Silica cycle

The silica cycle is the biogeochemical cycle in which biogenic silica is transported between the Earth's systems. Silicon is considered a bioessential element and is one of the most abundant elements on Earth. The silica cycle has significant overlap with the carbon cycle (see carbonate–silicate cycle) and plays an important role in the sequestration of carbon through continental weathering, biogenic export and burial as oozes on geologic timescales. Silicon is the seventh most abundant element in the universe and the second most abundant element in the Earth's crust (the most abundant is oxygen). The weathering of the Earth's crust by rainwater rich in carbon dioxide is a key process in the control of atmospheric carbon dioxide. It results in the generation of silicic acid in aqueous environments. Silicic acid, Si(OH)4, is a hydrated form of silica found only as an unstable solution in water, yet it plays a central role in the silica cycle. Silicifiers are organisms that use silicic acid to precipitate biogenic silica, SiO2. Biogenic silica, also referred to as opal, is precipitated by silicifiers as internal structures and/or external structures. Silicifiers are among the most important aquatic organisms. They include micro-organisms such as diatoms, rhizarians, silicoflagellates and several species of choanoflagellates, as well as macro-organisms such as siliceous sponges. Phototrophic silicifiers, such as diatoms, globally consume vast amounts of silicon along with nitrogen (N), phosphorus (P), and inorganic carbon (C), connecting the biogeochemistry of these elements and contributing to the sequestration of atmospheric carbon dioxide in the ocean. Heterotrophic organisms like rhizarians, choanoflagellates, and sponges produce biogenic silica independently of the photoautotrophic processing of C and N. The diatoms dominate the fixation and export of particulate matter in the contemporary marine silica cycle. This includes the export of organic carbon from the euphotic zone to the deep ocean via the biological carbon pump.

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Silica cycle

Biogenic silica

Biogenic silica (bSi), also referred to as opal, biogenic opal, or amorphous opaline silica, forms one of the most widespread biogenic minerals. For example, microscopic particles of silica called phytoliths can be found in grasses and other plants. Silica is an amorphous metal oxide formed by complex inorganic polymerization processes. This is opposed to the other major biogenic minerals, comprising carbonate and phosphate, which occur in nature as crystalline iono-covalent solids (e.g.

Marine sediment

Marine sediment, or ocean sediment, or seafloor sediment, are deposits of insoluble particles that have accumulated on the seafloor. These particles 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. Additional deposits come from marine organisms and chemical precipitation in seawater, as well as from underwater volcanoes and meteorite debris.

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