Marine sediment


Marine sediment, or ocean sediment, or seafloor sediment, are deposits of insoluble particles that have accumulated on the seafloor. These particles either have their origins in soil and rocks and have been transported from the land to the sea, mainly by rivers but also by dust carried by wind and by the flow of glaciers into the sea, or they are biogenic deposits from marine organisms or from chemical precipitation in seawater, as well as from underwater volcanoes and meteorite debris.
Except within a few kilometres of a mid-ocean ridge, where the volcanic rock is still relatively young, most parts of the seafloor are covered in sediment. This material comes from several different sources and is highly variable in composition. Seafloor sediment can range in thickness from a few millimetres to several tens of kilometres. Near the surface seafloor sediment remains unconsolidated, but at depths of hundreds to thousands of metres the sediment becomes lithified.
Rates of sediment accumulation are relatively slow throughout most of the ocean, in many cases taking thousands of years for any significant deposits to form. Sediment transported from the land accumulates the fastest, on the order of one metre or more per thousand years for coarser particles. However, sedimentation rates near the mouths of large rivers with high discharge can be orders of magnitude higher. Biogenous oozes accumulate at a rate of about one centimetre per thousand years, while small clay particles are deposited in the deep ocean at around one millimetre per thousand years.
Sediments from the land are deposited on the continental margins by surface runoff, river discharge, and other processes. Turbidity currents can transport this sediment down the continental slope to the deep ocean floor. The deep ocean floor undergoes its own process of spreading out from the mid-ocean ridge, and then slowly subducts accumulated sediment on the deep floor into the molten interior of the earth. In turn, molten material from the interior returns to the surface of the earth in the form of lava flows and emissions from deep sea hydrothermal vents, ensuring the process continues indefinitely. The sediments provide habitat for a multitude of marine life, particularly of marine microorganisms. Their fossilized remains contain information about past climates, plate tectonics, ocean circulation patterns, and the timing of major extinctions.

Overview

Except within a few kilometres of a mid-ocean ridge, where the volcanic rock is still relatively young, most parts of the seafloor are covered in sediments. This material comes from several different sources and is highly variable in composition, depending on proximity to a continent, water depth, ocean currents, biological activity, and climate. Seafloor sediments can range in thickness from a few millimetres to several tens of kilometres. Near the surface, the sea-floor sediments remain unconsolidated, but at depths of hundreds to thousands of metres the sediment becomes lithified.
The various sources of seafloor sediment can be summarized as follows:
  • Terrigenous sediment is derived from continental sources transported by rivers, wind, ocean currents, and glaciers. It is dominated by quartz, feldspar, clay minerals, iron oxides, and terrestrial organic matter.
  • Pelagic carbonate sediment is derived from organisms living in the ocean water that make their shells out of carbonate minerals such as calcite.
  • Pelagic silica sediment is derived from marine organisms that make their tests out of silica.
  • Volcanic ash and other volcanic materials are derived from both terrestrial and submarine eruptions.
  • Iron and manganese nodules form as direct precipitates from ocean-bottom water.
The distributions of some of these materials around the seas are shown in the diagram at the [|start of this article ↑]. Terrigenous sediments predominate near the continents and within inland seas and large lakes. These sediments tend to be relatively coarse, typically containing sand and silt, but in some cases even pebbles and cobbles. Clay settles slowly in nearshore environments, but much of the clay is dispersed far from its source areas by ocean currents. Clay minerals are predominant over wide areas in the deepest parts of the ocean, and most of this clay is terrestrial in origin. Siliceous oozes are common in the south polar region, along the equator in the Pacific, south of the Aleutian Islands, and within large parts of the Indian Ocean. Carbonate oozes are widely distributed in all of the oceans within equatorial and mid-latitude regions. In fact, clay settles everywhere in the oceans, but in areas where silica- and carbonate-producing organisms are prolific, they produce enough silica or carbonate sediment to dominate over clay.
Carbonate sediments are derived from a wide range of near-surface pelagic organisms that make their shells out of carbonate. These tiny shells, and the even tinier fragments that form when they break into pieces, settle slowly through the water column, but they don't necessarily make it to the bottom. While calcite is insoluble in surface water, its solubility increases with depth and at around 4,000 m, the carbonate fragments dissolve. This depth, which varies with latitude and water temperature, is known as the carbonate compensation depth. As a result, carbonate oozes are absent from the deepest parts of the ocean, but they are common in shallower areas such as the mid-Atlantic ridge, the East Pacific Rise, along the trend of the Hawaiian/Emperor Seamounts, and on the tops of many isolated seamounts.

