Carbonate–silicate Cycle

Carbonate–silicate Cycle

The carbonate–silicate geochemical cycle is the naturally occurring reversible chemical reaction with summary equation
CaSiO₃ (wollastonite) + CO₂ <=> CaCO₃ (calcite) + SiO₂.

Equilibrium of the carbonate-silicate reaction is generally shifted in the favor of carbonate formation under near surface temperature and pressure conditions, but shifts to silicate formation at temperatures above 300 °C. Therefore, at the Earth's surface silicates are converted to carbonate sediments, but these sediments are converted back to silicates during the subduction process. This process is far from being a closed loop, in Earth history generally the formation of carbonates significantly outpaces formation of silicates, effectively dissipating primordial carbon-dioxide-rich atmosphere. The situation is opposite for Venus due to higher temperatures, so Venus now has a high-density carbon-dioxide atmosphere.

The carbonate-silicate cycle is suspected as a reason for the ice ages, because it can create negative feedback on the global temperature with a typical time scale of a few million years, which effectively counters water vapor and carbon dioxide short-term positive feedback.