MARINE SEDIMENTS

 

4. Marine sediments

There are deposits of sediments on much of the ocean floor. These sediments come from a variety of sources and contain a record of about 200 million years of the earth's history, including the temperature of the overlying water.

Just as the source of marine sediments varies, so does the thickness of the sediments - it is very thick in trenches because they entrap sediments (9km); in the Pacific it is about 600m thick and from 500-1000m thick in the Atlantic.

Sediments are classified by size just as they are classified by source:

Collection of Sediments or Sediment Data

The simplest sampling device for softer marine sediments is a bottom grab . It operates by "biting" into the bottom when released on contact with the bottom. A larger area of the seafloor can be sampled using a box corer from which subsamples can be taken. A closeup view of the top of a sample show an irregular mud surface. Deeper penetration of the seafloor can be achieved using a piston core . This coring device is driven into the bottom by a heavy weight. Sediments in midwater can be collected using a sediment trap . Layering of marine sediments can be examined by towing geophysical cables which stream out behind a ship and emit pulses of sound. This is called seismic profiling and it allows us to map sub-bottom reflectors. Sediment texture

Determined mainly by grain size which is indicative of the energy which will cause transportation, erosion or deposition.

Well sorted sediments are those that are primarily in the same size classification. Beach sediments are usually well sorted. Glacial moraines are not well sorted.

As a sediment matures, it becomes more well sorted, with larger particles settling out and finer particles being moved on. Very fine sand settles at 1472 cm/hr and will travel 2.4 km while descending 1000m in a 1 cm/sec current silt, (the coarsest mud) settles almost 48 times more slowly and will travel 48 times further than the finest sand.

Sediment transport

The Hjulstrom curve shows the relationship between current velocity and particle size and erosion, transportation and deposition.

U.S. Rivers (shown in proportion to their volumes) (and in proportion to their sediment load worldwide) and glaciers carry sediments to the ocean margins. They either settle out in estuaries, become part of deltas or are spread across the shelf by currents. Most of this process is by longshore currents caused by breaking waves. Glacial transport is of importance in subpolar regions.

Clay particles require high currents speeds to erode or resuspend them due to their flat shape and high cohesiveness. They have a high surface area to volume ratio. Think to squirting a piece of clay with a hose.

Wind transport acts on particles less than 20 microns and can carry them out into the open ocean.

Classification of marine sediment

Sediment particles are also classified by their origin and composition:

1. Lithogenous or rock-derived sediments - primarily from soils on the continents and transported by rivers and winds. A second source of this sediment type is volcanoes, both terrestrial and undersea

2. Biogenous or sediments of biological origin from skeletal materials such as bones, teeth and shells. These are carbonate, silicate or phosphate

3. Hydrogenous sediments or those formed from chemical reactions in seawater or in the sediments i.e. manganese nodules

LITHOGENOUS - from weathering of rocks (freezing and thawing - which the Incas used to split rocks for building at Machu Picchu)

The elements and compounds that make up rocks are called minerals. Minerals are characterized by their chemical composition. The two most abundant elements in the earth's crust are oxygen and silicon. Together they account for over 74% of all elements. They form a silicate tetrahedron and together with iron and magnesium they form the ferromagnesium minerals. These are dark or black minerals with higher densities than other rock-forming minerals. Examples are olivine and biotite (mica). Densities range from 2.8 to 3.4 gms/cm3.

Three other common rock-forming minerals do not contain iron and magnesium and are called nonferromagnesium minerals. They are feldspar, muscovite and quartz. Their densities range from 2.6 to 3.1 gms/cm3.

Igneous rocks are classified by their texture. This is determined by the rate of cooling. Extrusive rocks such as basalt cool rapidly and are fine-grained. They can form sheet lava or pillow lava on the slopes of underwater volcanoes Intrusive igneous rocks cool very slowly and are coarse-grained. Granite is an example.

Common rock forming minerals are formed at higher temperatures and pressures than in atmospheric conditions. As a result, they are unstable and weather or breakdown when exposed to lower temperatures and pressures.

These particles are carried to the coast by rivers in suspension by the river currents. Fronts are often seen where rivers enter the ocean. The Brazos Rivers also forms these fronts. The particles are deposited when the currents are no fast enough to keep them suspended. Wave action sorts the particles by size moving the smaller particles the farthest offshore - much of the transport is longshore which tends to form bands of sediments parallel to the shore.

The muddy water off the Galveston beaches is a good example of this in that it is usually muddy due to the suspension of small lithogenous particles coming down the Mississippi River.

The finest particles take years to finally settle out and can be carried thousands of km from their entry point. They are found throughout the oceans, but as in the cases of river sediments, their accumulation rates (and their thickness) decrease as one gets further away from the continents.

