6.7: Sediment Transport, Deposition, and Accumulation

Sediment particles are transported by ocean currents and waves in the same way that dust and sand are blown around by winds. Strong winds and fast water currents both cause particles to be suspended and carried until the wind or current speed diminishes. Particles are then deposited on the ground or ocean floor, but they may be picked up again if the wind or current speed increases. CC4 explores the relationship between particle size and sinking rate, between current speed and sinking rate, and between current speed and resuspension of deposited particles.

Large particles sink rapidly, and high current speed is necessary to prevent them from being deposited. Once deposited, large particles are not resuspended unless the current speed is considerable. Many large particles are transported to the ocean by rivers during periods of peak river flow that follow flooding rains. When such particles reach the ocean, where currents are slower, they are either deposited or resuspended and transported by waves. In areas where wave energy is limited, the large particles are not resuspended so they are deposited at the river mouth.

Waves can generate orbital water motions that have higher speeds (orbital velocity) than ocean currents do. As a result, they can resuspend large particles in waters that are shallow enough for the wave energy to reach the seafloor, and sand-sized particles can be resuspended and transported in the nearshore zone. Waves can transport sand long distances along the coast in shallow water (Chap. 11). However, the speed of water motion in waves is reduced with depth below the surface (Chap. 8). Sand-sized particles that are transported offshore are deposited when they reach depths at which the wave orbital velocity is no longer high enough to resuspend them. Smaller sand and silt-sized particles that can be resuspended at lower water speeds are deposited in deeper waters than larger particles. Thus, waves tend to sort sand-sized particles in nearshore sediments: larger particles in shallow water and progressively smaller particles with increasing water depth.

Clay-sized particles are generally cohesive, but they tend to remain in suspension once resuspended, unless current speed is reduced substantially (CC4). The peak water velocity in waves is often sufficient to resuspend most cohesive sediments, and fine particles accumulate in nearshore areas only if these areas are well protected from waves and have slow currents. Such areas include both wetlands and fjords. Fine particles are transported by currents until they reach a low current area, where they are deposited permanently. The finest particles may be transported many thousands of kilometers and for many years before being deposited in the deep oceans, but they are often combined into clumps by electrostatic attraction or packaged into fecal pellets. The larger conglomerated particles are deposited more rapidly.

Aside from an occasional large particle (e.g., shark’s tooth or whalebone), deep-ocean sediments remote from coastal sediment inputs are fine-grained because almost all large particles are deposited in nearshore sediments, and most particles introduced directly to the deep ocean are silt and clay-sized. For example, most marine organisms are microscopic and therefore produce small particles. Meteorite dust particles, dust particles carried by winds, and most hydrogenous particles are also small.

Turbidity Currents

In some deep-ocean sediments, particularly those on abyssal plains adjacent to continental slopes, layers of coarse-grained sediments (sand and gravel) can be found. Layers of such sediments are separated by layers of fine-grained sediments that normally accumulate on the deep-ocean floor. The coarse-grained sediments are transported downslope to the abyssal plain by turbidity currents.

Turbidity currents are similar to avalanches. In an avalanche, snow accumulates on a mountainside until it becomes unstable, breaks loose, and tumbles down the slope as an avalanche. Similarly, turbidity currents occur when sediments accumulate on the continental shelf edge and slope until they become unstable, break loose, mix with the surrounding water, and flow down the continental slope in. Turbidity currents can be triggered by disturbances such as earthquake vibrations or sudden large discharges of sediment by rivers in the same way that avalanches can be triggered by noises or storm winds. Pockets or layers of methane or methane hydrates (Chap. 16) formed by the decay of organic matter in the sediments may rupture and be released when a turbidity current occurs. This may have the effect of providing a “lubricated” layer on which the turbidity current travels, which may enhance both the size and speed of the turbidity current.

Once a turbidity current has been triggered, the disturbed sediments flow down the continental slope. As it gathers momentum, the turbidity current entrains more sediment and water, just as an avalanche gathers more snow.

Turbidity currents can flow down the continental slope at speeds of 70 km·h–1 or more, which are sufficient to suspend and retain large particles in suspension. The speed of the turbidity current is reduced when it reaches the abyssal plain, and the entrained sediment particles are deposited from the suspended sediment cloud. The larger particles are deposited first, followed by progressively smaller particles, as speed and turbulence diminish. The result is the formation of a graded bed of sediments on the abyssal seafloor within which the largest grains are at the bottom and grain size progressively decreases upward (Fig. 6-14). Such graded beds, called “turbidite layers,” can be meters thick. In many locations, a number of them are separated by layers of finer sediment that were deposited slowly over the years, centuries, or millennia between successive turbidity currents.

Figure 6-14. There are a number of turbidite layers in these cores. Note the sharp change in grain size at the bottom of each turbidite layer and the gradual reduction in grain size above this boundary. The convex shape of the boundaries between layers is due to distortion of the core during the drilling process. Most turbidite layers would not exhibit the very large particles seen at the lower part of some of the turbidite layers. However, these cores are from the Antarctic continental slope, where large particles are carried onto the continental shelf by glaciers.

Turbidity currents can travel long distances on the abyssal plain before finally depositing all their terrigenous sediment load. Turbidity current deposits are generally thickest at the lower end of submarine canyons, where they form abyssal fans that decrease in thickness and median grain size in a seaward direction.

