Turbidity currents, and other types of underwater sediment density flow, are arguably the most important flow process for moving sediment across our planet. Direct monitoring provides the most ...reliable information on the varied ways in which these flows are triggered, and thus forms the basis for this contribution. Recent advances in flow monitoring make this contribution timely, although monitoring is biased towards more frequent flow types. Submarine deltas fed by bedload dominated rivers can be very active with tens of events each year. Larger events are generated by delta-lip failures, whilst smaller events can be associated with motion of up-slope migrating bedforms. River-fed submarine canyons are flushed every few years by powerful long run-out flows. Flows in river-fed delta and canyon systems tend to occur during months of elevated river discharge. However, many flows do not coincide with flood peaks, or occur where rivers do not reach hyperpycnal concentrations, and are most likely triggered by failure of rapidly deposited sediment. Plunging of hyperpycnal river floodwater commonly triggers dilute and slow moving flows in lakes and reservoirs, and has been shown to produce mm-thick fine-grained deposits. It is proposed here that such thin and fine deposits are typical of flows triggered by hyperpycnal river floods, rather than thicker sand layers with traction structure or displaying inverse-to-normal grading. Oceanographic canyons are detached from river mouths and fed by oceanographic processes (wave and tide resuspension, longshore drift, etc.). Most events in these canyons are associated with large wave heights. Up-slope migrating crescentic bedforms are seen, similar to those observed in river-fed deltas. Oceanographic processes tend to infill canyons, which are flushed episodically by much more powerful flows, inferred to result from slope failure. This filling and flushing model is less applicable to river-fed canyons in which flushing events are much more frequent. Oceanographic canyons may result from rapid sea level rise that detaches river mouths from canyon heads, and they can remain active during sea level highstands. Deep-water basin plains are often dominated by infrequent but very large flows triggered by failure of the continental slope. Recurrence intervals of these flows appear almost random, and only weakly (if at all) correlated with sea level change. Turbidites can potentially provide a valuable long-term record of major earthquakes, but widespread slope failure is the only reliable criteria for inferring seismic triggering. However, not all major earthquakes trigger widespread slope failure, so that the record is incomplete in some locations.
•These flows are most volumetrically important sediment transport process on Earth.•Summary of flow triggers, and flow types and frequencies, in different settings.•Direct monitoring provides most reliable information on flow timing and triggers.•Recent advances in direct monitoring make this contribution timely.
Large submarine landslides can have serious socioeconomic consequences as they have the potential to cause tsunamis and damage seabed infrastructure. It is important to understand the frequency of ...these landslides, and how that frequency is related to climate-driven factors such as sea level or sedimentation rate, in order to assess their occurrence in the future. Recent studies have proposed that more landslides occur during periods of sea level rise and lowstand, or during periods of rapid sedimentation. In this contribution we test these hypotheses by analysing the most comprehensive global data set of ages for large (>1 km3) late Quaternary submarine landslides that has been compiled to date. We include the uncertainties in each landslide age that arise from both the dating technique, and the typically larger uncertainties that result from the position of the samples used for dating. Contrary to the hypothesis that continental slope stability is linked to sea level change, the data set does not show statistically significant patterns, trends or clusters in landslide abundance. If such a link between sea level and landslide frequency exists it is too weak to be detected using the available global data base. It is possible that controlling factors vary between different geographical areas, and their role is therefore hidden in a global data set, or that the uncertainties within the dates is too great to see an underlying correlation. Our analysis also shows that there is no evidence for an immediate influence of rapid sedimentation on slope stability as failures tend to occur several thousand years after periods of increased sedimentation rates. The results imply that there is not a strong global correlation of landslide frequency with sea level changes or increases in local sedimentation rate, based on the currently available ages for large submarine landslides.
•Most comprehensive data base of ages of submarine landslides to date.•Includes uncertainty intervals to age estimates and local sedimentation rates.•No statistical evidence for link between sea level and submarine landslide abundance.•Little evidence for immediate influence of rapid sedimentation on slope stability.
