17.4: Particle Size, Sinking, Deposition, and Resuspension

Essential to Know

• The sinking rate of suspended particles in the ocean is determined primarily by particle size. Large particles sink quickly, and small particles sink more slowly.
• Large particles are deposited near where they enter the oceans (unless transported by turbidity currents). Very fine particles may be transported long distances by currents before they eventually settle on the seafloor.
• Particles can be resuspended after they reach the seafloor if the current speed is sufficiently high.
• Because fine-grained particles are cohesive, a high current speed is necessary to resuspend them from some fine-grained sediments.
• Particles may be alternately deposited and resuspended many times, where current speeds are variable.
• The particle size of grains within deposited sediment is determined by the range of current speeds and the size range of particles transported into the area. Areas with high maximum current speed generally have coarse-grained sediments. Fine-grained sediments are present only where the minimum current speed is low, and the maximum current speed is not extremely high.

Understanding the Concept

Solid particles, or suspended sediments, are transported through the water by currents in the same way that dust particles are carried through the air by winds. Just as dust particles settle when the air is calm, waterborne particles (suspended sediments) sink to the seafloor when currents are slow or absent. 

Particles sink through the water in response to gravity. However, particles do not all sink at the same rate, because of friction between them and the water molecules they must push aside. Higher-viscosity fluids have higher frictional resistance to sinking particles. The viscosity of water is low compared with, for example, molasses or motor oil. 

Large, dense particles are not slowed significantly by friction as they fall through the water column. However, viscosity becomes more important as particle size decreases. To understand why smaller particles are more affected by viscosity, we must consider three factors. First, the gravitational attraction on a particle falling through the water column is directly proportional to its mass (for particles of the same density, it would be directly proportional to the volume). Second, because viscous friction occurs at the particle surface, particles with larger surface area are subject to greater friction. Third, the ratio of the surface area of a particle to its volume generally increases as particle size is reduced. The ratio of viscous friction to gravitational force, therefore, increases as particle size is reduced. Consequently, for small particles, such as those in suspended sediment, the particle’s settling rate decreases progressively (the particle sinks more slowly) as the particle size decreases. 

There are some exceptions to this rule. First, the density of the particle is important. Less dense particles sink more slowly than denser particles. This fact explains how some marine organisms avoid sinking (Chap. 14). Second, the shape of the particle is important. A parachute or feather falls more slowly through the atmosphere than a rock or rice grain because the surface area of a parachute or feather is large in relation to its volume and weight. Thus, air resistance or viscous friction is enhanced. Similarly, some marine organisms have body designs that maximize their surface area to prevent them from sinking (Chap. 14).

Most suspended particles (mineral grains) in the oceans, other than organic detritus, are of similar density. Hence, they sink at rates that are determined primarily by their diameter. Table CC4-1 reports typical sinking or settling rates for particles of varying sizes. Sand-sized particles sink rapidly, and smaller particles sink much more slowly. Organic detritus particles are generally of lower density than mineral grains and sink more slowly.

TABLE CC4-1. Settling Velocity of Particles of Average Density and Approximately Spherical Shape When No Current Is Present

Property	Sand	Silt	Clay
Particle diameter	0.1 mm	0.01 mm	0.001 mm
Settling velocity	2.5 cm•s^–1	0.025 cm•s^–1	0.00025 cm•s^–1
Time to settle 4 km  (average ocean depth)	1.8 days	6 months (185 days)	50 years

Particle-sinking rates are modified by the presence of currents. As current speed increases, turbulence increases and particle sinking slows. This effect can be seen in a glass of orange juice. If the glass sits undisturbed for a while, the particles of orange pulp settle to the bottom of the glass. If the orange juice in the glass is gently stirred to keep the liquid in motion, the pulp will not settle. The effect of currents on particle-sinking velocities depends on both current speed and particle size. For a given particle size, sinking rate is reduced as current speed increases until the turbulence is sufficient to prevent the particle from sinking at all. The current speed at which particles no longer sink varies with particle size. This relationship is shown in Figure CC4-1a. At high current speeds, only large particles settle to the seafloor, and smaller particles remain in suspension. At lower current speeds, smaller particles also settle to the seafloor.

Figure CC4-1. Particle settling, resuspension, and current velocity. (a) Settling rate changes with current velocity. (b) Particle size and current velocity affect particle resuspension.

Once a particle has settled to the seafloor, it can be resuspended if the current speed at the seafloor is sufficiently high. Figure CC4-1b shows the current speeds needed to resuspend sediment particles of different sizes from sediments that consist primarily of grains of that size range. For all particle sizes, the speed needed to resuspend them is higher than the speed needed to prevent them from sinking (Fig. CC4-1). High current speeds are necessary to resuspend large particles, and somewhat lower speeds are needed to prevent them from sinking once resuspended.

As particle size decreases, both the resuspension speed and the speed required to prevent sinking decrease at approximately the same rate until silt- or clay-size particles are reached. The current speed needed to resuspend these smaller particles increases with decreasing particle size, so the speed needed to resuspend the finest sediment particles is higher than that required to resuspend sand-sized sediments. The reason is that fine sediment particles are irregularly shaped and have a very large surface area relative to their volume (size). The grains are so close to each other that they are held by electrostatic attraction between the individual particle surfaces. In addition, they tend to lock together because of their irregular shapes. Organic matter may also help the particles stick to each other. Because they cling together so tightly, fine-grained sediments are considered cohesive.

Figure CC4-2 shows how suspended sediments of various grain sizes behave at different current speeds. The settling and resuspension threshold lines on this chart are the same as those in Figure CC4-1. Three areas are shown in Figure CC4-2: one where conditions are such that a particle would sink to the seafloor; another where conditions are such that particles would remain suspended; and a third where conditions are such that particles would not only remain suspended, but would also be resuspended if they were deposited on the seafloor. Referring to Figure CC4-2, consider an ocean area with no local sources of particles, where the current speed ranges from A to B. No sediment of grain size larger than d₂ is brought into this area by currents. Particles of grain diameters between d₃ and d₄ would be deposited, and sediments would consist of these grain sizes. Particles smaller than d₁ would be transported through this area and would not settle.

Figure CC4-2. Depiction of why most sediments are sorted by grain size, as explained in the text. Note how grain size sorting is determined by the range of wave and/or current speeds at a particular point on the seafloor.

If the current speed in Figure CC4-2 ranged from A to D, particles between sizes d₁ and d₅ could enter the area and be deposited, but finer-grained particles would not be deposited. Particles between sizes d₆ and d₇ could be resuspended and removed, but fine-grained sediments between sizes d₆ and d₁, once deposited, could not be resuspended. Particles of size range d₄ to d₇ also would accumulate. However, in most locations, fine-grained particles far outnumber larger particles in the suspended sediment. Therefore, at this site the sediments would be fine-grained muds consisting primarily of grains between d₁ and d₆ in size.

From Figure CC4-2, we can conclude that large-grained particles cannot be carried far from areas where they are introduced to the oceans unless maximum current speeds are very high (but they could be transported by turbidity currents; Chap. 6). In contrast, fine-grained particles tend to be carried long distances before settling in an area where the minimum current speed is low, and they can not accumulate in any area where the minimum current speed is high. The diagram further demonstrates that the sediment particles that accumulate in a given location are restricted to a range of sizes determined by the current regime.