000 km3. This is five times greater than the neighboring tsunamigenic Holocene Storegga Slide's volume, and makes the Solsikke Slide one of the largest landslides yet discovered on our planet. The Solsikke Slide remobilized deposits from the previous three mega-slides within the North Sea Fan, including the Tampen Slide that is thought to have occurred ca. 120 kyr BP. Together with the overlying deposits, this constrains the timing of the Solsikke Slide to between 120 and 57 kyr BP (MIS 5-3), which indicates that mega-slides may happen more frequently than previously thought along this margin. Furthermore, we identify a network of faults that extend up from the acoustic basement to the base of the Solsikke headwall, and seismic evidence of lateral fluid migration within the underlying unit. These observations suggest seismic shaking and fluid migration, together with sediment loading, likely played an important role in pre-conditioning and/or triggering failure. Our findings thus have significant implications for understanding the hazard posed by mega-slides in the Northern Atlantic, in terms of their frequency, potential magnitude, and pre-conditioning factors.

The largest canyons on Earth occur on the seafloor, and seabed sediment flows called turbidity currents play a key role in carving these submarine canyons. However, the processes by which turbidity currents erode submarine canyons are very poorly documented and understood. Here we analyse the first detailed time-lapse bathymetric surveys of a large submarine canyon, and its continuation as a less-deeply incised channel. These are also the most comprehensive time-lapse surveys before and after a major canyon-channel flushing turbidity current. These unique field data come from the Congo Submarine Fan offshore West Africa, where canyon flushing turbidity currents between 2019 and 2020 eroded 2.65 km3 of seabed sediment, as they travelled for over 1100 km at speeds of 5-8 m/s. This eroded sediment volume is equivalent to 19-33 % of global sediment flux from all rivers to the oceans. The time-lapse surveys cover 40 % of the 1100 km long submarine canyonchannel. They show that erosion was predominantly (94 %) along the canyon-channel axis, with only 6 % from failures along canyon or channel flanks. However, erosion along the canyon-channel floor was very patchy; some areas were eroded to depths of 10-20 m, whilst intervening areas showed no significant change. Knickpoints with up-slope migrating headscarps account for 22 % of the total eroded volume. One knickpoint in the deep-sea channel migrated by 21 km in one year, making it the fastest moving submarine knickpoint yet documented. Most (62 %) eroded sediment was in zones extending across the canyon or channel floor, without distinct headscarps as is the case for knickpoints. Erosion restricted to outer bends only comprised 10 % of the total, suggesting processes of erosion differ significantly from meandering rivers in which outer bend erosion is more important. Patchy seabed erosion appears to be mainly due to flow-bed processes (e.g. knickpoints), but spatial variations in seabed sediment properties may also play a role. The irregular seabed erosion occurs despite near-uniform flow speeds observed between moorings and submarine cable breaks with spacing of tens to hundreds of kilometers. Patchy and localised erosion has important implications for assessing hazards to seabed telecommunication cables, which are more likely to break in areas of deep erosion, and for creating appropriate numerical models of seabed erosion and turbidity current behaviour, or how to interpretate ancient submarine canyons and channels in rock outcrops.

The transfer of carbon from land to the near-coastal ocean is increasingly being recognized in global carbon budgets. However, a more direct transfer of terrestrial organic carbon to the deep sea is comparatively overlooked. Among systems that connect coastal to deep-sea environments, the submarine Congo Canyon is of particular interest since the canyon head starts 30 km into the Congo River estuary, which delivers similar to 7 % of the dissolved and particulate organic carbon from the world's rivers. However, sediment and particulate organic carbon transport mechanisms that operate in the Congo Canyon and submarine canyons more globally are poorly constrained compared to rivers because monitoring of deep-sea canyons remains challenging. Using a novel array of acoustic instruments, sediment traps, and cores, this study seeks to understand the hydrodynamic processes that control delivery of particulate organic carbon via the submarine Congo Canyon to the deep sea. We show that particulate organic carbon transport in the canyon axis is modulated by two processes. First, we observe periods where the canyon dynamics are dominated by tides, which induce a background oscillatory flow (speeds of up to 0.15 ms-1) through the water column, keeping muds in suspension, with a net upslope transport direction. Second, fast-moving (up to 8 ms-1) turbidity currents occur for 35 % of the time during monitoring periods and transport particulate organic carbon with mud and sand at an estimated transit flux that is more than 3 to 6 times the flux induced by tides. Organic carbon transported and deposited in the submarine canyon has a similar isotopic composition to organic carbon in the Congo River and in the deep-sea fan at 5 km of water depth. Episodic turbidity currents thus promote efficient transfer of river-derived particulate organic carbon in the Congo submarine fan, leading to some of the highest terrestrial carbon preservation rates observed in marine sediments globally.

