Carbonate–silicate cycle

The carbonate–silicate geochemical cycle, also known as the inorganic carbon cycle, describes the long-term transformation of silicate rocks to carbonate rocks by weathering and sedimentation, and the transformation of carbonate rocks back into silicate rocks by metamorphism and volcanism. Carbon dioxide is removed from the atmosphere during burial of weathered minerals and returned to the atmosphere through volcanism. On million-year time scales, the carbonate-silicate cycle is a key factor in controlling Earth's climate because it regulates carbon dioxide levels and therefore global temperature. The carbonate–silicate geochemical cycle, also known as the inorganic carbon cycle, describes the long-term transformation of silicate rocks to carbonate rocks by weathering and sedimentation, and the transformation of carbonate rocks back into silicate rocks by metamorphism and volcanism. Carbon dioxide is removed from the atmosphere during burial of weathered minerals and returned to the atmosphere through volcanism. On million-year time scales, the carbonate-silicate cycle is a key factor in controlling Earth's climate because it regulates carbon dioxide levels and therefore global temperature. However the rate of weathering is sensitive to factors that modulate how much land is exposed. These factors include sea level, topography, lithology, and vegetation changes. Furthermore, these geomorphic and chemical changes have worked in tandem with solar forcing, whether due to orbital changes or stellar evolution, to determine the global surface temperature. Additionally, the carbonate-silicate cycle has been considered a possible solution to the Faint young Sun paradox. The carbonate-silicate cycle is the primary control on carbon dioxide levels over long timescales. It can be seen a branch of the carbon cycle, which also includes the organic carbon cycle, in which biological processes convert carbon dioxide and water into organic matter and oxygen via photosynthesis. The inorganic cycle begins with the production of carbonic acid (H2CO3) from rainwater and gaseous carbon dioxide. Carbonic acid is a weak acid, but over long timescales, it can dissolve silicate rocks (as well as carbonate rocks). Most of the Earth's crust (and mantle) is composed of silicates. These substances break down into dissolved ions as a result. For example, calcium silicate CaSiO3, or wollastonite, reacts with carbon dioxide and water to yield a calcium ion, Ca2+, a bicarbonate ion, HCO3-, and dissolved silica. This reaction structure is representative of general silicate weathering of calcium silicate minerals. The chemical pathway is as follows: 2 C O 2 + H 2 O + C a S i O 3 → C a 2 + + 2 H C O 3 − + S i O 2 {displaystyle 2CO_{2}+H_{2}O+CaSiO_{3} ightarrow Ca^{2+}+2HCO_{3}^{-}+SiO_{2}} River runoff carries these products to the ocean, where marine calcifying organisms use Ca2+ and HCO3- to build their shells and skeletons, a process called carbonate precipitation: C a 2 + + 2 H C O 3 − → C a C O 3 + C O 2 + H 2 O {displaystyle Ca^{2+}+2HCO_{3}^{-} ightarrow CaCO_{3}+CO_{2}+H_{2}O} Two molecules of CO2 are required for silicate rock weathering; marine calcification releases one molecule back to the atmosphere. The calcium carbonate (CaCO3) contained in shells and skeletons sinks after the marine organism dies and is deposited on the ocean floor. The final stage of the process involves the movement of the seafloor. At subduction zones, the carbonate sediments are buried and forced back into the mantle. Some carbonate may be carried deep into the mantle where high pressure and temperature conditions allow it to combine metamorphically with SiO2 to form CaSiO3 and CO2, which is released from the interior into the atmosphere via volcanism, thermal vents in the ocean, or soda springs, which are natural springs that contain carbon dioxide gas or soda water: