Turbidity current

Source: Wikipedia, the free encyclopedia.
Turbidites
are deposited in the deep ocean troughs below the continental shelf, or similar structures in deep lakes, by turbidity currents which slide down the slopes.
Longitudinal section through an underwater turbidity current

A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process.[1] Turbidity currents can also occur in other fluids besides water.

Researchers from the Monterey Bay Aquarium Research Institute found that a layer of water-saturated sediment moved rapidly over the seafloor and mobilized the upper few meters of the preexisting seafloor. Plumes of sediment-laden water were observed during turbidity current events but they believe that these were secondary to the pulse of the

seafloor sediment moving during the events. The belief of the researchers is that the water flow is the tail-end of the process that starts at the seafloor.[1]

In the most typical case of oceanic turbidity currents, sediment laden waters situated over sloping ground will flow down-hill because they have a higher density than the adjacent waters. The driving force behind a turbidity current is gravity acting on the high density of the sediments temporarily suspended within a fluid. These semi-suspended solids make the average density of the sediment bearing water greater than that of the surrounding, undisturbed water.

As such currents flow, they often have a "snow-balling-effect", as they stir up the ground over which they flow, and gather even more sedimentary particles in their current. Their passage leaves the ground over which they flow scoured and eroded. Once an oceanic turbidity current reaches the calmer waters of the flatter area of the abyssal plain (main oceanic floor), the particles borne by the current settle out of the water column. The sedimentary deposit of a turbidity current is called a turbidite.

Seafloor turbidity currents are often the result of sediment-laden river outflows, and can sometimes be initiated by earthquakes, slumping and other soil disturbances. They are characterized by a well-defined advance-front, also known as the current's head, and are followed by the current's main body. In terms of the more often observed and more familiar above sea-level phenomenon, they somewhat resemble flash floods.

Turbidity currents can sometimes result from submarine

submarine trench slopes of convergent plate margins, continental slopes and submarine canyons
of passive margins. With an increasing continental shelf slope, current velocity increases, as the velocity of the flow increases, turbulence increases, and the current draws up more sediment. The increase in sediment also adds to the density of the current, and thus increases its velocity even further.

Definition

Turbidity currents are traditionally defined as those sediment gravity flows in which sediment is suspended by fluid turbulence.[2][3][4] However, the term "turbidity current" was adopted to describe a

interstitial fluid is a liquid (generally water); a pyroclastic current is one in which the interstitial fluid is gas.[5]

Triggers

Hyperpycnal plume

When the concentration of suspended sediment at the mouth of a

floods, glacier outbursts, dam breaks, and lahar flows. In fresh water environments, such as lakes, the suspended sediment concentration needed to produce a hyperpycnal plume is quite low (1 kg/m3).[7]

Sedimentation in reservoirs

The

permeable obstacles with the right design.[8]

Earthquake triggering

Turbidity currents are often triggered by

tectonic disturbances of the sea floor. The displacement of continental crust in the form of fluidization and physical shaking both contribute to their formation. Earthquakes have been linked to turbidity current deposition in many settings, particularly where physiography favors preservation of the deposits and limits the other sources of turbidity current deposition.[9][10] Since the famous case of breakage of submarine cables by a turbidity current following the 1929 Grand Banks earthquake,[11] earthquake triggered turbidites have been investigated and verified along the Cascadia subduction Zone,[12] the Northern San Andreas Fault,[13] a number of European, Chilean and North American lakes,[14][15][16] Japanese lacustrine and offshore regions[17][18] and a variety of other settings.[19][20]

Canyon-flushing

When large turbidity currents flow into

littoral drift, storms or smaller turbidity currents. Canyon-flushing associated with surge-type currents initiated by slope failures may produce currents whose final volume may be several times that of the portion of the slope that has failed (e.g. Grand Banks).[22]

Slumping

Sediment that has piled up at the top of the

continental slope, particularly at the heads of submarine canyons can create turbidity current due to overloading, thus consequent slumping
and sliding.