Submarine canyons and channels are globally important pathways for sediment, organic carbon, nutrients and pollutants to the deep sea, and they form the largest sediment accumulations on Earth. However, studying these remote submarine systems comprehensively remains a challenge. In this study, we used the only complete-coverage and repeated bathymetric surveys yet for a very large submarine system, which is the Congo Fan off West Africa. Our aim is to understand channel-modifying features such as subaqueous landslides, meander-bend evolution, knickpoints and avulsions by analyzing their morphometric characteristics. We used a new approach to identify these channel-modifying features via morphometric fingerprints, which allows a systematic and efficient search in low-resolution bathymetry data. These observations have led us to identify three morphodynamic reaches within the Congo Canyon-Channel. The upper reach of the system is characterized by landslides that can locally block the channel, storing material for extended periods and re-excavating material through a new incised channel. The middle reach of the system is dominated by the sweep and swing of meander bends, although their importance depends on the channel’s age, and the time since the last up-channel avulsion. In the distal and youngest part of the system, an upstream migrating knickpoint is present, which causes multi-stage sediment transport and overspill through an underdeveloped channel with shallow depths. These findings complement previous less-detailed morphometric analyses of the Congo Canyon-Channel, offering a clearer understanding of how submarine canyon-channels can store sediment (due to channel-damming landslides, meander point bars, levee building due to overspill), re-excavate that sediment (via thalweg incision, meander propagation, knickpoint migration) and finally transport it to the deep sea. This improved understanding of the morphodynamics of the Congo Canyon-Channel may help to understand the evolution of other submarine canyon-channels, and assessment of hazards faced by seabed infrastructure such as telecommunication cables.

Burial of organic carbon in marine sediments is a long-term sink of atmospheric CO2, and submarine turbidity currents are volumetrically the most important sediment transport process on Earth. Yet the processes, amounts, and efficiency of organic carbon transfer by turbidity currents through submarine canyons to the deep sea are poorly documented and understood. We present an organic carbon budget for the submarine Congo Canyon, offshore West Africa, constrained with time-lapse bathymetry, sediment cores, and flow monitoring, including the effects of two >1000-km-runout canyon-flushing turbidity currents. In one year, flows eroded an estimated 6.09 +/- 2.70 Mt of previously buried terrestrial organic carbon in the canyon, primarily from fine-grained and vegetation-rich muddy sand facies with high organic carbon contents (as high as 11%). The age and composition of organic carbon in the Congo Canyon is comparable to those in the Congo River, indicating that transfer is efficient. Over the whole canyon-channel system, we extrapolate that 43 +/- 15 Mt of organic carbon was eroded and transported to the deep (>5 km) sea, equivalent to 22% of the annual global particulate organic carbon export from rivers to oceans and 54%-108% of the predicted annual terrestrial organic carbon burial in the oceans. Canyon-flushing turbidity currents carried a globally significant mass of terrestrial organic carbon down just one submarine canyon in a single year, indicating their importance for redistribution and delivery of organic carbon to the deep sea.

Abstract 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.