Texture

Sediment texture can be examined in several ways. The first way is grain size. Sediments can be classified by particle size according to the Wentworth scale. Clay sediments are the finest with a grain diameter of less than.004 mm and boulders are the largest with grain diameters of 256 mm or larger. Among other things, grain size represents the conditions under which the sediment was deposited. High energy conditions, such as strong currents or waves, usually results in the deposition of only the larger particles as the finer ones will be carried away. Lower energy conditions will allow the smaller particles to settle out and form finer sediments.
File:Sand under electron microscope.jpg|thumb|Scanning electron micrograph showing grains of silica sand
Sorting is another way to categorize sediment texture. Sorting refers to how uniform the particles are in terms of size. If all of the particles are of a similar size, such as in beach sand, the sediment is well-sorted. If the particles are of very different sizes, the sediment is poorly sorted, such as in glacial deposits.
A third way to describe marine sediment texture is its maturity, or how long its particles have been transported by water. One way which can indicate maturity is how round the particles are. The more mature a sediment the rounder the particles will be, as a result of being abraded over time. A high degree of sorting can also indicate maturity, because over time the smaller particles will be washed away, and a given amount of energy will move particles of a similar size over the same distance. Lastly, the older and more mature a sediment the higher the quartz content, at least in sediments derived from rock particles. Quartz is a common mineral in terrestrial rocks, and it is very hard and resistant to abrasion. Over time, particles made from other materials are worn away, leaving only quartz behind. Beach sand is a very mature sediment; it is composed primarily of quartz, and the particles are rounded and of similar size.

Origins

Marine sediments can also classified by their source of origin. There are four types:
  • Lithogenous sediments, also called terrigenous sediments, are derived from preexisting rock and come from land via rivers, ice, wind and other processes. They are referred to as terrigenous sediments since most comes from the land.
  • Biogenous sediments are composed of the remains of marine organisms, and come from organisms like plankton when their exoskeletons break down
  • Hydrogenous sediments come from chemical reactions in the water, and are formed when materials that are dissolved in water precipitate out and form solid particles.
  • Cosmogenous sediments are derived from extraterrestrial sources, coming from space, filtering in through the atmosphere or carried to Earth on meteorites.

    Lithogenous

Lithogenous or terrigenous sediment is primarily composed of small fragments of preexisting rocks that have made their way into the ocean. These sediments can contain the entire range of particle sizes, from microscopic clays to large boulders, and they are found almost everywhere on the ocean floor. Lithogenous sediments are created on land through the process of weathering, where rocks and minerals are broken down into smaller particles through the action of wind, rain, water flow, temperature- or ice-induced cracking, and other erosive processes. These small eroded particles are then transported to the oceans through a variety of mechanisms:
Streams and rivers: Various forms of runoff deposit large amounts of sediment into the oceans, mostly in the form of finer-grained particles. About 90% of the lithogenous sediment in the oceans is thought to have come from river discharge, particularly from Asia. Most of this sediment, especially the larger particles, will be deposited and remain fairly close to the coastline, however, smaller clay particles may remain suspended in the water column for long periods of time and may be transported great distances from the source.
Wind: Windborne transport can take small particles of sand and dust and move them thousands of kilometres from the source. These small particles can fall into the ocean when the wind dies down, or can serve as the nuclei around which raindrops or snowflakes form. Aeolian transport is particularly important near desert areas.
Glaciers and ice rafting: As glaciers grind their way over land, they pick up lots of soil and rock particles, including very large boulders, that get carried by the ice. When the glacier meets the ocean and begins to break apart or melt, these particles get deposited. Most of the deposition will happen close to where the glacier meets the water, but a small amount of material is also transported longer distances by rafting, where larger pieces of ice drift far from the glacier before releasing their sediment.
Gravity: Landslides, mudslides, avalanches, and other gravity-driven events can deposit large amounts of material into the ocean when they happen close to shore.
Waves: Wave action along a coastline will erode rocks and will pull loose particles from beaches and shorelines into the water.
Volcanoes: Volcanic eruptions emit vast amounts of ash and other debris into the atmosphere, where it can then be transported by wind to eventually get deposited in the oceans.
Gastroliths: Another, relatively minor, means of transporting lithogenous sediment to the ocean are gastroliths. Gastrolith means "stomach stone". Many animals, including seabirds, pinnipeds, and some crocodiles deliberately swallow stones and regurgitate them latter. Stones swallowed on land can be regurgitated at sea. The stones can help grind food in the stomach or act as ballast regulating buoyancy. Mostly these processes deposit lithogenous sediment close to shore. Sediment particles can then be transported farther by waves and currents, and may eventually escape the continental shelf and reach the deep ocean floor.
; Composition
Lithogenous sediments usually reflect the composition of whatever materials they were derived from, so they are dominated by the major minerals that make up most terrestrial rock. This includes quartz, feldspar, clay minerals, iron oxides, and terrestrial organic matter. Quartz is one of the most common minerals found in nearly all rocks, and it is very resistant to abrasion, so it is a dominant component of lithogenous sediments, including sand.