The word “pelagic” comes from an ancient Greek word that means “open sea.” Pelagic sediments are those that have settled slowly through the waters of the open ocean to collect on the seafloor. These sediments mainly consist of fine-grained clay and the skeletons of water-dwelling microscopic organisms. Fine-grained pelagic deposits can be found almost everywhere on the ocean bottom, but near land their presence is hidden by the abundant sediment delivered to the ocean basins by rivers and beach erosion. Therefore, pelagic sediments form recognizable deposits only well away from land.
Pelagic sediments fall into two broad groups, those that are made up of particles of existing rocks and those that are collections of skeletons of microscopic life. The two groups are called lithogenous and biogenous, respectively.
Lithogenous pelagic sediments mostly reach the oceans from one of two sources. Some are dust blown from continental land masses, called eolian (or aeolian) sediment, and the rest is volcanic ash blown into the atmosphere by violent eruptions. A small amount of other sediment, called cosmogenous dust, constantly rains into the oceans from space or from the remains of meteors destroyed in the atmosphere. In high latitudes near land, sediment that floated out to sea on glacial icebergs can also be found on the ocean bottom. Although these last two have interesting origins, eolian and volcanic sediment account for the great majority of lithogenous pelagic sediment.
Biogenous pelagic sediments are the remains of tiny species of life called plankton, which float around in the uppermost few tens of feet of the water. These animals secrete shells or “tests” that are made of one of two common minerals. When the animals die, their bodies stop floating and the shells drift down to the seafloor. Although all of these animals are tiny, they are present in vast numbers throughout the oceans, forming a continuous rain of shells that collects in layers on the seafloor over millions of years.
Some plankton build shells with calcite, or calcium carbonate (CaCO3). Calcite shells that collect on the bottom form a layer of muddy ooze that eventually becomes the rock limestone. They are especially likely to form the limestone known as chalk, such as the white rocks exposed in the cliffs of Dover in England. Deposits of this “carbonate ooze” account for about half of all pelagic sediments found around the world.
Other, less numerous, microscopic animals build their shells out of silica (SiO2), usually a form of quartz known as the mineral opal. This siliceous ooze is made up of the skeletons of organisms called diatoms and radiolarians. When turned into rock, these deposits sometimes become layers of hard, dense chert. Other deposits do not become chert layers; these instead form deposits of a soft sedimentary rock called diatomaceous earth. Large deposits of diatomaceous earth formed from pelagic sediments are mined in California, USA.
Although pelagic sediment accumulates constantly, it builds up slowly. The layers build up at a rate of less than a few centimeters every 1,000 years. Over millions of years, however, this means that great thicknesses of these sediments can pile up. The slow rate of sedimentation provides a valuable clue in plate tectonic theory: the thickness of pelagic sediments increases with distance from the mid-Atlantic ridge and other spreading centers. This trend supports the hypothesis that the ocean crust near these spreading centers is younger than more distant crust, and thus has had less time to accumulate pelagic deposits.