Submarine sediment density flows are one of the most important processes for moving sediment across our planet, yet they are extremely difficult to monitor directly. The speed of long run‐out ...submarine density flows has been measured directly in just five locations worldwide and their sediment concentration has never been measured directly. The only record of most density flows is their sediment deposit. This article summarizes the processes by which density flows deposit sediment and proposes a new single classification for the resulting types of deposit. Colloidal properties of fine cohesive mud ensure that mud deposition is complex, and large volumes of mud can sometimes pond or drain‐back for long distances into basinal lows. Deposition of ungraded mud (TE‐3) most probably finally results from en masse consolidation in relatively thin and dense flows, although initial size sorting of mud indicates earlier stages of dilute and expanded flow. Graded mud (TE‐2) and finely laminated mud (TE‐1) most probably result from floc settling at lower mud concentrations. Grain‐size breaks beneath mud intervals are commonplace, and record bypass of intermediate grain sizes due to colloidal mud behaviour. Planar‐laminated (TD) and ripple cross‐laminated (TC) non‐cohesive silt or fine sand is deposited by dilute flow, and the external deposit shape is consistent with previous models of spatial decelerating (dissipative) dilute flow. A grain‐size break beneath the ripple cross‐laminated (TC) interval is common, and records a period of sediment reworking (sometimes into dunes) or bypass. Finely planar‐laminated sand can be deposited by low‐amplitude bed waves in dilute flow (TB‐1), but it is most likely to be deposited mainly by high‐concentration near‐bed layers beneath high‐density flows (TB‐2). More widely spaced planar lamination (TB‐3) occurs beneath massive clean sand (TA), and is also formed by high‐density turbidity currents. High‐density turbidite deposits (TA, TB‐2 and TB‐3) have a tabular shape consistent with hindered settling, and are typically overlain by a more extensive drape of low‐density turbidite (TD and TC,). This core and drape shape suggests that events sometimes comprise two distinct flow components. Massive clean sand is less commonly deposited en masse by liquefied debris flow (DCS), in which case the clean sand is ungraded or has a patchy grain‐size texture. Clean‐sand debrites can extend for several tens of kilometres before pinching out abruptly. Up‐current transitions suggest that clean‐sand debris flows sometimes form via transformation from high‐density turbidity currents. Cohesive debris flows can deposit three types of ungraded muddy sand that may contain clasts. Thick cohesive debrites tend to occur in more proximal settings and extend from an initial slope failure. Thinner and highly mobile low‐strength cohesive debris flows produce extensive deposits restricted to distal areas. These low‐strength debris flows may contain clasts and travel long distances (DM‐2), or result from more local flow transformation due to turbulence damping by cohesive mud (DM‐1). Mapping of individual flow deposits (beds) emphasizes how a single event can contain several flow types, with transformations between flow types. Flow transformation may be from dilute to dense flow, as well as from dense to dilute flow. Flow state, deposit type and flow transformation are strongly dependent on the volume fraction of cohesive fine mud within a flow. Recent field observations show significant deviations from previous widely cited models, and many hypotheses linking flow type to deposit type are poorly tested. There is much still to learn about these remarkable flows.
The original discovery of active submarine landslides and turbidity currents in the deep ocean was made in the 1950s through analysis of breaks in transoceanic communications cables. Further insights ...regarding the causes, frequency, and behavior of damaging submarine flows are presented here, based on recent disruptions of modern communications cables in the Strait of Luzon off southern Taiwan. In 2006, the Pingtung earthquake triggered landslides and at least three sediment density flows (a general term covering turbidity currents and similar flows). These flows sped down submarine canyons and into the Manila Trench at 12.7–5.6 m s⁻¹ (45–20 km h⁻¹), resulting in 22 cable breaks. In 2009, the cables were again damaged, this time by extreme river discharge associated with Typhoon Morakot. Two cables were damaged during the main flood when debris-charged river waters dived to the seabed and down Gaoping Canyon. A second, more damaging sediment density flow formed three days later when river levels were near normal and seismic activity was low. It is suggested that this second flow resulted from deposited flood sediment that was remobilized possibly by internal wave activity. Further breaks were reported in 2010 and 2012. While historical cable break databases are incomplete, they imply that since at least 1989, density flows capable of breaking cables have been infrequent, but they increased markedly after the 2006 Pingtung earthquake—a time that coincided with a transition to more extreme rainfall associated with northward migration of typhoon tracks to Taiwan.
