The carbonate–silicate geochemical cycle describes the transformation of silicate rocks to carbonate rocks by weathering and sedimentation at Earth's surface and the transformation of carbonate rocks back into silicates by metamorphism and magmatism. It plays a large part in the carbon cycle, since the equilibrium point of the carbonate-silicate cycle dictates the pace of carbon release from the lithosphere. In addition to helping maintain the earth's carbon dioxide levels at average levels, it also promotes geological change.
The carbonate-silicate cycle impacts the global carbon cycle, as carbon dioxide is removed from the Earth's surface through the burial of weathered minerals in deep ocean sediments and returned to the atmosphere through metamorphism and volcanism. However, this process is far from being a closed loop. In Earth history generally, the formation of carbonates significantly outpaces the formation of silicates, effectively removing carbon dioxide from the atmosphere. Because carbon dioxide is a potent greenhouse gas, the carbonate-silicate cycle is suspected to initiate ice ages by creating a negative feedback on the global temperature with a typical timescale of a few million years that is capable of countering water vapor and carbon dioxide short-term positive feedback on global temperature.
An overview of the cycle
The carbonate-silicate cycle throughout time has been a key factor in controlling the Earth's climate, it has served to control carbon dioxide levels and by proxy, temperature. The process begins with atmospheric carbon dioxide dissolving and being absorbed into clouds and converting to carbonic acid, allowing for acidic rainfall. This rainfall as it makes contact with land causes an effect called "weathering"; it causes erosion and the dissolving of minerals including calcium, magnesium, bicarbonate and silicate. The minerals, specifically calcium carbonate, make their way to bodies of water where they settle into sedimentary layers or get incorporated into the ecosystem via aquatic organisms. The leftover carbonate gets pulled into subduction zones past the ocean floor where they recombine with the silicate particles. These particles are then vented along with other silicates to release gaseous carbon dioxide which is vented through volcanoes.
On a chemical level, the carbonate-silicate cycle involves many chemical reactions that occur in different environments. In the atmosphere, gaseous carbon dioxide (CO2) dissolves in rainwater, forming natural carbonic acid (H2CO3). This weak acid weathers silicate rocks on continents, slowly dissolving the rock and releasing aqueous minerals through the chemical reaction CaSiO3(s) (wollastonite) + 2CO2(g) + H2O(l) <=> Ca2+
(aq) + 2HCO−
3 (aq) (bicarbonate) + SiO2(aq) (dissolved silica). These dissolved minerals are eventually carried by water to the ocean, where they are used by living organisms such as foraminifera, radiolarians, coccolithopores, and diatoms to create shells of CaCO3 (calcite) or SiO2 (opal) through the reactions Ca2+ (aq) + 2HCO3−
(aq) → CaCO3(s) + CO2(g) + H2O(l) (for calcite precipitation) and SiO2(aq) → SiO2(s) (for opal precipitation). When these organisms die, many shells are remineralized but some shells fall all the way to the sea floor and are buried. The cycle is completed when the sea floor is subducted and carbonate minerals recombine with silicate minerals at temperatures above 300 °C to reform calcium silicates and release gaseous CO2 through volcanism (CaCO3(s) + SiO2(s) <=> CaSiO3(s) + CO2(g)).
Carbonate-silicate cycle and other planets
On Mars, the analysis of the carbonate-silicate cycle is used to gain insight as to how dense the carbon dioxide filled atmosphere is and if it is conducive to allowing water on Mars. Evidence from carbonate deposits in the soil of the planet shows that water, albeit highly acidic, precipitated on the planet. The carbonate-silicate cycle equilibrium point is shifted on Venus due to surface temperatures above 300°C, which favor the formation of calcium silicates over weathering. Thus, Venus has a high-density carbon-dioxide atmosphere.
- Carbon cycle
- Gaia hypothesis
- Runaway greenhouse effect
- Major ice ages
- Snowball earth
- Lake Nyos
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