Evolution of Turbidity Currents: New insights from direct field measurements

Underwater sediment density flows, including turbidity currents, are capable of transporting vast amounts of sediment, nutrients and pollutants to the deep-sea. These flows can be powerful, causing damage to seafloor infrastructure. Understanding how the flow velocity and magnitude develops over distance is thus important for risk assessments, as well as determining sediment fluxes. However, as few direct measurements are available, these flows remain rather poorly understood. This thesis aims to use three direct monitoring datasets from three different oceanographic settings worldwide that have captured turbidity currents in unusual detail, allowing for unique analysis of their flow evolution. Detailed measurements of turbidity currents in Monterey Canyon, offshore California, show that their evolution depends on the initial velocity and the availability of an easily erodible substrate. Turbidity currents exceeding a velocity threshold can plateau with near-uniform velocities, and thus run out over greater distances. A new model is proposed for how these near-uniform velocities are obtained. In the Var Canyon-River system, France, nearshore measurements are used to analyse turbidity current velocity structures, and how these develop over distance. Turbidity currents are shown to self-organise over short distances by amalgamation of velocity peaks, which is partly controlled by erodible substrate availability. This efficient self-organisation occurs within 10 km, after which the original trigger is indiscernible. This has important implications for interpreting turbidity current deposits. Bute Inlet, British Columbia, is one of the most complete studies, where source-to-sink direct measurements are combined with sediment cores. These data allow for a unique analysis of turbidity current activity over space and time. The current-day channelized system is highly active with yearly events, although these events are low magnitude. In contrast, distally the system shows high magnitude events occurring on centennial time scales. These data suggest that infrequent mechanisms control large-scale events currently not observed directly. This thesis provides a detailed analysis of turbidity current development over distance, essential for determination of sediment fluxes and hazard assessment

Time-lapse surveys reveal patterns and processes of erosion by exceptionally powerful turbidity currents that flush submarine canyons: A case study of the Congo Canyon

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 canyon-channel. 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

Morphometric fingerprints and downslope evolution in bathymetric surveys: insights into morphodynamics of the Congo canyon-channel

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

The submarine Congo Canyon as a conduit for microplastics to the deep sea

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

Luminiscensdatering av marina raviner. Tillämpningpå Monterey Canyon, Kalifonien

Submarine canyons are major geomorphic features, transporting large quantities of sediments from land to the deep sea. These sediments contain nutrients, enabling life in the deep sea and potentially forming hydrocarbon reservoirs. The transport of sediments towards the deep sea is also important as it links into fundamental concepts of the Earth’s system, such as the global carbon cycle and land surface denudation. Concepts based on the assumption of direct transport of sediments from land to the deep sea. However, how this transport occurs, on what timescales, and if there is potential storage of sediments along the way, is actually poorly known. The current theory is that gravity flow events, such as turbidity events, are the main mechanism behind canyon formation and maintenance. Luminescence dating, an absolute dating method, has been used in an earlier study to look at sediment transport via turbidity currents down the Monterey Canyon, off the coast of California, USA (Stevens et al., 2014). An active upper canyon was found at 1093 metres depth, with frequent events. At 3555 and 3612 metres depth the turbidity events dated were older and indications of major reworking of sediments were found. To pinpoint this change in environments, the present study used luminescence dating in order to get an age estimate of sediments at 2920 meter depth, creating a sequence of ages in the canyon. These cores have captured sediments that have been transported via sand waves, not turbidites. Sand waves are related to the frequent passing of turbidity events, but exact understanding of the mechanisms at hand is poorly understood. Single grain analysis on quartz is used to obtain the individual properties and ages of grains. This gives representative canyon entry ages of the sediments in addition to intrinsic grain properties. The data shows a skewed distribution of grain ages with a narrow, dominant peak between 180 and 240 years within a single core, indicating frequent flushing events and minor reworking of sediments. This is a similar pattern to the core at 1093 meter depth, albeit with increased age, suggesting temporary storage of sediment to at least a depth of 2920 metres. It is proposed that there is a gradual increase in ages down canyon towards 2900 meters depth with a more abrupt transition in environments with increased storage of sediments between 2900 and 3500 meter depth. Sand waves, and the exact relation to turbidites, remain a poorly understood transport mechanisms, but are potentially capable to transport vast amounts of sediments towards the deep sea.Marina raviner transporterar stora mängder sand från land till havets botten. Men hur sker detta? I nuläget förklaras sandtransport med gravitationsflöde, att gravitationen drar ner sandkornen mot bottnen. Men sker detta vid ett enda stort skede eller i små gradvisa steg? Och kan det vara så att sand, på sin väg till botten, lagras i ravinerna? Det är dessa frågor som jag försöker att kasta ljus på i och med detta projekt.Målet var att komplettera vår kunskap om hur sand förflyttar sig från land till havsbotten genom att studera ifall det har skett en gradvis eller en abrupt transport av sand i ravinen Monterey Canyon vid Kaliforniens kust. Detta har jag gjort genom att datera åldern av sand vid ett djup av 2 920 meter, som i det här fallet deponerats av sandvågor, och sedan jämfört denna ålder med åldrarna på sanden från ett mindre (1 100 m) och större (3 500 m och djupare) djup, vilka analyserades i en tidigare studie (Stevens et al., 2014). Transportmekanism i den studien skiljer sig med min då det rör sig om gravitationsflöden.Med endast några få sandkorn av annan ålder så var den dominerande åldern på kornen i mitt prov mellan 180 till 240 år gamla. Provet vid 1 100 m djup visade sig också ha en liten spridning av ålder vilket tyder på att det skett återkommande utspolning av gammal sand som istället ersatts av ny. Man kan se att det ändå sker en tillfällig lagring av sediment mellan det minsta, studerade djupet och det nyligen tagna provet på 2 900 m, då en svag ökning i ålder kan mätas. Åldern på sandkornen i proven som kom från ett större djup var istället mycket spridd och generellt mycket högre än de från mindre djup. Att det finns en mix av ålder vid ett och samma djup tyder på att sanden vid återkommande tillfällen omfördelats i ravinen.Denna studie antyder att ravinen i fråga är aktiv upp till ett djup av minst 2 920 meter, med bara en svag stegring i ålder med ökande djup. Mellan 2 920 och 3 500 meter ändras miljön vilket gör att sprid-ningen av ålder ökar. Därutöver var det nya provet taget från en plats med ett annorlunda transport-mekanism, sandvågor istället för gravitationsflöde. Sandvågor, och dess relation till gravitationsflöden, är fortsättningsvis en dåligt förstådd transportmekanism som potentiellt är kapabel till att flytta stora mängder sediment till havets botten