Airborn particles can be spread over long distances by high altitude winds. If we look at stratospheric aerosols before the eruption of Mt. Pinatubo in the Phillipines in early May 1991, we see worldwide bands covering all but polar areas.

We can track the progression of the sulfur dioxide emitted by the volcano as it forms a global band in the equatorial region. During 14 June to 20 July this band can also be seen in the stratospheric aerosols. This demonstrates how windborn particles can be found in most marine sediments. Wind-transported particles are centered in two east-west belts centered around 30o N and 30oS - the major sources being the high mountains and the deserts

Rates of accumulation: 1-3mm in 1,000 years (abyssal, or red, clays) far from the continents, 1-100m per 1,000 yrs near major rivers

Glaciers are responsible for the transport of large cobbles and boulders out onto the continental shelf - this occurs through a process called rafting which grinds down rocks as it moves over them and also incorporates some into the bottom surface. When icebergs break off, they drift out over the shelf and slowly melt, depositing these particles on the bottom - these types of deposits are most common on the Antarctic continental shelf and off New England.

Turbidity currents also play an important role in transporting coarser-grained sediments into deeper water than otherwise would be expected - these deposits are called turbidites

BIOGENOUS

three major groups based on chemical composition. In order of abundance:

1. calcareous sediments - come from plants called coccolithophores and animals called foraminifera and pteropods. Over geologic time over 90% of the carbon dioxide added to the atmosphere by volcanic activity has been removed and deposited in marine sediments through the action of these types of organisms. This system is called the biological pump (more later).

2. siliceous sediments - come from plants called diatoms and animals called radiolarians. On land we can see exposed white cliffs composed of sediments formed from their remains. In the ocean, these sediments are widespread.

3. phosphatic sediments - come from bones, teeth and scales of fishes and other marine vertebrates

a biogenous sediment is also called an ooze and is any deposit which contains more than 30% biogenous constituents by weight. They are influenced by: production in the overlying waters, dilution with other types of sediments and chemical changes including dissolution - calcareous oozes are shallower than siliceous oozes due to the carbonate compensation depth (ca 4000m)

HYDROGENOUS (also called authigenic sediments)

Manganese nodules which are known to cover 20-50% of the Pacific Ocean bottom - these are pea to coconut sized cobbles - they are rich in several minerals (MnO2 and Fe2O3)and are under serious consideration as an undersea mining venture by several companies. They form very slowly around nucleation centers such as pieces of bones or teeth.

Estimates are up to 1.6x1012 metric tons

The DOMES project was undertaken to monitor the effects of the sediment plume on the organisms in the water column rates average 1 cm/103 years

Phosphorite or P2O5 is common as a thin crust on the continental shelf and banks where the depth is < 1,000m. They seem to be related to upwelling areas

Glauconite is a greenish silicate hydrate and includes potassium, magnesium and iron. It may form by submarine weathering of biotite. It is usually found to depths of 2500m, but is more common on topographic highs near the coastline. When it is abundant, the sediments are referred to as green sands or green muds.

Carbonates of nonbiogenous origin are rare, but are found in the Bahama Banks. More about carbonate equilibrium in the chemical section.

Extraterrestrial

About 10,000 to 100,000 tons of meteorites and cosmic dust fall on the earth's surface everyday - can be found everywhere

Distribution of marine sediments - This diagram show general sediment distribution patterns. Clays are typical of abyssal plain regions, calcareous oozes are found in the remainder of the bottom fom low to mid- latitudes and siliceous oozes are found in high latitudes.

Even in the deep sea, sediments can be disturbed by physical or biological processes. Resuspension of the fine-grained sediments causes a near-bottom turbid layer called the nepheloid layer. On the sediments surface, burrows can ofter be seen. They represent the presence of borrowing organisms which mix the top several centimeters of the sediments, a process called bioturbation.

This chart shows a global view of sediment thickness, regardless of its origin. You can see the thicker sediments in the shelf regions as opposed to the relatively thin cover on most of the deep sea floor.

5. Coastlines and coastal processes

River of Sand film - sediment transport, changing beaches, longshore currents, effects of man-made structures, rip currents, wave refraction

Important Terms: particle sources, bioturbation, extraterrestrial, terrigenous, biogenous, authigenic, manganese nodule, nepheloid layer, calcareous oozes, siliceous oozes, radiolarians, foraminifera, diatoms, coccolithophores, turbidites, rafting, glaciated continental shelves, Hjulstrom curve, piston core, seismic profiling, bottom grab, box corer, sediment traps

 

Top of Page