Submarine canyons are important conduits for microplastic transport to the deep sea via turbidity currents. However, other near-bed oceanographic flows and sub-seafloor processes may play an important role in the transport and burial of microplastics. We use sediment push-cores for microplastic and sediment grain-size analysis from two transects across the Whittard Canyon, UK, to show that complex process-interactions control the transport and burial of microplastics and semi-synthetic microfibres in the thalweg and on the canyon flanks. Microplastic pollution is pervasive across the canyon at both transects, from the thalweg and from 500 m higher on the flanks, despite turbidity currents being confined to the canyon thalweg. Furthermore, we calculate sediment accumulation rates from 210Pb dating and show that microplastic concentrations remain similar at sediment depths down to 10 cm. This reveals that the huge global-increase in plastic production rates over time is not recorded, and that microplastics are present in sediments that pre-date the mass-production of plastic. The interaction of turbidity currents, deep-tidally-driven currents, and sub-seafloor processes shreds any potential signal that microplastics may provide as indicators of historical plastic production rates, which undermines the utility of microplastics as reliable markers of the onset of the Anthropocene.

Abstract Submarine channels are key features for the transport of flow and nutrients into deep water. Previous studies of their morphology and channel evolution have treated these systems as abiotic, and therefore assume that physical processes are solely responsible for morphological development. Here, a unique dataset is utilised that includes spatial measurements around a channel bend that hosts active sediment gravity flows. The data include flow velocity and density, alongside bed grain size and channel‐floor benthic macrofauna. Analysis of these parameters demonstrate that while physical processes control the broadest scale variations in sedimentation around and across the channel, benthic biology plays a critical role in stabilising sediment and trapping fines. This leads to much broader mixed grain sizes than would be expected from purely abiotic sedimentation, and the maintenance of sediment beds in positions where all the sediment should be actively migrating. Given that previous work has also shown that submarine channels can be biological hotspots, then the present study suggests that benthic biology probably plays a key role in channel morphology and evolution, and that these need to be considered both in the modern and when considering examples preserved in the rock record.

Submarine channels are the primary conduits for land-derived material, including organic carbon, pollutants, and nutrients, into the deep-sea. The flows (turbidity currents) that traverse these systems can pose hazards to seafloor infrastructure such as cables and pipelines. Here we use a novel combination of repeat seafloor surveys and turbidity current monitoring along a 50 km-long submarine channel in Bute Inlet, British Columbia, and discharge measurements from the main feeding river. These source-to-sink observations provide the most detailed information yet on magnitude-frequency-distance relationships for turbidity currents, and the spatial-temporal patterns of sediment transport within a submarine channel-lobe system. This analysis provides new insights into mass redistribution, and particle residence times in submarine channels, as well as where particles are eventually buried and how that is recorded in the deposits. We observe stepwise sediment transport down the channel, with turbidity currents becoming progressively less frequent with distance. Most flows dissipate and deposit within the proximal (< 11 km) part of the system, whilst longer run-out flows then pick up this sediment, 'shuffling' it further downstream along the channel. This shuffling occurs mainly through upstream migration of knickpoints, which can generate sediment bypass along the channel over timescales of 10-100 yrs. Infrequent large events flush the channel and ultimately transport sediment onto the lobe. These flushing events can occur without obvious triggers, and thus might be internally generated. We then present the first ever sediment budget analysis of an entire submarine channel system, which shows that the river input and lobe aggradation can approximately balance over decadal timescales. We conclude by discussing the implication of this sediment shuffling for seafloor geohazards and particle burial. (c) 2022 The Author(s). Published by Elsevier B.V.& nbsp;

