Carbonate-silicate cycle
Encyclopedia
The carbonate–silicate geochemical cycle is the naturally occurring reversible chemical reaction with summary equation
CaSiO3
+CO2
<=>CaCO3
+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
.
CaSiO3
Calcium silicate
Calcium silicate is the chemical compound Ca2SiO4, also known as calcium orthosilicate and sometimes formulated 2CaO.SiO2. It is one of group of compounds obtained by reacting calcium oxide and silica in various ratios e.g. 3CaO.SiO2, Ca3SiO5; 2CaO.SiO2, Ca2SiO4; 3CaO.2SiO2, Ca3Si2O7 and...
+CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
<=>CaCO3
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
+SiO2
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
.
Equilibrium
Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...
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
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
process. This process is far from being a closed loop
Closed ecological system
Closed ecological systems are ecosystems that do not rely on matter exchange with any part outside the system.The term is most often used to describe small manmade ecosystems...
, 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
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
due to higher temperatures, so Venus now has a high-density carbon-dioxide atmosphere
Atmosphere of Venus
The atmosphere of Venus is much denser and hotter than that of Earth. The temperature at the surface is 740 K , while the pressure is 93 bar. The Venusian atmosphere supports opaque clouds made of sulfuric acid, making optical Earth-based and orbital observation of the surface impossible...
.
The carbonate-silicate cycle is suspected as a reason for the ice ages, because it can create negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
on the global temperature with a typical time scale of a few million years, which effectively counters water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
short-term positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...
.
See also
- Carbon cycleCarbon cycleThe carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...
- DaisyworldDaisyworldDaisyworld, a computer simulation, is a hypothetical world orbiting a star whose radiant energy is slowly increasing. It is meant to mimic important elements of the Earth-Sun system, and was introduced by James Lovelock and Andrew Watson in a paper published in 1983 to illustrate the plausibility...
- Gaia hypothesisGaia hypothesisThe Gaia hypothesis, also known as Gaia theory or Gaia principle, proposes that all organisms and their inorganic surroundings on Earth are closely integrated to form a single and self-regulating complex system, maintaining the conditions for life on the planet.The scientific investigation of the...
- Runaway greenhouse effect
- Major ice ages