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The silica cycle is the biogeochemical cycle in which silica is transported between the Earth's systems. Opal silica (SiO2) is a chemical compound of silicon, and is also called silicon dioxide. 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.
Terrestrial Silica Cycling
Silica is an important nutrient utilized by plants, trees, and grasses in the terrestrial biosphere. Silicate is transported by rivers and can be deposited in soils in the form of various siliceous polymorphs. Plants can readily uptake silicate in the form of H4SiO4 for the formation of phytoliths. Phytoliths are tiny rigid structures found within plant cells that aid in the structural integrity of the plant. Phytoliths also serve to protect the plants from consumption by herbivores who are unable to consume and digest silica-rich plants efficiently. Silica release from phytolith degradation or dissolution is estimated to occur at a rate double that of global silicate mineral weathering. Considering biogeochemical cycling within ecosystems, the import and export of silica to and from terrestrial ecosystems is small.
Silicate minerals are abundant in rock formations all over the planet, comprising approximately 90% of the Earth's crust. The primary source of silicate to the terrestrial biosphere is weathering. An example of the chemical reaction for this weathering is:
Wollastonite (CaSiO3) and enstatite (MgSiO3) are examples of silicate-based minerals. The weathering process is important for carbon sequestration on geologic timescales. The process of and rate of weathering is variable dependent upon rainfall, runoff, vegetation, lithology, and topography.
The major sink of the terrestrial silica cycle is export to the ocean by rivers. Silica that is stored in plant matter or dissolved can be exported to the ocean by rivers. The rate of this transport is approximately 6 Tmol Si yr−1. This is the major sink of the terrestrial silica cycle, as well as the largest source of the marine silica cycle. A minor sink for terrestrial silica is silicate that is deposited in terrestrial sediments and eventually exported to the Earth's crust.
Marine Silica Cycling
Siliceous organisms in the ocean, such as diatoms and radiolaria, are the primary sink of dissolved silicic acid into opal silica. Once in the ocean, dissolved Si molecules are biologically recycled roughly 25 times before export and permanent deposition in marine sediments on the seafloor. This rapid recycling is dependent on the dissolution of silica in organic matter in the water column, followed by biological uptake in the photic zone. The estimated residence time of the silica biological reservoir is about 400 years. Opal silica is predominately undersaturated in the world's oceans. This undersaturation promotes rapid dissolution as a result of constant recycling and long residence times. The estimated turnover time of Si is 1.5x104 years. The total net inputs and outputs of silica in the ocean are 9.4 ± 4.7 Tmol si yr−1 and 9.9 ± 7.3 Tmol Si yr−1, respectively.
Biogenic silica production in the photic zone is estimated to be 240 ± 40 Tmol si year −1. Dissolution in the surface removes roughly 135 Tmol Si year−1, while the remaining Si is exported to the deep ocean within sinking particles. In the deep ocean, another 26.2 Tmol Si Year−1 is dissolved before being deposited to the sediments as opal rain. Over 90% of the silica here is dissolved, recycled and eventually upwelled for use again in the euphotic zone.
The major sources of marine silica include rivers, groundwater flux, seafloor weathering inputs, hydrothermal vents, and atmospheric deposition (aeolian flux). Rivers are by far the largest source of silica to the marine environment, accounting for up to 90% of all the silica delivered to the ocean. A source of silica to the marine biological silica cycle is silica that has been recycled by upwelling from the deep ocean and seafloor.
Deep seafloor deposition is the largest long-term sink of the marine silica cycle (6.3 ± 3.6 Tmol Si year−1), and is roughly balanced by the sources of silica to the ocean. The silica deposited in the deep ocean is primarily in the form of siliceous ooze, which is eventually subducted under the crust and metamorphosed in the upper mantle. Under the mantle, silicate minerals are formed in oozes and eventually uplifted to the surface. At the surface, silica can enter the cycle again through weathering. This process can take tens of millions of years. The only other major sink of silica in the ocean is burial along continental margins (3.6 ± 3.7 Tmol si year −1), primarily in the form of siliceous sponges. Due to the high degrees of uncertainty in source and sink estimations, it's difficult to conclude if the marine silica cycle is in equilibrium. The residence time of silica in the oceans is estimated to be about 10,000 years. Silica can also be removed from the cycle by becoming chert and being permanently buried.
The rise in agriculture of the past 400 years has increased the exposure rocks and soils, which has resulted in increased rates of silicate weathering. In turn, the leaching of amorphous silica stocks from soils has also increased, delivering higher concentrations of dissolved silica in rivers. Conversely, increased damming has lead to a reduction in silica supply to the ocean due to uptake by freshwater diatoms behind dams. The dominance of non-siliceous phytoplankton due to anthropogenic nitrogen and phosphorus loading and enhanced silica dissolution in warmer waters has the potential to limit silicon ocean sediment export in the future.
Role in Climate Regulation
The silica cycle plays an important role in long term global climate regulation. The global silica cycle also has large effects on the global carbon cycle through the Carbonate-Silicate Cycle. The process of silicate mineral weathering transfers atmospheric CO2 to the hydrologic cycle through the chemical reaction displayed above. Over geologic timescales, the rates of weathering change due to tectonic activity. During a time of high uplift rate, silicate weathering increases which results in high CO2 uptake rates, offsetting increased volcanic CO2 emissions associated with the geologic activity. This balance of weathering and volcanoes is part of what controls the greenhouse effect and ocean pH over geologic time scales.
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