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.
The carbonate-silicate cycle involves several 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 under temperatures above 300 °C to reform calcium silicates and release gaseous CO2 through volcanism (CaCO3(s) + SiO2(s) → CaSiO3(s) + CO2(g)).
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 time scale of a few million years that is capable of countering water vapor and carbon dioxide short-term positive feedback on global temperature.
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.