Convective sedimentation beneath river plumes

Laboratory images of how convective sedimentation beneath a buoyant sediment-laden surface can initiate a secondary turbidity current.[23]

A buoyant sediment-laden river plume can induce a secondary turbidity current on the ocean floor by the process of convective sedimentation.[24][4] Sediment in the initially buoyant hypopycnal flow accumulates at the base of the surface flow,[25] so that the dense lower boundary become unstable. The resulting convective sedimentation leads to a rapid vertical transfer of material to the sloping lake or ocean bed, potentially forming a secondary turbidity current. The vertical speed of the convective plumes can be much greater than the Stokes settling velocity of an individual particle of sediment.[26] Most examples of this process have been made in the laboratory,[24][27] but possible observational evidence of a secondary turbidity current was made in Howe Sound, British Columbia,[28] where a turbidity current was periodically observed on the delta of the Squamish River. As the vast majority of sediment laden rivers are less dense than the ocean,[7] rivers cannot readily form plunging hyperpycnal flows. Hence convective sedimentation is an important possible initiation mechanism for turbidity currents.[4]

An example of steep submarine canyons carved out by turbidity currents, located along California's Central Coast

Effect on ocean floor

Large and fast-moving turbidity currents can

reinforcing
the cables in vulnerable areas.

When turbidity currents interact with regular ocean currents, such as

Gulf of Cadiz, where the ocean current leaving the Mediterranean Sea (also known as the Mediterranean outflow water) pushes turbidity currents westward. This has changed the shape of submarine valleys and canyons in the region to also curve in that direction.[29]

Deposits

interbedded with finegrained dusky-yellow sandstone and gray clay shale that occur in graded beds, Point Loma Formation
, California.

When the energy of a turbidity current lowers, its ability to keep suspended sediment decreases, thus sediment deposition occurs. When the material comes to rest, it is the sand and other coarse material which settles first followed by mud and eventually the very fine particulate matter. It is this sequence of deposition that creates the so called

turbidite
deposits.

Because turbidity currents occur underwater and happen suddenly, they are rarely seen as they happen in nature, thus turbidites can be used to determine turbidity current characteristics. Some examples: grain size can give indication of current velocity, grain lithology and the use of foraminifera for determining origins, grain distribution shows flow dynamics over time and sediment thickness indicates sediment load and longevity.

Turbidites are commonly used in the understanding of past turbidity currents, for example, the Peru-Chile Trench off Southern Central Chile (36°S–39°S) contains numerous turbidite layers that were cored and analysed.[30] From these turbidites the predicted history of turbidity currents in this area was determined, increasing the overall understanding of these currents.[30]

Antidune deposits

Some of the largest

wavelengths of 110–2600 m and wave heights of 1–15 m.[31] Turbidity currents responsible for wave generation are interpreted as originating from slope failures on the adjacent Venezuela, Guyana and Suriname continental margins.[31] Simple numerical modelling has been enabled to determine turbidity current flow characteristics across the sediment waves to be estimated: internal Froude number = 0.7–1.1, flow thickness = 24–645 m, and flow velocity = 31–82 cm·s−1.[31] Generally, on lower gradients beyond minor breaks of slope, flow thickness increases and flow velocity decreases, leading to an increase in wavelength and a decrease in height.[31]

Reversing buoyancy

The behaviour of turbidity currents with

brackish interstitial water entering the sea) has been investigated to find that the front speed decreases more rapidly than that of currents with the same density as the ambient fluid.[32] These turbidity currents ultimately come to a halt as sedimentation results in a reversal of buoyancy, and the current lifts off, the point of lift-off remaining constant for a constant discharge.[32] The lofted fluid carries fine sediment with it, forming a plume that rises to a level of neutral buoyancy (if in a stratified environment) or to the water surface, and spreads out.[32] Sediment falling from the plume produces a widespread fall-out deposit, termed hemiturbidite.[33] Experimental turbidity currents [34] and field observations [35]
suggest that the shape of the lobe deposit formed by a lofting plume is narrower than for a similar non-lofting plume

Prediction

Prediction of erosion by turbidity currents, and of the distribution of turbidite deposits, such as their extent, thickness and grain size distribution, requires an understanding of the mechanisms of sediment transport and deposition, which in turn depends on the fluid dynamics of the currents.