Den vinddrivna snöackumuleringens variabilitet och terräng : Fastställande av sambandet med hjälp av markpenetrerande radar på Svalbard

Snow accumulation patterns can be highly variable depending on terrain and wind. Knowledge of spatial variability of snow accumulation is of high relevance for mass balance modelling. By not incorporating the variability in snow cover, the estimation in mass fluxes and the surface melt are incorrectly presented, affecting the eventual estimation of for instance contribution to sea level rise. Additionally, knowledge of snow accumulation variability is essential for assessing the reliability of point-wise mass balance measurements. Using ground penetrating radar (GPR), the spatial variability of snow can be mapped with both a great spatial and temporal resolution. GPR enables tracing of summer surface melt layers, resulting in a 2D reconstruction of past snow accumulation and associated variability. GPR measurements have been done on Svalbard, during 2012, 2013 and 2014. Based on the selected 2009 summer surface in the GPR measurements, accumulation rates were estimated between 2009-2012; 2009-2013 and 2009-2014. In addition, several terrain parameters are computed by combining DEM calculations with wind direction, resulting in a sheltering index, slope and curvature. We explore relationships between the found accumulation pattern and the terrain parameters with varying wind directions. Correlations between terrain and accumulation depend on the selected wind angle, which appears to change with elevation. The results suggest that localized wind patterns prevail on the glacier and shape the snow cover. Katabatic winds form at low elevations on the glacier and are oriented in the glacier direction of approximately 20 degrees. At intermediate elevation, winds from the east-southeast regulate the accumulation pattern. On the upper parts of the glacier, the terrain is more exposed and winds from large-scale atmospheric circulation, at 240 degrees, become more important in formation of the snow accumulation pattern. Correlations are overall high, indicating a strong influence of terrain features on the accumulation distribution. No distinction can be made between the different terrain parameters and accumulation, all returning similar correlations with accumulation except for curvature, which overall returns slightly lower correlations. In addition, the results found great spatial variability in snow accumulation, underlining the importance of incorporating snow accumulation variability in glacier mass balance models. Snow accumulation patterns can be highly variable depending on terrain and wind. Knowledge of spatialvariability of snow accumulation is of high relevance for mass balance modelling. By not incorporating the variability in snow cover, the estimation in mass fluxes and the surface melt are incorrectlypresented, affecting the eventual estimation of for instance contribution to sea level rise. Additionally,knowledge of snow accumulation variability is essential for assessing the reliability of point-wise mass balance measurements.Using ground penetrating radar (GPR), the spatial variability of snow can be mapped with both agreat spatial and temporal resolution. GPR enables tracing of summer surface melt layers, resulting in a 2D reconstruction of past snow accumulation and associated variability. GPR measurements have been done on Svalbard, during 2012, 2013 and 2014. Based on the selected 2009 summer surface in the GPR measurements, accumulation rates were estimated between 2009-2012; 2009-2013 and 2009-2014. In addition, several terrain parameters are computed by combining DEM calculations with wind direction, resulting in a sheltering index, slope and curvature. We explore relationships between the found accumulation pattern and the terrain parameters with varying wind directions.Correlations between terrain and accumulation depend on the selected wind angle, which appears to change with elevation. The results suggest that localized wind patterns prevail on the glacier and shape the snow cover. Katabatic winds form at low elevations on the glacier and are oriented in the glacier direction of approximately 20 degrees. At intermediate elevation, winds from the east-southeast regulate the accumulation pattern. On the upper parts of the glacier, the terrain is more exposed and winds from large-scale atmospheric circulation, at 240 degrees, become more important in formation of the snow accumulation pattern. Correlations are overall high, indicating a strong influence of terrain features on the accumulation distribution. No distinction can be made between the different terrain parameters and accumulation, all returning similar correlations with accumulation except for curvature, which overall returns slightly lower correlations. In addition, the results found great spatial variability in snowaccumulation, underlini