Subsea cables underpin global communications, carrying more than 99 % of all digital data traffic worldwide. While this >1.6 million km-long network has been designed to be highly resilient, subsea cables can be damaged by a number of natural hazards that occur across all water depths in the ocean. Here, we explore the diversity of natural hazards that can damage cables, considering a broad frequency-magnitude spectrum. This paper is the first global perspective of actual and potential hazards affecting cables. As such, it is an accessible overview of the regional variability and complexity of hazards. Relatively rare and extreme events, such as super typhoons, submarine landslides or associated turbidity currents and volcanic eruptions, can synchronously cause widespread damage to multiple systems, in some cases disconnecting entire countries or dramatically slowing data traffic. We show that damage is rarely linked to an initial event, instead arising from cascades of processes that can lag by years. Not all instances of cable damage that relate to natural processes are linked to extreme events. We show that much smaller intensity meteorological and oceanographic processes such as storms and continuous seafloor currents that have been overlooked by previous studies can also damage subsea cables. New analysis of past instances of cable damage reveals that a significant proportion of previously unattributed faults may relate to such low-level but sustained impacts. It is these hazards that are most likely to change in frequency and magnitude in response to ongoing climate change but are also more predictable. Through mapping of exposure to these different hazards, we identify geographically-constrained hazard hotspots and identify various mitigation measures to enhance the evidence base and further strengthen subsea telecommunications network resilience.

Submarine channels are conduits for sediment-laden flows called turbidity currents, which play a globally significant role in the offshore transport of sediment and organic carbon and pose a hazard to critical seafloor infrastructure. Time-lapse repeat surveys of active submarine channels have recently shown that upstream-migrating knickpoints can dominate channel evolution. This finding contrasts with many studies of ancient outcrops and subsurface geophysical data that inferred channel bends migrate laterally, as occurs in meandering rivers. Here, we aim to test these two contrasting views by analysing two high-resolution repeat seafloor surveys acquired 13 years apart across the entirety of an active submarine channel in Knight Inlet, British Columbia. We find that two main mechanisms control channel evolution, with the normalised channel radius of curvature (specifically, R* - channel radius of curvature normalised to channel width) explaining which of these mechanisms dominate. Pronounced outer bend migration only occurs at tight bends (R*<1.5). In contrast, at broader bends and straighter sections (R*>1.5), erosion is focused within the channel axis, where upstream-migrating knickpoints dominate. High centrifugal accelerations at tight bends promote super-elevation of flows on the outer channel flank, thus, enhancing outer bend erosion. At R*>1.5, flow is focused within the channel axis, promoting knickpoints that migrate upstream at an order of magnitude faster than the rate of outer bend erosion at tight bends. Despite the dominance of knickpoints in eroding the channel axis, their stratigraphic preservation is very low. In contrast, the lateral migration of channel bends results in much higher preservation via lateral accretion of deposits on the inner bend. We conclude that multiple mechanisms can control evolution at different channel reaches and that the role of knickpoints has been underestimated from past studies that focused on deposits due to their low preservation potential.

Large submarine flows of sediment (sand and mud), known as turbidity currents, transfer and bury significant amounts of organic carbon and pollutants to the deep sea via submarine canyons. They are also significant geohazards, regularly breaking networks of seabed telecommunications cables that carry > 99% of global data that underpin the internet. Despite this, key parameters (notably their sediment concentration) in these flows are yet to be directly measured in real-time due to their inherently harsh environment that is unsuitable for commercial conductivity sensors. To address this issue, a subsea datalogger (SSDL) is developed with a planar conductivity sensor head that can measure the sediment concentration within dense turbidity currents. Unlike conventional sensors, the planar design of the SSDL’s sensor prevents clogging at high sediment concentrations, allowing for continuous measurements within turbidity currents. The conductivity sensor is developed with a temperature sensor which is measured using an external 16-Bit ADC which is controlled with a SAMD21 32-Bit ARM microcontroller. The SSDL measures the temperature and the conductivity of the seawater once every 4 seconds for over a year. In an initial device test, the SSDL can record a turbidity current within the Bute Inlet, Canada. It is found that the seawater’s conductivity increases with salinity concentration and decreases with sediment concentration. The SSDL developed here can thus be used for both conventional subsea datalogging applications and high turbidity current applications.

