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Silica cycle

Silica cycle is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Silica cycle rather than just read about it. In short: The silica cycle is the biogeochemical cycle in which biogenic silica is transported between the Earth's systems. Silicon is one of the most abundant elements on Earth, and is considered necessary for life.

Silica cycle — main illustration
Silica cycle — illustration

Key takeaways

  • Silica cycle belongs to chemistry; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Silica cycle to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Silica cycle from memory before moving on to harder problems.

Reference excerpt

The silica cycle is the biogeochemical cycle in which biogenic silica is transported between the Earth's systems. Silicon is one of the most abundant elements on Earth, and is considered necessary for life. 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.

Overview

Silicon is the eighth 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. As a result, diatoms, and other silica-secreting organisms play crucial roles in the global carbon cycle by sequestering carbon in the ocean. The connection between biogenic silica and organic carbon, together with the significantly higher preservation potential of biogenic siliceous compounds compared to organic carbon makes opal accumulation records of interest in paleoceanography and paleoclimatology. Understanding the silica cycle is important for understanding the functioning of marine food webs, biogeochemical cycles, and the biological pump. Silicic acid is delivered to the ocean through six pathways as illustrated in the diagram above, which all ultimately derive from the weathering of the Earth's crust.

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.

Weathering 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. The process and rate of weathering is variable, depending on rainfall, runoff, vegetation, lithology, and topography. Given sufficient time, rainwater can dissolve even a highly resistant silicate-based mineral such as quartz. Water breaks the bonds between atoms in the crystal:

The overall reaction for the dissolution of quartz results in silicic acid

SiO2 + 2H2O → H4SiO4 Another example of a silicate-based mineral is enstatite (MgSiO3). Rainwater weathers this to silicic acid as follows:

MgSiO 3 ( s ) + 2 CO 2 ( g ) + H 2 O ( l ) = Mg 2 + ( aq ) + 2 HCO 3 − ( aq ) + SiO 2 ( aq ) {\displaystyle {\ce {MgSiO3(s) + 2CO2(g) + H2O(l) = Mg2+(aq) + 2HCO3- (aq) + SiO2(aq)}}}

Reverse weathering

… excerpt ends here. Continue reading the full article.

Illustrations

Silica cycle: Silicon cycle and balance in the modern world ocean [1] Input, output, and biological silicon fluxes, with possible balance. Total silicon inputs = total silicon outputs = 15.6 Tmol Si yr−1 in reasonable agreement with the individual range of each flux. White arrows represent fluxes of net sources of dissolved silicic acid and/or of dissolvable amorphous silica and of dissolved silicic acid recycled fluxes. Orange arrows represent sink fluxes of silicon, either as biogenic silica or as authigenic silica. Green arrows correspond to biological (pelagic) fluxes. Values of flux as published by Tréguer & De La Rocha.[1] Fluxes in teramoles of silicon per year (Tmol Si yr−1).
Silicon cycle and balance in the modern world ocean [1] Input, output, and biological silicon fluxes, with possible balance. Total silicon inputs = total silicon outputs = 15.6 Tmol Si yr−1 in reasonable agreement with the individual range of each flux. White arrows represent fluxes of net sources of dissolved silicic acid and/or of dissolvable amorphous silica and of dissolved silicic acid recycled fluxes. Orange arrows represent sink fluxes of silicon, either as biogenic silica or as authigenic silica. Green arrows correspond to biological (pelagic) fluxes. Values of flux as published by Tréguer & De La Rocha.[1] Fluxes in teramoles of silicon per year (Tmol Si yr−1).
Silica cycle illustration
Silica cycle illustration
Silica cycle illustration
Silica cycle: Marine[28] and terrestrial[3][29][30][31][18] contributions to the silica cycle are shown, with the relative movement (flux) provided in units of Tmol Si/yr.[20] Marine biological production primarily comes from diatoms.[32] Estuary biological production is due to sponges.[33] Values of flux as published by Tréguer & De La Rocha.[20] Reservoir size of silicate rocks, as discussed in the sources section, is 1.5 × 1021 Tmol.[34]
Marine[28] and terrestrial[3][29][30][31][18] contributions to the silica cycle are shown, with the relative movement (flux) provided in units of Tmol Si/yr.[20] Marine biological production primarily comes from diatoms.[32] Estuary biological production is due to sponges.[33] Values of flux as published by Tréguer & De La Rocha.[20] Reservoir size of silicate rocks, as discussed in the sources section, is 1.5 × 1021 Tmol.[34]

Worked examples

Example 1 — a first encounter with Silica cycle

Start with the simplest possible case. Write down what Silica cycle claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Silica cycle before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Silica cycle ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Silica cycle

In research
Silica cycle appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Silica cycle in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Silica cycle is common in secondary-school and first-year university syllabi. It links to neighbouring topics Biogeochemical cycle, Silicon, so understanding it makes those chapters shorter.
In everyday life
Look for Silica cycle outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Silica cycle in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Silica cycle means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Silica cycle out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Silica cycle in simple terms?

The silica cycle is the biogeochemical cycle in which biogenic silica is transported between the Earth's systems. Silicon is one of the most abundant elements on Earth, and is considered necessary for life.

Why does Silica cycle matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Silica cycle?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Silica cycle.

Tags

  • Biogeochemical cycle
  • Silicon

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