How distinctive are flood-triggered turbidity currents?

Turbidity currents triggered at river mouths form an important highway for sediment, organic carbon, and nutrients to the deep sea. Consequently, it has been proposed that the deposits of these flood-triggered turbidity currents provide important long-term records of past river floods, continental erosion, and climate. Various depositional models have been suggested to identify river-flood-triggered turbidite deposits, which are largely based on the assumption that a characteristic velocity structure of the flood-triggered turbidity current is preserved as a recognizable vertical grain size trend in their deposits. Four criteria have been proposed for the velocity structure of flood-triggered turbidity currents: prolonged flow duration; a gradual increase in velocity; cyclicity of velocity magnitude; and a low peak velocity. However, very few direct observations of flood-triggered turbidity currents exist to test these proposed velocity structures. Here we present direct measurements from the Var Canyon, offshore Nice in the Mediterranean Sea. An acoustic Doppler current profiler was located 6 km offshore from the river mouth, and provided detailed velocity measurements that can be directly linked to the state of the river. Another mooring, positioned 16 km offshore, showed how this velocity structure evolved down-canyon. Three turbidity currents were measured at these moorings, two of which are associated with river floods. The third event was not linked to a river flood and was most likely triggered by a seabed slope failure. The multi-pulsed and prolonged velocity structure of all three (flood- and landslide-triggered) events is similar at the first mooring, suggesting that it may not be diagnostic of flood triggering. Indeed, the event that was most likely triggered by a slope failure matched the four flood-triggered criteria best, as it had prolonged duration, cyclicity, low velocity, and a gradual onset. Hence, previously assumed velocity-structure criteria used to identify flood-triggered turbidity currents may be produced by other triggers. Next, this study shows how the proximal multi-pulsed velocity structure reorganizes down-canyon to produce a single velocity pulse. Such rapid-onset, single-pulse velocity structure has previously been linked to landslide-triggered events. Flows recorded in this study show amalgamation of multiple velocity pulses leading to shredding of the flood signal, so that the original initiation mechanism is no longer discernible at just 16 km from the river mouth. Recognizing flood-triggered turbidity currents and their deposits may thus be challenging, as similar velocity structures can be formed by different triggers, and this proximal velocity structure can rapidly be lost due to self-organization of the turbidity current

Morphometric fingerprints and downslope evolution in bathymetric surveys: insights into morphodynamics of the Congo canyon-channel

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

Carbon and sediment fluxes inhibited in the submarine Congo Canyon by landslide-damming

Landslide-dams, which are often transient, can strongly affect the geomorphology, and sediment and geochemical fluxes, within subaerial fluvial systems. The potential occurrence and impact of analogous landslide-dams in submarine canyons has, however, been difficult to determine due to a scarcity of sufficiently time-resolved observations. Here we present repeat bathymetric surveys of a major submarine canyon, the Congo Canyon, offshore West Africa, from 2005 and 2019. We show how an ~0.09 km3 canyon-flank landslide dammed the canyon, causing temporary storage of a further ~0.4 km3 of sediment, containing ~5 Mt of primarily terrestrial organic carbon. The trapped sediment was up to 150 m thick and extended >26 km up-canyon of the landslide-dam. This sediment has been transported by turbidity currents whose sediment load is trapped by the landslide-dam. Our results suggest canyon-flank collapses can be important controls on canyon morphology as they can generate or contribute to the formation of meander cut-offs, knickpoints and terraces. Flank collapses have the potential to modulate sediment and geochemical fluxes to the deep sea and may impact efficiency of major submarine canyons as transport conduits and locations of organic carbon sequestration. This has potential consequences for deep-sea ecosystems that rely on organic carbon transported through submarine canyons

What determines the downstream evolution of turbidity currents?

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