Volcanic eruptions on land create hot and fast pyroclastic density currents, triggering tsunamis or surges that travel over water where they reach the ocean. However, no field study has documented what happens when large volumes of erupted volcanic material are instead delivered directly into the ocean. We show how the rapid emplacement of large volumes of erupted material onto steep submerged slopes triggered extremely fast (122 kilometers per hour) and long-runout (>100 kilometers) seafloor currents. These density currents were faster than those triggered by earthquakes, floods, or storms, and they broke seafloor cables, cutting off a nation from the rest of the world. The deep scours excavated by these currents are similar to those around many submerged volcanoes, providing evidence of large eruptions at other sites worldwide.

Submarine landslides pose a hazard to coastal communities and critical seafloor infrastructure, occurring on all of the world's continental margins, from coastal zones to hadal trenches. Offshore monitoring has been limited by the largely unpredictable occurrence of submarine landslides and the need to cover large regions. Recent subsea monitoring has provided new insights into the preconditioning and run-out of submarine landslides using active geophysical techniques. However, these tools measure a small spatial footprint and are power- and memory-intensive, thus limiting long-duration monitoring. Most landslide events remain unrecorded. In this chapter, we first show how passive acoustic and seismologic techniques can record acoustic emissions and ground motions created by terrestrial landslides. This terrestrial-focused research has catalyzed advances in characterizing submarine landslides using onshore and offshore networks of broadband seismometers, hydrophones, and geophones. We discuss new insights into submarine landslide preconditioning, timing, location, velocity, and down-slope evolution arising from these advances. Finally, we outline challenges, emphasizing the need to calibrate seismic and acoustic signals generated by submarine landslides. Passive seismic and acoustic sensing has a strong potential to enable more complete hazard catalogs to be built and open the door to emerging techniques (such as fiber-optic sensing) to fill key knowledge gaps.

Here we show how ultra‐high resolution seabed mapping using new technology can help to understand processes that sculpt submarine canyons. Time‐lapse seafloor surveys were conducted in the axis of Monterey Canyon, ∼50 km from the canyon head (∼1,840 m water depth) over an 18‐month period. These surveys comprised 5‐cm resolution multibeam bathymetry, 1‐cm resolution lidar bathymetry, and 2‐mm resolution stereophotographic imagery. Bathymetry data reveal centimeter‐scale textures that would be undetectable by more traditional survey methods. Upward‐looking Acoustic Doppler Current Profilers at the site recorded the flow character of internal tides and the passage of three turbidity currents, while sediment cores collected from the site record flow deposits. Combined with flow and core data, the bathymetry shows how turbidity currents and internal tides modify the seabed. The turbidity currents drape sediment across the site, infilling bedform troughs and smoothing erosional features carved by the internal tides (e.g., rippled scours). Turbidity currents with speeds of 0.9–3.3 m/s failed to cause notable bedform movement, which is surprising given that flows with similar speeds produced rapid bedform migration elsewhere, including the upper Monterey Canyon. The lack of migration may be related to the character of the underlying substrate or indicate that turbidity currents at the site lack dense, near‐bed layers. The scale of scours produced by the internal tides (≤0.7 m/s) approaches the scale of features recorded in the ancient rock record. Thus, these results illustrate how the scale gap between seabed mapping technology and the rock record may eventually be bridged.

Seafloor sediment flows, called turbidity currents, form the largest sediment accumulations, deepest canyons and longest channels on Earth. It was once thought that turbidity currents were impractical to measure in action, especially given their ability to damage sensors in their path, but direct monitoring since the mid-2010s has measured them in detail. In this Review, we summarize knowledge of turbidity currents gleaned from this direct monitoring. Monitoring identifies triggering mechanisms from dilute river plumes, and shows how rapid sediment accumulation can precondition slope failure, but the final triggers can be delayed and subtle. Turbidity currents are consistently more frequent than predicted by past sequence-stratigraphic models, including at sites >300 km from any coast. Faster flows (more than ~1.5 m s–1) are driven by a dense near-bed layer at their front, whereas slower flows are entirely dilute. This frontal layer sometimes erodes large (>2.5 km3) volumes of sediment, yet maintains a near-uniform speed, leading to a travelling-wave model. Monitoring shows that flows sculpt canyons and channels through fast-moving knickpoints, and shows how deposits originate. Emerging technologies with reduced cost and risk can lead to widespread monitoring of turbidity currents, so their sediment and carbon fluxes can be compared with other major global transport processes. Seafloor turbidity currents form Earth’s largest sediment accumulations, deepest canyons and longest channels, but their destructive nature makes them notoriously difficult to measure in action. This Review explores how insights from detailed direct measurements have advanced understanding of turbidity currents.