The extreme complexity of most turbidite systems and beds has promoted the development of quantitative models of turbidity current behaviour inferred solely from their deposits. Small-scale laboratory experiments therefore offer one of the best means of studying their dynamics. Mathematical models can also provide significant insights into current dynamics. In the long term, numerical techniques are most likely the best hope of understanding and predicting three-dimensional turbidity current processes and deposits. In most cases, there are more variables than governing

2003 Tokachi-oki earthquake a large turbidity current was observed by the cabled observatory which provided direct observations, which is rarely achieved.[36]

Oil exploration

Oil and gas companies are also interested in turbidity currents because the currents deposit

hydrocarbons. The use of numerical modelling and flumes are commonly used to help understand these questions.[37] Much of the modelling is used to reproduce the physical processes which govern turbidity current behaviour and deposits.[37]

Modeling approaches

Shallow-water models

The so-called depth-averaged or shallow-water models are initially introduced for compositional gravity currents [38] and then later extended to turbidity currents.[39][40] The typical assumptions used along with the shallow-water models are: hydrostatic pressure field, clear fluid is not entrained (or detrained), and particle concentration does not depend on the vertical location. Considering the ease of implementation, these models can typically predict flow characteristic such as front location or front speed in simplified geometries, e.g. rectangular channels, fairly accurately.

Depth-resolved models

With the increase in computational power, depth-resolved models have become a powerful tool to study gravity and turbidity currents. These models, in general, are mainly focused on the solution of the

Navier-Stokes equations
for the fluid phase. With dilute suspension of particles, a Eulerian approach proved to be accurate to describe the evolution of particles in terms of a continuum particle concentration field. Under these models, no such assumptions as shallow-water models are needed and, therefore, accurate calculations and measurements are performed to study these currents. Measurements such as, pressure field, energy budgets, vertical particle concentration and accurate deposit heights are a few to mention. Both Direct numerical simulation (DNS) [41] and Turbulence modeling[42] are used to model these currents.

Notable examples of turbidity currents

  • Within minutes after the
    continental slope from the earthquake's epicenter, snapping the cables as it passed.[43] Subsequent research of this event have shown that continental slope sediment failures mostly occurred below 650 meter water depth.[44] The slumping that occurred in shallow waters (5–25 meters) passed down slope into turbidity currents that evolved ignitively.[44] The turbidity currents had sustained flow for many hours due to the delayed retrogressive failure and transformation of debris flows into turbidity currents through hydraulic jumps.[44]
  • The Cascadia subduction zone, off the northwestern coast of North America, has a record of earthquake triggered turbidites[9][12][45] that is well-correlated to other evidence of earthquakes recorded in coastal bays and lakes during the Holocene.[46][47][48][49][50] Forty–one Holocene turbidity currents have been correlated along all or part of the approximately 1000 km long plate boundary stretching from northern California to mid-Vancouver island. The correlations are based on radiocarbon ages and subsurface stratigraphic methods. The inferred recurrence interval of Cascadia great earthquakes is approximately 500 years along the northern margin, and approximately 240 years along the southern margin.[45]
  • bathymetric slope. Current velocities were 20 m/s (45 mph) on the steepest slopes and 3.7 m/s (8.3 mph) on the shallowest slopes.[51]
  • One of the earliest observations of a turbidity currents was by François-Alphonse Forel. In the late 1800s he made detailed observations of the plunging of the Rhône river into Lake Geneva[52] at Port Valais. These papers were possibly the earliest identification of a turbidity current[53] and he discussed how the submarine channel formed from the delta. In this freshwater lake, it is primarily the cold water that leads to plunging of the inflow. The sediment load by itself is generally not high enough to overcome the summer thermal stratification in Lake Geneva.
  • The longest turbidity current ever recorded occurred in January 2020 and flowed for 1,100 kilometers (680 mi) through the Congo Canyon over the course of two days, damaging two submarine communications cables. The current was a result of sediment deposited by the 2019–2020 Congo River floods.[54]

See also

References

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External links

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