Submarine channels are the primary conduits for terrestrial sediment, organic carbon, and pollutant transport to the deep sea. Submarine channels are far more difficult to monitor than rivers, and ...thus less well understood. Here we present 9 years of time-lapse mapping of an active submarine channel along its full length in Bute Inlet, Canada. Past studies suggested that meander-bend migration, levee-deposition, or migration of (supercritical-flow) bedforms controls the evolution of submarine channels. We show for the first time how rapid (100-450 m/year) upstream migration of 5-to-30 m high knickpoints can control submarine channel evolution. Knickpoint migration-related changes include deep (>25 m) erosion, and lateral migration of the channel. Knickpoints in rivers are created by external factors, such as tectonics, or base-level change. However, the knickpoints in Bute Inlet appear internally generated. Similar knickpoints are found in several submarine channels worldwide, and are thus globally important for how channels operate.
Seafloor sediment flows (turbidity currents) are among the volumetrically most important yet least documented sediment transport processes on Earth. A scarcity of direct observations means that basic ...characteristics, such as whether flows are entirely dilute or driven by a dense basal layer, remain equivocal. Here we present the most detailed direct observations yet from oceanic turbidity currents. These powerful events in Monterey Canyon have frontal speeds of up to 7.2 m s
, and carry heavy (800 kg) objects at speeds of ≥4 m s
. We infer they consist of fast and dense near-bed layers, caused by remobilization of the seafloor, overlain by dilute clouds that outrun the dense layer. Seabed remobilization probably results from disturbance and liquefaction of loose-packed canyon-floor sand. Surprisingly, not all flows correlate with major perturbations such as storms, floods or earthquakes. We therefore provide a new view of sediment transport through submarine canyons into the deep-sea.
Submarine landslides on open continental slopes can be prodigious in scale. They are an important process for global sediment fluxes, and can generate very damaging tsunamis. Submarine landslides are ...far harder to monitor directly than terrestrial landslides, and much greater uncertainty surrounds their preconditioning factors and triggers. Submarine slope failure often occurs on remarkably low (< 2°) gradients that are almost always stable on land, indicating that particularly high excess pore pressures must be involved. Earthquakes trigger some large submarine landslides, but not all major earthquakes cause widespread slope failure. The headwalls of many large submarine landslides appear to be located in water depths that are too deep for triggering by gas hydrate dissociation. The available evidence indicates that landslide occurrence is either weakly (or not) linked to changes in sea level or atmospheric methane abundance, or the available dates for open continental slope landslides are too imprecise to tell. Similarly, available evidence does not strongly support a view that landslides play an important role in methane emissions that cause climatic change. However, the largest and best-dated open continental slope landslide (the Storegga Slide) coincides with a major cooling event 8,200 years ago. This association suggests that caution may be needed when stating that there is no link between large open slope landslides and climate change.