The increasing plastic pollution of the world’s oceans represents a serious threat to marine ecosystems and has become a well-known topic garnering growing public attention. The global input of plastic waste into the oceans is estimated to be approximately 10 million tons per year and predicted to rise by one order of magnitude by 2025. More than 90% of the plastic that enters the oceans is thought to end up on the seafloor, and seafloor sediment samples show that plastics are concentrated in confined morphologies and sedimentary environments such as submarine canyons. These canyons are occasionally flushed by powerful gravity-driven sediment flows called turbidity currents, which transport vast volumes of sediment to the deep sea and deposit sediment in deep-sea fans. As such, turbidity currents may also transport plastics present in the canyon and bury plastics in deep-sea fans. These fans may therefore act as sinks for seafloor plastics. Here we present a comprehensive dataset showing the spatial distribution of microplastics in seafloor sediments from the Congo Canyon, offshore West Africa. Multicores taken from 16 locations along the canyon, sampled different sedimentary sub-environments including the canyon thalweg, canyon terraces, and distal lobe. Microplastics were extracted from the sediments by density separation and the polymer type, size, and shape of all individual microplastic particles were analysed using laser-direct infrared-spectroscopy (LDIR). Microplastic number concentrations in the sediments of the distal lobe are significantly higher than in the canyon, indicating that the Congo Canyon system is a highly efficient conduit for microplastic transport to the deep sea. Moreover, microplastic concentrations of >20,000 particles per kg of dry sediment were recorded in the lobe, which represent some of the highest ever recorded microplastic number concentrations in seafloor sediments. This shows that deep-sea fans can serve as hotspots and potential terminal sinks for seafloor microplastics.

Quantification of the controls on turbidity current recurrence is required to better constrain land to sea fluxes of sediment, carbon and pollutants, and design resilient infrastructure that is vulnerable to such flows. This is particularly important offshore from river deltas, where sediment supply is high. Numerous mechanisms can trigger turbidity currents, even at a single river mouth. However quantitative analysis of recurrence and triggers has been limited to an individual trigger for each turbidity current due to the low number of precisely timed (via direct monitoring) flows. We are therefore yet to quantify if and how coincident processes combine to generate turbidity currents, and their relative importance. Here, we analyse the timing and causes of 113 turbidity currents directly-monitored from the source of turbidity current initiation to depositional sink in a single submarine channel. This submarine channel is located offshore from glacial-fed river-deltas at Bute Inlet, a fjord in British Columbia, Canada. Using a multivariate statistical approach, we demonstrate the statistical significance of combined river discharge and tidal controls on turbidity current occurrence during 2018, from which we derive a statistical model that calculates turbidity current probability for any given input of river discharge and water level. This new model predicts turbidity current activity with >84% success offshore other river deltas where flow timing is precisely constrained by directly monitoring, including the Squamish and Fraser River-deltas in British Columbia. We suggest that this model will be applicable for turbidity current prediction at glacial meltwater-fed fjords in many other regions worldwide.

Submarine turbidity currents form the largest sediment accumulations on Earth, raising the question of their role in global carbon cycles. It was previously inferred that terrestrial organic carbon was primarily incinerated on shelves and that most turbidity current systems are presently inactive. Turbidity currents were thus not considered in global carbon cycles, and the burial efficiency of global terrestrial organic carbon was considered low to moderate (∼10–44%). However, recent work has shown that burial of terrestrial organic carbon by turbidity currents is highly efficient (>60–100%) in a range of settings and that flows occur more frequently than once thought, although they were far more active at sea-level lowstands. This leads to revised global estimates for mass flux (∼62–90 Mt C/year) and burial efficiency (∼31–45%) of terrestrial organic carbon in marine sediments. Greatly increased burial fluxes during sea-level lowstands are also likely underestimated; thus, organic carbon cycling by turbidity currents could play a role in long-term changes in atmospheric CO 2 and climate.