Sands & Sediments
Sediment is defined as "solid fragments of inorganic or organic material that come from the weathering of rock and are carried and deposited by wind, water, or ice." (thefreedictionary.com)
Sediment is an integral part in helping us learn about biological productivity and ecological concerns in a particular area. As sediment settles, it forms layers. These layers can be used to study the past by looking at the composition of the layers on the ocean floor. Paleoceanography is the study of prehistoric ocean using microfossils and sediment layers.
Sediment is an integral part in helping us learn about biological productivity and ecological concerns in a particular area. As sediment settles, it forms layers. These layers can be used to study the past by looking at the composition of the layers on the ocean floor. Paleoceanography is the study of prehistoric ocean using microfossils and sediment layers.
Sediment can be sampled using several different methods.
1) Clamshell sampler: Allows for large sample of top sediment but doesn't reveal layers. 2)Box or Piston Corer: Open tube on a cable that is dropped from a ship. The weight of the tube drives it into the sediment. This method can take samples as deep as 25m and keeps the layers intact. 3) Drilling: Scientists use a drill to obtain sediment samples deep in the Earth's crust. Chikyu, a drilling vessel, can drill up to 7,000m deep! 4) Seismic Refraction: Scientists will shoot an airgun into the water and allow it to penetrate the sea floor. As the sound is refracted, scientists use a hydrophone (a device that picks up noise underwater) to pick up the noise. This is a form of SONAR. |
Clamshell Sampler
Piston Corer Chikyu Drilling Vessel Seismic Refraction |
Types of Sediments
Particle Size
Particle size is the diameter of an individual particle. When we look at ocean sediments, the smaller the particle, the slower it settles. Larger particles require more energy to move. Because of this, currents and waves tend to distribute sediment based on particle size. One exception is clay which clumps together so it may settle earlier with the large particles.
Particle size is the diameter of an individual particle. When we look at ocean sediments, the smaller the particle, the slower it settles. Larger particles require more energy to move. Because of this, currents and waves tend to distribute sediment based on particle size. One exception is clay which clumps together so it may settle earlier with the large particles.
Sedimentation
Continental Shelf Sediments: Tides, waves and currents strongly affect continental shelf sedimentation. Shoreline turbulence is one of the biggest influences because it prevents smaller particles from settling. It carries the smallest particles out to sea which is why most beaches are sandy and not muddy (look at wentworth scale; clay is much smaller than sand).
**Side note: if you've ever been in a "wading pool" at the beach and it's all mushy--this is why! There is very little turbulence in this area so the clay settles to the bottom.**
Waves and tides have less effect in deep water so mud can be present further off shore. As you move out from shore (towards deeper water), sediments tend to have smaller particles. The "ideal" continental shelf sediment should be sand-->muddy sand-->sandy mud-->mud; but, this is NOT what you get! You may see this pattern but you will find coarse sediment again. This is because the sea level has changed over time. Recent sediments have accumulated since the sea level stabilized while relict sediments are the ones that were left when the sea level was lower.
Deep Ocean Sediments: Deep ocean sediments are high in biogenous material. This is due to less lithogenous sediments and lots of plankton. Areas that contain 30% or more biogenous sediment are called oozes. The oozes are named based upon the material. Siliceous oozes are primarily siliceous plankton while calcareous ooze are made of calcareous plankton. Oozes accumulate very slowly (1-6cm per 1000 years); however, this is still 10x faster than deep ocean lithogenous sediments. Other deep ocean sediments include clay, wind-born dust and volcanic ash.
Continental Shelf Sediments: Tides, waves and currents strongly affect continental shelf sedimentation. Shoreline turbulence is one of the biggest influences because it prevents smaller particles from settling. It carries the smallest particles out to sea which is why most beaches are sandy and not muddy (look at wentworth scale; clay is much smaller than sand).
**Side note: if you've ever been in a "wading pool" at the beach and it's all mushy--this is why! There is very little turbulence in this area so the clay settles to the bottom.**
Waves and tides have less effect in deep water so mud can be present further off shore. As you move out from shore (towards deeper water), sediments tend to have smaller particles. The "ideal" continental shelf sediment should be sand-->muddy sand-->sandy mud-->mud; but, this is NOT what you get! You may see this pattern but you will find coarse sediment again. This is because the sea level has changed over time. Recent sediments have accumulated since the sea level stabilized while relict sediments are the ones that were left when the sea level was lower.
Deep Ocean Sediments: Deep ocean sediments are high in biogenous material. This is due to less lithogenous sediments and lots of plankton. Areas that contain 30% or more biogenous sediment are called oozes. The oozes are named based upon the material. Siliceous oozes are primarily siliceous plankton while calcareous ooze are made of calcareous plankton. Oozes accumulate very slowly (1-6cm per 1000 years); however, this is still 10x faster than deep ocean lithogenous sediments. Other deep ocean sediments include clay, wind-born dust and volcanic ash.