Volcanic flank collapses and explosive eruptions are among the largest and most destructive processes on Earth. Events at Mount St. Helens in May 1980 demonstrated how a relatively small (<5 km
) ...flank collapse on a terrestrial volcano could immediately precede a devastating eruption. The lateral collapse of volcanic island flanks, such as in the Canary Islands, can be far larger (>300 km
), but can also occur in complex multiple stages. Here, we show that multistage retrogressive landslides on Tenerife triggered explosive caldera-forming eruptions, including the Diego Hernandez, Guajara and Ucanca caldera eruptions. Geochemical analyses were performed on volcanic glasses recovered from marine sedimentary deposits, called turbidites, associated with each individual stage of each multistage landslide. These analyses indicate only the lattermost stages of subaerial flank failure contain materials originating from respective coeval explosive eruption, suggesting that initial more voluminous submarine stages of multi-stage flank collapse induce these aforementioned explosive eruption. Furthermore, there are extended time lags identified between the individual stages of multi-stage collapse, and thus an extended time lag between the initial submarine stages of failure and the onset of subsequent explosive eruption. This time lag succeeding landslide-generated static decompression has implications for the response of magmatic systems to un-roofing and poses a significant implication for ocean island volcanism and civil emergency planning.
Flutes and tool marks are commonly observed sedimentary structures on the bases of sandstones in deep‐water successions. These sole structures are universally used as palaeocurrent indicators but, in ...sharp contrast to most sedimentary structures, they are not used in palaeohydraulic reconstructions or to aid prediction of the spatial distribution of sediments. Since Kuenen's famous 1953 paper, flutes and tool marks in deep‐water systems have been linked to turbidity currents, as reflected in the standard Bouma sequence taught to generations of geologists. Yet, these structures present a series of unaddressed enigmas. Detailed field studies in the 1960s and early 1970s observed that flutes are typically associated with thicker, more proximal beds, whilst tools are generally prevalent in thinner, more distal, beds. Additionally, flutes and tool marks are rarely observed on the same surfaces, and flutes are seen to change downstream from larger wider parabolic to smaller narrower spindle‐shaped forms. No model has been proposed that explains these field‐based observations. This contribution undertakes a radical re‐examination of the formative flow conditions of flutes and tool marks, and demonstrates that they are the products of a wide range of sediment gravity flows, from turbulent flows, through transitional clay‐rich flows, to debris flows. Flutes are not solely the product of turbulent flows, but can continue to form in transitional flows. Grooves are shown to be formed by debris flows, slumps and slides, not turbidity currents, and in many cases the debris flows are linked to the debritic component of hybrid flows. Discontinuous tool marks, including skim (bounce) marks, prod marks and skip marks, are shown to be formed by transitional mud‐rich flows. Consequently, the observed spatial distribution of flutes and tool marks can be explained by a progressive increase in flow cohesivity downstream. This model of flutes and tool marks dovetails with models of hybrid flows that predict such a longitudinal increase in flow cohesivity. However, some deposits show grooves preferentially associated with Bouma TA beds, and these are likely formed by flows transforming from higher to lower cohesion, and are present in basins where hybrid beds are absent or rare. The recognition that sole structures may have no genetic link to the later overlying turbidity current deposits, and can be formed by a wide range of flow types, indicates that the existing pictorial description of the Bouma sequence is incorrect. A modified Bouma sequence is proposed here that addresses these points. In utilizing the advances in fluid dynamics since Kuenen's pioneering research, this study demonstrates that it is possible to use flutes and tool marks to interpret flow type at the point of formation, the nature of flow transformations, and the mechanics of the basal layer. These advances suggest that it is then possible to predict the nature of deposit type down‐dip. This new understanding, in combination with further testing in outcrop of the proposed relationships between sole marks and palaeohydraulics, opens up a wealth of possibilities for improving the understanding of deep‐water clastic environments, with implications for developing more complete facies models, assessing subaqueous geohazards and the resilience of seafloor infrastructure, and advancing our understanding of deep‐water sediments as archives of palaeoenvironmental change.
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.
Key Points
Here we document for the first time how very dilute (up to 0.07 kg/m3) river plumes can generate powerful turbidity currents
Such low sediment concentrations are 20 times lower than those predicted by past theory and experiments
Therefore, turbidity currents are likely to be much more frequent and occur at a far wider range of locations than previously thought
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