There are two major sedimentary processes that supply sediment to the deep ocean.  The continual rain of fine-grained sediment and plankton from the upper levels of ocean and the input of materials from continental shelf through turbidity flows.  Particle transport of sediments is of major interest to many different fields of research and industries.  In the 1950’s a concept was hypothesized and developed into a theory of sediment gravity transportation by fluidal means and called it turbidity currents. Turbidity currents are a type of density current that reveal a viscous fluid mechanical behavior solely supported by fluid turbulence (Boggs, 2000).   In laboratory experiments it is found that turbidity currents can be recreated by hyperpycnal flow into a basin.  In nature this is indicative of sediment rich outflow of rivers into coastal waters.  Turbidity currents can also be formed by seismic activity or volcanic eruptions that create submarine landslides along coastal shelves.  Under natural conditions this sediment gravity flow occurs in lakes and on continental shelves, most notably near the heads of submarine canyons.  Turbidity currents can play a significant role in affecting environmental habitats of marine life and disrupting platforms and other mechanical equipment that are fixed on the continental slopes.  Significant research has gone into sediment gravity flows due to the hazards that they pose when dealing with slope stability.  Also, large oil and gas reservoirs are found within turbidites (turbidity current deposits).  The benefits of studying turbidity currents have become clear and profitable.  Therefore, a new industry of locating areas of both modern and ancient turbidites has evolved.  A modern method of detecting turbidites is to map the landscape of margins on the sub-sea floor.  This helps to determine possible depositional areas of turbidity currents and landslide triggering mechanisms thus preventing some of the risk associated with studying turbidity currents.
        Turbidity currents flow because the sediment water mixture is denser than surrounding water.  There are two main types of turbidity currents: high-density and low-density.  Low-density flows are made up of clay, silt and fine to medium grained sand size particles while high-density flows contain greater than 30 percent grains of coarse grained sands and pebble to cobble size clasts (Lowe, 1982).   Turbidity currents can either move as surges or as steady uniform flows depending upon what initiated the density current.  Seismic activity, high deposition rates during river floods, and oversteepened delta fronts can generate surge turbidity currents.  This initiation causes turbulence in the water that leads to the entrainment of sediment particles.  As the density current moves down-slope it picks up more sediment, eroding away substrate while increasing in velocity.  As a surge fully develops it moves into three main parts, the head, body, and tail.  When the head of a turbidity current mixes with fresh, sediment reduced water and marine water the velocity of flow is drastically reduced.  Only the re-supply of sediment can keep the flow moving.  The head is characterized by its abundance of sediment, extreme turbulence (vortices), and size and thickness that is about twice that of the rest of the flow (Boggs, 2000).  The body flows faster than the head, which causes the head to roll over onto itself.  The body is constant in its size and shape.  The tail of the flow contains the least amount of sediment and thins away from the body lagging behind and depositing sediments in the low flow regime.  As velocity decreases, turbidity currents in marine environments that started at the edge of submarine canyons erode and accumulate sediments on fans at the ends of submarine feeder channels (Hampton, 1996).
        Turbidites tend to be very repetitious and thus Bouma (1962) was able to generalize an ideal stratigraphy of deposition called the Bouma sequence.  These sedimentary structures record the decay of flow velocity as it passes a point from the upper flow regime to the lower flow regime commonly showing a fining upwards facies appearance.  The Bouma sequence contains five structural units of low-density flows and of high-density flows.  The five structural units are given the labels A through E.  A description of the turbidite sequence beginning from the basal succession is as follows: A has a scoured base, poorly sorted and massive.   This part of the sequence was laid down in the upper flow regime however turbulence was reduced owing to factors relating to the boundary layer.  B contains laminated sand without “fines” signifying still in the upper flow regime.  This unit contains finer grain size and more well sorted than A.  Unit C contains cross-laminations with medium to fine sand.  In some sequences climbing ripples have formed symptomatic of a moderate flow regime.  Unit D has sand and silt deposits with less defined laminations as opposed to unit C.  The uppermost unit E consists of fines from sediment as the turbidity current finally wanes in velocity depositing the suspended sediment.