Where a deep-ocean trench is present at the base of the continental slope, turbidity currents are intercepted by the trench and do not reach beyond it to the abyssal plain. This is one reason why the topography of the Atlantic Ocean abyssal floor is flatter than that of the Pacific Ocean. The topography of the Atlantic Ocean floor is buried by turbidites that have reached the oceanic ridge since the Atlantic Ocean first began to form. Because most of the Pacific Ocean floor was created after subduction zones formed around this ocean, turbidite deposits in the Pacific are rare.

Turbidity currents must be common events in the oceans, particularly where continental shelves are narrow and terrigenous sediment inputs are high. They are difficult to observe because they last only a few hours at most and occur unpredictably, although some cause tsunamis, marking their occurrence. Nevertheless, their destructive power must be respected. In fact, this destructive power is what led to the first quantitative observation of the speed and geographic extent of turbidity currents. In 1929, an earthquake occurred in the Atlantic Ocean off Nova Scotia. The turbidity current triggered by the earthquake plunged down the continental slope and snapped and buried several undersea telephone cables (Fig. 6-15). Because the precise times at which successive cables broke were known, the peak turbidity current speed was later estimated to be at least 70 km·h^–1. This turbidity current, like many others, moved primarily down a submarine canyon and traveled more than 600 km before slowing and depositing a turbidite layer across the abyssal plain. Because turbidity currents often flow down submarine canyons, it has been suggested that their scouring action maintains or even creates the canyons.

Figure 6-15. Submarine cables in the North Atlantic were severed in succession by an earthquake-triggered turbidity current in 1929. (a) The earthquake epicenter was located on the continental slope. Turbidity currents caused by the earthquake spread turbidite sediments across large areas of the deep-sea floor, extending more than 1000 km from the source. (b) The turbidity currents broke several submarine telephone cables at precisely known times. From these data, it was found that the turbidity currents had reached speeds in excess of 70 km•h^–1.

By studying the layers of sediments, scientists have determined that extremely large turbidity currents have occurred in the past and may therefore occur again. For example, about 20,000 years ago, a turbidity current occurred in the western Mediterranean that was estimated to have deposited 500 km³ of sediments on the deep-sea floor, enough material to cover all of Texas with nearly 2 m of mud and sand. Another slide off the coast of Norway occurred between 30,000 and 50,000 years ago, involved more than twice the volume of sediment as the Mediterranean example, and left a scar on the continental slope that is larger than the state of Maryland.

Turbidity currents carry shallow-water organisms to great depths, where some, particularly microorganisms, may survive and adapt. Their occurrence may also affect the food web. After the 1989 Loma Prieta earthquake in California, dense schools of fish were observed feasting on the sediment-dwelling organisms exposed on the floor and sides of Monterey Canyon, where the sediment slumped and presumably triggered a turbidity current.

Debris fields surrounding some volcanic islands, such as Hawaii, provide evidence that massive slides occur periodically when parts of the volcano that created the island collapse, carrying volcanic rocks far from the island. Such avalanches may cause intense turbidity currents and tsunamis (Chap. 9).

Accumulation Rates

All sediments are mixtures of particles from many different sources. The accumulation rate and type of sediment are determined by the relative quantities of particles from each source deposited at each location. For example, Figure 6-16 schematically illustrates the accumulation of sediments at two hypothetical locations. The input rate of biogenous particles to the sediment is the same at each location, but the input rate of lithogenous sediment is much higher at the first location than at the second. The sediments that accumulate at Site A are predominantly lithogenous, whereas the sediments at Site B are predominantly biogenous. However, the accumulation rate is much faster at A than at B. Because sediments are characterized by their predominant material, the Site A sediment would be called “lithogenous sediment,” and the Site B sediment would be called “biogenous sediment,” or ooze. An ooze is a sediment that contains more than 30% biogenous particles by volume. Where one type of organism is responsible for most of the biogenous particles, the ooze can be named after this type: pteropod ooze, radiolarian ooze, diatom ooze, foraminiferal ooze, and so on.

Figure 6-16. The relationships between the accumulation rates of different types of particles, and the accumulation rates and characteristics of the sediments.

Lithogenous particles are the dominant input to ocean sediments. Most lithogenous material is discharged to the oceans from land. Relatively large particles are deposited near river mouths and glaciers and in estuaries and wetlands. Sediment accumulation rates in these nearshore regions range from about 100 cm per 1000 years up to extreme rates such as the 7 m per year found in the delta of the Fraser River in British Columbia, Canada. Somewhat less but still large quantities of lithogenous sediment are transported offshore and are deposited on continental shelves. Such sediment can also reach the deep-ocean floor in areas where the continental shelves are narrow, or when turbidites flow down a continental shelf. Many continental shelves are areas of high biological productivity (Chap. 13). Accordingly, sedimentation rates on the continental shelf and slope and within marginal seas, such as the Mediterranean, generally are about 10 to 100 cm per 1000 years.

In the deep oceans, remote from land, lithogenous inputs are much less, and biogenous material, especially calcium carbonate, is dissolved before it can settle and be buried. Therefore, sedimentation rates in the deep-ocean basins are very low, approximately 0.1 cm per 1000 years. Under highly productive areas, on shallow seamounts or on oceanic plateaus remote from land, the increased sedimentation rate of biogenous material input can raise the overall sedimentation rate by about an order of magnitude, to approximately 1 cm per 1000 years.

Although sediments accumulate very slowly on a human timescale, they accumulate to substantial thickness in some places (Fig. 6-17). At a sedimentation rate of 0.5 cm per 1000 years, sediments approaching 1 km thick (850 m) can accumulate in 170 million years, which is the approximate age of the oldest (except for some remnants) oceanic crust.

Figure 6-17. Thickness of accumulated sediments on the world’s oceanic crust.