Carbonate-silicate cycle
The carbonate-silicate cycle , alternatively also called the inorganic carbon cycle , is used in chemistry to describe the geochemical , cyclic conversion of free carbon dioxide and silicates to carbonates and silicon dioxide (and vice versa) under the influence of carbonic acid or silica . The carbonate-silicate cycle regulates the amount of carbon dioxide in the atmosphere over longer periods of time . This is because carbon dioxide increases the overall temperature of the earth via the greenhouse effect , but this increased temperature accelerates the weathering of silicate-rich rocks, which again reduce the carbon dioxide content through the precipitation of limestone ( negative feedback ). As a result of the cycle, the entire carbon dioxide content in the atmosphere is exchanged once every 500,000 years and any deviations from the equilibrium that may have occurred in the meantime are compensated for.
This cycle is just as important for petrology as it is for geoecology .
The cycle
The cycle begins with atmospheric carbon dioxide and rainwater, which together form carbonic acid:
The carbonated rainwater erodes silicate rocks by dissolving calcium silicate minerals (compounds of calcium , silicon and oxygen ) from it, whereby the released calcium and hydrogen carbonate ions get into the groundwater. The equation for the conversion of the feldspar anorthite by carbonic acid to form kaolinite serves as an example :
![{\ mathrm {2 \; H_ {2} CO_ {3} \ + Ca [Al_ {2} Si_ {2} O_ {8}] \ + H_ {2} O \ \ rightarrow \ Al_ {2} [(OH ) _ {4} | Si_ {2} O_ {5}] \ + Ca ^ {{2 +}} \ +2 \; HCO_ {3} ^ {-} \}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/91150c8153101a889b3d50d37e181cbbf7ae0d2e)
The ions are transported into the sea via flowing waters. In the sea, various living beings, planktonic and sessile animals , use calcium, hydrogen carbonate and carbonate ions to build inner and outer skeletons from calcium carbonate (CaCO 3 ); alternatively calcium carbonate can also be precipitated inorganically, but this only takes place on earth under special conditions. (Silicon dioxide dissolved in water is also built into skeletons by other organisms such as diatoms and radiolarians and sedimented with them.) After they die, most of them are dissolved again by sinking below the compensation depth , but some of them remain and form carbonate sediments on the sea floor . Over the millennia, this sea floor is transported from the mid-ocean ridges to the subduction zones at the continental margins as a result of plate tectonics . There it migrates into the interior of the earth together with the sinking oceanic plate. At high pressure and high temperatures, the calcium carbonate reacts with the silicon dioxide (carbonate metamorphism) and silicate minerals are formed again with the splitting off of carbon dioxide. The equation for the conversion of calcite (calcium carbonate) with silicon dioxide to wollastonite and carbon dioxide serves as an example :
![{\ mathrm {3 \; CaCO_ {3} \ +3 \; SiO_ {2} \ \ rightarrow \ Ca_ {3} [Si_ {3} O_ {9}] \ +3 \; CO_ {2} \}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/81d956c1f0c8338e28c0d28effb9ad5a9be83b22)
(The simple calcium silicate wollastonite was only used here for explanation, in fact any other silicates with a complicated composition can be formed in the same way, but this would complicate the sum equations.) The carbon dioxide finally gets through volcanism , either via the submarine volcanism of the mid-ocean ridges or via continental volcanism, back into the atmosphere as carbon dioxide gas, closing the cycle.
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- James F. Kasting, David Catling: Evolution of a Habitable Planet . In: Annual Review of Astronomy and Astrophysics . tape 41 , no. 1 , September 2003, p. 429-463 , doi : 10.1146 / annurev.astro.41.071601.170049 ( PDF ).
- Robert A. Berner , Antonio C. Lasaga, Robert M. Garrels : The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years . In: American Journal of Science . tape 283 , no. 7 , September 1983, p. 641–683 , doi : 10.2475 / ajs.283.7.641 ( PDF ). (so-called BLAG model)