The delivery and burial of terrestrial particulate organic carbon (OC) in marine sediments is important to quantify, because this OC is a food resource for benthic communities, and if buried it may lower the concentrations of atmospheric CO 2 over geologic timescales. Analysis of sediment cores has previously shown that fjords are hotspots for OC burial. Fjords can contain complex networks of submarine channels formed by seafloor sediment flows, called turbidity currents. However, the burial efficiency and distribution of OC by turbidity currents in river‐fed fjords had not been investigated previously. Here, we determine OC distribution and burial efficiency across a turbidity current system within Bute Inlet, a fjord in western Canada. We show that 62% ± 10% of the OC supplied by the two river sources is buried across the fjord surficial (30–200 cm) sediment. The sandy subenvironments (channel and lobe) contain 63% ± 14% of the annual terrestrial OC burial in the fjord. In contrast, the muddy subenvironments (overbank and distal basin) contain the remaining 37% ± 14%. OC in the channel, lobe, and overbank exclusively comprises terrestrial OC sourced from rivers. When normalized by the fjord’s surface area, at least 3 times more terrestrial OC is buried in Bute Inlet, compared to the muddy parts of other fjords previously studied. Although the long‐term (>100 years) preservation of this OC is still to be fully understood, turbidity currents in fjords appear to be efficient at storing OC supplied by rivers in their near‐surface deposits.

The exceptionally preserved fossils entombed in the deposits of sediment-gravity flows in the Cambrian Burgess Shale of British Columbia have been fundamental for understanding the origin of major animal groups during the Cambrian explosion. More recently, they have been used to investigate the evolution of community structure; however, this assumes that the fossil assemblage represents an in-life community. Here we test this assumption for the first time based on experimental and field approaches. We use flume experiments to create analog flows and show that transport of the polychaete Alitta virens over tens of kilometers does not induce significantly more damage beyond that already experienced due to normal decay processes. Integration of experimental results with taphonomic assessment of fossils and sedimentological analysis suggests that the organisms of the Burgess Shale in the classic Walcott Quarry locality could have undergone substantial transport and may represent a conflation of more than one community.

Submarine channels deliver globally important volumes of sediments, nutrients, contaminants and organic carbon into the deep sea. Knickpoints are significant topographic features found within numerous submarine channels, which most likely play an important role in channel evolution and the behaviour of the submarine sediment‐laden flows (turbidity currents) that traverse them. Although prior research has linked supercritical turbidity currents to the formation of both knickpoints and smaller crescentic bedforms, the relationship between flows and the dynamics of these seafloor features remains poorly constrained at field‐scale. This study investigates the distribution, variation and interaction of knickpoints and crescentic bedforms along the 44 km long submarine channel system in Bute Inlet, British Columbia. Wavelet analyses on a series of repeated bathymetric surveys reveal that the floor of the submarine channel is composed of a series of knickpoints that have superimposed, higher‐frequency, crescentic bedforms. Individual knickpoints are separated by hundreds to thousands of metres, with the smaller superimposed crescentic bedforms varying in wavelengths from ca 16 m to ca 128 m through the channel system. Knickpoint migration is driven by the passage of frequent turbidity currents, and acts to redistribute and reorganize the crescentic bedforms. Direct measurements of turbidity currents indicate the seafloor reorganization caused by knickpoint migration can modify the flow field and, in turn, control the location and morphometry of crescentic bedforms. A transect of sediment cores obtained across one of the knickpoints show sand–mud laminations of deposits with higher aggradation rates in regions just downstream of the knickpoint. The interactions between flows, knickpoints and bedforms that are documented here are important because they likely dominate the character of preserved submarine channel‐bed deposits.

The scale of submarine channels can rival or exceed those formed on land and they form many of the largest sedimentary deposits on Earth. Turbidity currents that carve submarine channels pose a major hazard to offshore cables and pipelines, and transport globally significant amounts of organic carbon. Alongside the primary channels, many systems also exhibit a range of headless channels, which often abruptly terminate at steep headscarps. These enigmatic features are widespread in lakes and ocean floors, either as branches off the main submarine channel thalweg or as isolated secondary channels. Prior research has proposed that headless channels may be associated with early and incipient stages of channel development, but their formation and evolution remain poorly understood. Here, we investigate the morphology, origin and development of headless channels by examining repeat bathymetric surveys spanning a period from 1986 to 2018, in Bute Inlet, Canada. We show how channel switching processes, the extension of turbidity currents across distal fans, along with overbanking turbidity currents, are able to initiate headless channels in submarine settings. We discuss how the evolution of headless channels plays an important role in shaping submarine channels, promoting channel extension and modifying the overall longitudinal profile, as well as impacting the character of sedimentary records in channel-lobe transition zones.

