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. 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.
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 seismic instability, which is common with steep underwater slopes, and especially with 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.Sanders, J.E. 1965 Primary sedimentary structures formed by turbidity currents and related resedimentation mechanisms. In: Primary Sedimentary Structures and Their Hydro-Dynamic Interpretation – a Symposium Middleton, G. V.), SEPM Spec. Publishers, 12, 192–219.Meiburg, E. & Kneller, B. 2010, "Turbidity currents and their deposits", Annual Review of Fluid Mechanics, vol. 42, pp. 135–156. However, the term "turbidity current" was adopted to describe a natural phenomenon whose exact nature is often unclear. The turbulence within a turbidity current is not always the support mechanism that keeps the sediment in suspension; however it is probable that turbulence is the primary or sole grain support mechanism in dilute currents (<3%).Kneller, B. & Buckee, C. 2000, "The structure and fluid mechanics of turbidity currents: A review of some recent studies and their geological implications", Sedimentology, vol. 47, no. SUPPL. 1, pp. 62–94. Definitions are further complicated by an incomplete understanding of the turbulence structure within turbidity currents, and the confusion between the terms turbulent (i.e. disturbed by eddies) and turbid (i.e. opaque with sediment).McCave, I.N. & Jones, K.P.N. 1988 Deposition of ungraded muds from high-density non-turbulent turbidity currents. Nature, 333, 250–252. Kneller & Buckee, 2000 define a suspension current as 'flow induced by the action of gravity upon a turbid mixture of fluid and (suspended) sediment, by virtue of the density difference between the mixture and the ambient fluid'. A turbidity current is a suspension current in which the interstitial fluid is liquid (generally water); a pyroclastic current is one in which the interstitial fluid is gas.
Triggers
Hyperpycnal plume
When the concentration of suspended sediment at the mouth of a river is so large that the density of river water is greater than the density of sea water a particular kind turbidity current can form where the sediment-laden freshwater from the river continues as a submarine current across the bottom of a lake or sea. This is called a hyperpycnal plume.Mulder, T. & Syvitski, J.P.M. 1995, "Turbidity currents generated at river mouths during exceptional discharges to the world oceans", Journal of Geology, vol. 103, no. 3, pp. 285–299. The average concentration of suspended sediment for most river water that enters the ocean is much lower than the sediment concentration needed to form a hyperpycnal plume. Although some rivers can have continuously high sediment load that can create a continuous hyperpycnal plume, such as the Haile River (China), which has an average suspended concentration of 40.5 kg/m3. The sediment concentration needed to produce a hyperpycnal plume in marine water is 35 to 45 kg/m3, depending on the water properties within the coastal zone. Most rivers produce hyperpycnal flows only during exceptional events, such as storms, 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).
Sedimentation in reservoirs
The transport and deposition of the sediments in narrow alpine reservoirs is often caused by turbidity currents. They follow the thalweg of the lake to the deepest area near the dam, where the sediments can affect the operation of the bottom outlet and the intake structures.Oehy, C.D. & Schleiss, A.J. 2007, "Control of turbidity currents in reservoirs by solid and permeable obstacles", Journal of Hydraulic Engineering, vol. 133, no. 6, pp. 637–648. Controlling this sedimentation within the reservoir can be achieved by using solid and permeable obstacles with the right design.
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.Adams, J., 1990, Paleoseismicity of the Cascadia subduction zone: Evidence from turbidites off the Oregon-Washington Margin: Tectonics, v. 9, p. 569–584.Goldfinger, C., 2011, Submarine Paleoseismology Based on Turbidite Records: Annual Review of Marine Science, v. 3, p. 35–66. Since the famous case of breakage of submarine cables by a turbidity current following the 1929 Grand Banks earthquake,Heezen, B.C., and Ewing, M., 1952, Turbidity currents and submarine slumps, and the 1929 Grand Banks earthquake: American Journal of Science, v. 250, p. 849–873. earthquake triggered turbidites have been investigated and verified along the Cascadia subduction Zone,Goldfinger, C., Nelson, C.H., and Johnson, J.E., 2003, Holocene Earthquake Records From the Cascadia Subduction Zone and Northern San Andreas Fault Based on Precise Dating of Offshore Turbidites: Annual Review of Earth and Planetary Sciences, v. 31, p. 555–577. the Northern San Andreas Fault,Goldfinger, C., Grijalva, K., Burgmann, R., Morey, A.E., Johnson, J.E., Nelson, C.H., Gutierrez-Pastor, J., Ericsson, A., Karabanov, E., Chaytor, J.D., Patton, J., and Gracia, E., 2008, Late Holocene Rupture of the Northern San Andreas Fault and Possible Stress Linkage to the Cascadia Subduction Zone Bulletin of the Seismological Society of America, v. 98, p. 861–889. a number of European, Chilean and North American lakes,Schnellmann, M., Anselmetti, F.S., Giardini, D., and Ward, S.N., 2002, Prehistoric earthquake history revealed by lacustrine slump deposits: Geology, v. 30, p. 1131–1134.Moernaut, J., De Batist, M., Charlet, F., Heirman, K., Chapron, E., Pino, M., Brümmer, R., and Urrutia, R., 2007, Giant earthquakes in South-Central Chile revealed by Holocene mass-wasting events in Lake Puyehue: Sedimentary Geology, v. 195, p. 239–256.Brothers, D.S., Kent, G.M., Driscoll, N.W., Smith, S.B., Karlin, R., Dingler, J.A., Harding, A.J., Seitz, G.G., and Babcock, J.M., 2009, New Constraints on Deformation, Slip Rate, and Timing of the Most Recent Earthquake on the West Tahoe-Dollar Point Fault, Lake Tahoe Basin, California: Bulletin of the Seismological Society of America, v. 99, p. 499–519. Japanese lacustrine and offshore regionsNakajima, T., 2000, Initiation processes of turbidity currents; implications for assessments of recurrence intervals of offshore earthquakes using turbidites: Bulletin of the Geological Survey of Japan, v. 51, p. 79–87.Noda, A., TuZino, T., Kanai, Y., Furukawa, R., and Uchida, J.-i., 2008, Paleoseismicity along the southern Kuril Trench deduced from submarine-fan turbidites: Marine Geology, v. 254, p. 73–90. and a variety of other settings.Huh, C.A., Su, C.C., Liang, W.T., and Ling, C.Y., 2004, Linkages between turbidites in the southern Okinawa Trough and submarine earthquakes: Geophysical Research Letters, v. 31.Gràcia, E., Vizcaino, A., Escutia, C., Asiolic, A., Garcia-Orellanad, J., Pallàse, R., Lebreiro, S., and Goldfinger, C., 2010, Holocene earthquake record offshore Portugal (SW Iberia): Applying turbidite paleoseismology in a slow-convergence margin: Quaternary Science Reviews, v. 29, p. 1156–1172.
Canyon-flushing When large turbidity currents flow into canyons they may become self-sustaining,Pantin, H.M. 1979 Interaction between velocity and effective density in turbidity flow: phase-plane analysis, with criteria for autosuspension. March Geol., 31, 59–99. and may entrain sediment that has previously been introduced into the canyon by 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).Piper, D.J.W. & Aksu, A.E. 1987 The source and origin of the 1929 Grand Banks turbidity current inferred from sediment budgets. Geo-March Lett., 7, 177–182.
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