The Geological Carbon Cycle

The origin atmosphere of the Earth was rich in reduced gases including methane, CH4. The carbon content of the Earth steadily increased over eons as a result of collisions with carbon-rich meteors. As the oxygen content of the atmosphere increase, the carbon-containing molecules were oxidized to CO2.

Carbon dioxide, an acidic oxide, and carbonic acid have slowly but continuously combined with calcium and magnesium oxides, basic oxides, in the crust to form insoluble carbonates.

Carbon dioxide in water, or carbonic acid, also reacts with silicate rock. The chemical weather of calcium silicates by carbonic acid produces calcium carbonate and silicon dioxide.

Through the process of erosion, these carbonates are washed into the ocean and eventually settle to the bottom. These materials are drawn into the mantle by subduction (a process in which one lithospheric plate descends beneath another, often as a result of folding or faulting) at the edges of continental plates. The heat and pressure within the Earth causes the metal carbonates to react with silica to form metal silicates, such as Ca2SiO4, and CO2. The carbon is then returned to the atmosphere as carbon dioxide during volcanic eruptions.

The balance between weathering, subduction, and volcanism controls atmospheric carbon dioxide concentrations over time periods of hundreds of millions of years but the concentration of CO2 has changed dramatically. The oldest geologic sediments suggest that, before life evolved, the concentration of atmospheric carbon dioxide may have been one-hundred times that of the present, providing a substantial greenhouse effect during a time of low solar output. Ice core samples taken in Antarctica and Greenland have shown that carbon dioxide concentrations during the last ice age were only half of what they are today.

Over the lifetime of the earth, roughly 75 % of the carbon injected into the atmosphere by volcanoes has found its way into deposits of calcium carbonate (limestone). Limestone tends to accumulate on the beds of shallow seas where the acidity of sea water is reduced. The acidity is higher on the deep ocean floor and the shells and skeletons of marine organisms dissolve as fast as they precipitate.

Weathering of limestone deposits by rain tends to return carbon atoms to the short term reservoirs and to atmospheric carbon dioxide. Weathering of silicate rocks by carbonic acid is faster in a warmer climate because rainfall amounts tend to be greater. By providing calcium ions, weathering promotes limestone formation and removal of carbon dioxide from the atmosphere. An increase in average temperature would, eventually, favor decreasing atmospheric carbon dioxide concentrations and reduce global temperatures, however this geochemical process is too slow to have an effect on human-produced global warming.

Professor Patricia Shapley, University of Illinois, 2010