Supplementary methods (ramped oxidation system and sedimentation rate estimates).

Seabed sediment flows called turbidity currents form some of the largest sediment accumulations, deepest canyons and longest channel systems on Earth. Only rivers transport comparable sediment volumes over such large areas; but there are far fewer measurements from turbidity currents, ensuring they are much more poorly understood. Turbidity currents differ fundamentally from rivers, as turbidity currents are driven by the sediment that they suspend. Fast turbidity currents can pick up sediment, and self-accelerate (ignite); whilst slow flows deposit sediment and dissipate. Self-acceleration cannot continue indefinitely, and flows might reach a near-uniform state (autosuspension). Here we show how turbidity currents evolve using the first detailed measurements from multiple locations along their pathway, which come from Monterey Canyon offshore California. All flows initially ignite. Typically, initially-faster flows then achieve near-uniform velocities (autosuspension), whilst slower flows dissipate. Fractional increases in initial velocity favour much longer runout, and a new model explains this bifurcating behaviour. However, the only flow during less-stormy summer months is anomalous as it self-accelerated, which is perhaps due to erosion of surficial-mud layer mid-canyon. Turbidity current evolution is therefore highly sensitive to both initial velocities and seabed character.

Seafloor sediment density flows are the primary mechanism for transporting sediment to the deep sea. These flows are important because they pose a hazard to seafloor infrastructure and deposit the largest sediment accumulations on Earth. The cohesive sediment content of a flow (i.e., clay) is an important control on its rheological state (e.g., turbulent or laminar); however, how clay becomes incorporated into a flow is poorly understood. One mechanism is by the abrasion of (clay-rich) mud clasts. Such clasts are common in deep-water deposits, often thought to have traveled over large (more than tens of kilometers) distances. These long travel distances are at odds with previous experimental work that suggests that mud clasts should disintegrate rapidly through abrasion. To address this apparent contradiction, we conduct laboratory experiments using a counter rotating annular flume to simulate clast transport in sediment density flows. We find that as clay clasts roll along a sandy floor, surficial armoring develops and reduces clast abrasion and thus enhances travel distance. For the first time we show armoring to be a process of renewal and replenishment, rather than forming a permanent layer. As armoring reduces the rate of clast abrasion, it delays the release of clay into the parent flow, which can therefore delay flow transformation from turbidity current to debris flow. We conclude that armored mud clasts can form only within a sandy turbidity current; hence where armored clasts are found in debrite deposits, the parent flow must have undergone flow transformation farther up slope.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

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. Plain Language Summary Seafloor-hugging flows of sediment-laden water, called turbidity currents, transport large volumes of sediment to the deep sea and pose a hazard to seafloor infrastructure such as pipelines and telecommunication cables. However, these flows remain poorly understood because of the limited field data available and the difficulty of measuring sediment concentration. It is sediment concentration that drives the flows, and this information is critical to modelers who seek to understand how fast and how far the flows are capable of running out along the seafloor. Recent field studies of turbidity currents have used acoustic flow meters that measure flow velocity through vertical profiles above the seabed. These instruments also record the magnitude of the sound reflected by the moving particles within the flow. This magnitude is related to both the concentration and grain size of the sediment. We take this information and determine the sediment concentration of 10 flows at 2,000 m water depth in the Congo Canyon, offshore West Africa. Our results indicate that sediment concentrations are very dilute in most of the flow. We show how the retarding force of friction is lower than expected, meaning that current flow models are likely to underestimate how fast and far the flows runout.

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.

