Ocean Sediment: Meaning and Various Types

Ocean sediments are particles and fragments of dirt, dust, and other debris that have settled out of the water and accumulated on the ocean floor. The crucial importance of marine sediments is that they reveal much about Earth’s history. Marine sediments provide clues to past climates, movements of the ocean floor, ocean circulation patterns, and nutrient supplies for marine organisms.

Marine sediments can also be helpful in ascertaining the timing of major extinctions, global climate change, and the movement of plates.

More than half of the rocks exposed on the continents are sedimentary rocks deposited in ancient ocean environments and uplifted onto land by plate tectonic processes. Even the tallest mountains on the continents far from any ocean contain marine fossils, which indicate that these rocks originated on the ocean floor in the geologic past. Mount Everest consists of limestone, which is a type of rock that originated as sea floor deposits.

Over time, sediments can become lithified and turned to rock and form sedimentary rock. Particles of sediment come from worn pieces of rocks, as well as living organisms, minerals dissolved in water, and outer space.

There are several types of marine sediments such as:

• Lithogenous sediment
• Biogenous Sediment
• Hydrogenous sediment

Lithogenous Sediment

This type of sediment is derived from pre-existing rock material that originates on the continents or islands from erosion, volcanic eruptions, or blown dust. The another term used for lithogenous sediment is terrigenous sediment. Obviously, the origin of Lithogenous Sediment begins as rocks on continents or islands. Over time, weathering agents such as water, temperature extremes, and chemical effects break rocks into smaller pieces.

Neritic Deposits versus Pelagic Deposits

Marine sedimentary deposits can also be categorized as either neritic or pelagic. Neritic deposits are found on continental shelves and in shallow water near islands; these deposits are generally coarse grained. Alternatively, pelagic deposits are found in the deep ocean basins and are typically fine grained.

When rocks are in smaller pieces, they can be more easily eroded and transported. This eroded material is the basic component of which all lithogenous sediment is composed. Eroded material from the continents is carried to the oceans by streams, wind, glaciers, and gravity. The transported sediment can be deposited in many environments, including bays or lagoons near the ocean, as deltas at the mouths of rivers, along beaches at the shoreline, or further offshore across the continental margin. It can also be carried beyond the continental margin to the deep-ocean basin by turbidity currents. The greatest quantity of lithogenous material is found around the margins of the continents, where it is constantly moved by high-energy currents along the shoreline and in deeper turbidity currents

The majority of lithogenous deposits such as beach sands are composed primarily of quartz.

Biogenous Sediment / Ooze

Biogenous sediment is derived from the remains of hard parts of once living organisms. Origin of Biogenous Sediment Biogenous sediment begins as the hard parts (shells, bones, and teeth) of living organisms ranging from minute algae and protozoans to fish and whales. When organisms that produce hard parts die, their remains settle onto the ocean floor and can accumulate as biogenous sediment. Biogenous sediment can be classified as either macroscopic or microscopic. Macroscopic biogenous sediment is large enough to be seen without the aid of a microscope and includes shells, bones, and teeth of large organisms.

Ooze

The microscopic organisms produce tiny shells called tests that begin to sink after the organisms die and continually rain down in great numbers onto the ocean floor. These microscopic tests can accumulate on the deep ocean floor and form deposits called ooze. Ooze resembles very fine grained, mushy material like mud.  The organisms that contribute to biogenous sediment are chiefly algae and protozoans. Algae are primarily aquatic, eukaryotic, photosynthetic organisms, ranging in size from microscopic single cells to large organisms like giant kelp. Protozoans are any of a large group of single-celled, eukaryotic, usually microscopic organisms that are generally not photosynthetic.

Opal

The two most common chemical compounds in biogenous sediment are calcium carbonate (which forms the mineral calcite) and silica. Silica is often chemically combined with water to produce the hydrated form of silica, which is called opal.

Diatoms and Diatomaceous Earth

Most of the silica in biogenous ooze comes from microscopic algae called diatoms and protozoans called radiolarians. Because diatoms photosynthesize, they need strong sunlight and are found only within the upper, sunlit surface waters of the ocean. Most diatoms are free floating, or planktonic. The living organism builds a glass greenhouse out of silica as a protective covering and lives inside.  Where diatoms are abundant at the ocean surface, thick deposits of diatom rich ooze can accumulate below on the ocean floor. When this ooze lithified, it becomes diatomaceous earth, which is a lightweight white rock composed of diatom tests and clay.

Hydrogenous Sediment

Hydrogenous sediment is derived from the dissolved material in water. Chemical reactions within seawater cause certain minerals to precipitate. Precipitation usually occurs when there is a change in conditions, such as a change in temperature or pressure or the addition of chemically active fluids.

Manganese Nodules

Manganese nodules are rounded, hard lumps of manganese plus iron plus other metals typically 5 cms to 20 cms in diameter. When cut in half, they often reveal a layered structure formed by precipitation around a central nucleation object, which might be a piece of lithogenous sediment, coral, volcanic rock, a fish bone, or a shark s tooth.

Manganese nodules are found on the deep-ocean floor. The major components of these nodules are manganese dioxide (around 30% by weight) and iron oxide (around 20%). Other accessory metals present in manganese nodules include copper, nickel and cobalt. Although the concentration of these accessory metals is usually less than 1%, they can exceed 2% by weight, which may make them attractive exploration targets in the future.

Phosphates

Phosphates occur abundantly as coatings on rocks and as nodules on the continental shelf and on banks at shallow depths. Concentrations of phosphates in such deposits indicate abundant biological activity in surface water above where they accumulate. Because phosphates are valuable as fertilizers, ancient marine phosphate deposits that have been uplifted onto land are extensively mined to supply agricultural needs.

Carbonates

The two most important carbonate minerals in marine sediment are aragonite and calcite. Both are composed of calcium carbonate but aragonite has a different crystalline structure that is less stable and changes into calcite over time.  Most carbonate deposits are biogenous in origin. However, hydrogenous carbonate deposits can precipitate directly from seawater in tropical climates to form aragonite crystals

Metal Sulfides

Deposits of metal Sulfides are associated with hydrothermal vents and black smokers along the mid-ocean ridge. These deposits contain iron, nickel, copper, zinc, silver, and other metals in varying proportions. Transported away from the mid-ocean ridge by sea floor spreading, these deposits can be found throughout the ocean floor and can even be uplifted onto continents.