        Biological communities that rarely experience severe disturbance are likely to lose many species because their selection regimes have not filtered out organisms with low resistance or resilience.  In general, the frequency of severe disturbances to biological community decreases sharply with increasing depth; continental slopes areas have few or no natural agents of disturbance except for occurrences of turbidity currents.  This effect is greater owing to the organisms that live in these waters have not adapted to this type of disturbance.  However, the intensity and severity of turbidity currents on organism’s communities can be considered low due to the fact that recorded observations of turbidity currents are not as frequent in modern oceans than in past seas, therefore, gauging their destructiveness on multi-cellular life is a difficult task.  No significant physical method of determining the devastation and recuperation on communities has been invented (Wilber, 1983).
        Economically many oil companies are interested in the thickness and the sorting of the grains in sedimentary deposits of ancient turbidity currents.  These are major exploration targets for oil companies because some of these turbidites have themselves become reservoir rocks (Ritchie et al., 2000).  Major oil companies have recently renewed their interest in turbidites as oil exploration and drilling has been taken into deeper waters offshore.  In the Gulf of Mexico, turbidite reservoirs hold $900 billion (USD) worth of oil (Barley, 1999).
        Deepwater landslides seldom affect humans, however, the most notorious occurrence of a large turbidity current happened on November 18, 1929, at 017:02 Newfoundland time when an earthquake occurred of the coast of Grand Banks.  The after-effect of the earthquake was the most baffling; trans-Atlantic telephone cables on the upper slope of the Grand Banks broke simultaneously with the earthquake while cables on the shelf remained unbroken.  Lower down on the slope in a relatively linear sequence trans-Atlantic telephone cables began snapping.  Thirteen hours after the first cable break the last cable 600 km away snapped.  The Grand Banks turbidity current was recorded to travel as fast as 65 km per hour in an area of 250 km wide in 300-meter deep water (Heezen, 1952).  The construction of deep ocean projects is at a rise and with that comes concern as to how to protect structures from damage so another Grand Banks incident does not occur.  The costs of replacing damaged property and hazards to human life due to contributions from density flows has been a true concern for engineers and scientists as demand more offshore construction increases. (van der Vink et al., 1998).  Oil companies and scientists interested in slope stability for building platforms or drilling must take caution and assess the risk and replacement costs due to submarine landslides.  The use of Geographical Information Systems (GIS) is currently being used to map the steep slopes associated with submarine landslides.
        Turbidity currents are a natural process that supply sediment to the deep ocean floor and carve out submarine canyons.  The flow mechanisms are becoming better understood with more advanced mathematical models and equations to recreate density flows under various conditions.  This allows people to assess the risk involved in construction, research and exploration more accurately with a greater knowledge of the properties and destruction abilities that turbidity currents can posses.  To date, human impact has been minimal and the effect to biological communities is hard to determine.   However, the abundance of petroleum reservoirs in some geologic units of turbidity currents has made research into turbidites very profitable, and has ensured funding for future research of fluidal sediment density flows.
 

References

Barley B., Deepwater problems around the world. Leading Edge, 18, 133-149, 1999
Boggs, Sam.  Principles of Sedimentology and Stratigraphy, 3rd edition: Prentice-Hall 2000
Dillon W.P., Zimmerman H.B., Erosion by Biological Activity in Two New England    Submarine Canyons.  Journal of Sedimentary Petrology, Vol 40. No. 2, p. 542-547, June, 1970.
Hampton, M. A., H. J. Lee, and J. Locat, Submarine Landslides, Reviews of Geophysics, vol. 34, no. 1, p. 33-59, 1996
Heezen, B. C. and M. Ewing, Turbidity currents and submarine slumps, and the 1929 Grand Banks earthquake, American Journal of Science, vol. 250, p. 849-873, 1952.
Lowe, D.R., Sedimentary gravity flows: II. Depositional models with special reference to the deposits of high-density turbidity currents: Jour. Sed. Petrology, v. 52, p. 279-297. 1982.
Ritchie, L.J., Batey, J., McDonald, K., Gladstone, C., Sparks, R.S.J. and Woods, A.W.  Experimental study of stratified gravity currents.  2000.
Van der Vink, R. M. and others, Why the United States is Becoming More Vulnerable to Natural Disasters, American Geophysical Union, Vol. 79, no. 44, November 3, 1998.
Wilber, C.G. Turbidity in the Aquatic Environment: An Environmental Factor in Fresh and Oceanic Waters. Charles C. Thomas Publishers, Springfield IL. 1983.