Submarine canyons are globally important conduits for sediment and organic carbon transport into the deep sea. Using a novel dataset from Monterey Canyon, offshore central California, that includes an extensive array of water column sampling devices, we address how fine-grained sediment and organic carbon are transported, mixed, fractionated, and buried along a submarine canyon. Anderson-type sediment traps were deployed 10-300 m above the seafloor on a suite of moorings anchored between 278 and 1849 m water depths along the axial channel of Monterey Canyon during three consecutive 6-month deployments (2015-2017). Tidal currents within the canyon suspended and transported fine-grained sediment and organic carbon that were captured in sediment traps, which record the composition of sediment and organic carbon transport along the canyon. High sediment accumulation rates in traps increased up-canyon and near the seafloor, where fine-scale ( < 1 cm) layering was increasingly distinctive in CT scans. There was no along-canyon trend in the organic carbon composition (percent modern carbon and isotopic signatures) among trap locations, suggesting effective mixing. Organic carbon content (weight percent total organic carbon) and excess Pb-210 activities (dpm/g) increased down-canyon, reflecting reduced flux of sediment and organic carbon into deeper water, more distal traps. Differing organic carbon signatures in traps compared with previous measurements of seabed deposits along Monterey Canyon suggest that organic carbon transported through the canyon with internal tides may not be consistently recorded in seafloor deposits. First-order estimates from comparing organic carbon content of core and trap samples results in low organic carbon specific burial efficiency (ranging from similar to 26% to similar to 0.1%) and suggests that the modern upper Monterey Canyon may not be an effective sink for carbon. Organic carbon isotopic signatures from sediment traps in the water column show more marine influence than seafloor sediment cores; this is likely due to the deposition and reworking of seafloor deposits by sediment density flows and preferential consumption of fresh marine organic carbon on the seafloor, which is better preserved in the traps. Sediment and remaining organic carbon in canyon floor and lower flank deposits preferentially reflect episodic sediment density flow events that are unrelated to internal tides. This study provides a quantified example and conceptual model for internal-tide-related sediment and organic carbon transport, mixing, and burial trends along a submarine canyon that are likely to be similar in many canyons worldwide.

Submarine canyons are conduits for episodic and powerful sediment density flows (commonly called turbidity currents) that move globally significant amounts of terrestrial sediment and organic carbon into the deep sea, forming some of the largest sedimentary deposits on Earth. The only record available for most turbidity currents is the deposit they leave behind. Therefore, to understand turbidity current processes, we need to determine the degree to which these flows are represented by their deposits. However, linking flows and deposits is a major long-standing scientific challenge. There are few detailed measurements from submarine turbidity currents in action, and even fewer direct measurements that can be compared to resulting seabed deposits. Recently, an extensive array of moorings along Monterey Canyon, offshore California, took measurements and samples during sediment density flow events, providing the most comprehensive dataset to date of turbidity current flows and their deposits. Here, we use sediment trap samples, velocity measurements, and seafloor cores to document how sand is transported through a submarine canyon, and how the transported sediment is represented in seafloor deposits. Sediment trap samples from events contain primarily fine to medium-grained sand with sharp bases, normal grading, and muddy tops. Sediment captured from the water column during the flow shows normal grading, which is broadly consistent with the initial peak and waning of flow velocities measured at a single height within the flow, and may be enhanced by collapsing flows. Flow events contain coarser sand concentrated toward the seafloor and larger grain sizes on the seafloor or in the dense near-bed layer, possibly representative of stratified flows. Although flow velocity varies, sand grain sizes in sediment traps are similar over distances of 50 km down-canyon, suggesting that grain size is an unfaithful record of down-canyon changes in maximum flow speeds. Sand transported within flow events and sampled in sediment traps is similar to sand sampled from the seafloor shortly after the events, but traps do not contain pebbles and gravel common in seabed deposits. Seabed deposits thus appear to faithfully record the sand component that is transported in the water column during sub-annual turbidity currents.

Gravity currents are the primary means by which sediments, solutes and heat are transported across the ocean-floor. Existing theory of gravity current flow employs a statistically-stable model of turbulent diffusion that has been extant since the 1960s. Here we present the first set of detailed spatial data from a gravity current over a rough seafloor that demonstrate that this existing paradigm is not universal. Specifically, in contrast to predictions from turbulent diffusion theory, self-sharpened velocity and concentration profiles and a stable barrier to mixing are observed. Our new observations are explained by statistically-unstable mixing and self-sharpening, by boundary-induced internal gravity waves; as predicted by recent advances in fluid dynamics. Self-sharpening helps explain phenomena such as ultra-long runout of gravity currents and restricted growth of bedforms, and highlights increased geohazard risk to marine infrastructure. These processes likely have broader application, for example to wave-turbulence interaction, and mixing processes in environmental flows.