The global carbon cycle describes the movement of carbon between Earth’s major spheres: the atmosphere, oceans, biosphere, and lithosphere. It is divided into the fast cycle (biological and oceanic exchanges over days to millennia) and the slow carbon cycle. The slow cycle is a geological process that manages the exchange of carbon between the atmosphere, oceans, and the Earth’s crust and mantle. It operates on an immense timescale, requiring between 100 and 200 million years to complete a full circuit. This deep geological cycling of carbon is fundamental to the long-term regulation of Earth’s climate, acting as a natural thermostat that maintains stable conditions for life.
Identifying the Major Reservoirs
Sedimentary rocks, particularly carbonate rocks like limestone and dolomite, represent the single largest store of carbon in the slow cycle, holding approximately 65,500 billion metric tons. This reservoir dwarfs the amount of carbon held in the atmosphere.
The deep ocean contains far more dissolved inorganic carbon than the atmosphere. This deep-water store is separated from the surface by temperature and density layers, allowing carbon to be sequestered. Organic carbon from dead organisms buried in mud and sediment constitutes another significant geological reservoir. Over millions of years, heat and pressure convert these organic-rich sediments into sedimentary rock such as shale, or into fossil fuels like coal, oil, and natural gas.
Geological Uptake: Weathering and Sedimentation
Chemical weathering drives the removal of carbon from the atmosphere and its transfer into the lithosphere. This begins when atmospheric carbon dioxide (\(\text{CO}_2\)) dissolves into rainwater, forming a weak carbonic acid (\(\text{H}_2\text{CO}_3\)). This slightly acidic water then reacts with exposed silicate rocks. This silicate weathering reaction chemically breaks down the rock, consuming atmospheric \(\text{CO}_2\).
The carbon is converted into dissolved ions, predominantly bicarbonate (\(\text{HCO}_3^-\)). These ions are carried away from the land by rivers, eventually being transported into the ocean. The dissolved bicarbonate and calcium ions are utilized by tiny marine organisms, such as plankton and corals, to construct their shells and skeletons of calcium carbonate (\(\text{CaCO}_3\)).
When these organisms die, their remains sink to the ocean floor, accumulating as sediment. The weight of overlying water and successive layers compresses these deposits. This process, known as lithification, transforms the loose sediment into dense, solid carbonate rock, most notably limestone. This completes the uptake phase of the slow cycle.
Geological Release: Volcanic and Metamorphic Activity
This release is intimately linked with the dynamics of plate tectonics. At subduction zones, where one tectonic plate slides beneath another, sedimentary rocks that formed on the ocean floor are carried deep into the Earth’s interior.
As these deeply buried carbonate rocks descend, they are subjected to immense heat and pressure, triggering a process called metamorphism. Under these conditions, the carbonate minerals chemically decompose, releasing carbon dioxide in a process known as metamorphic decarbonation. This liberated \(\text{CO}_2\) then becomes trapped in the mantle and crustal rock of the subducting plate.
This carbon is returned to the surface through volcanic activity. The \(\text{CO}_2\) gas is expelled from the Earth’s interior, primarily through volcanoes located near subduction zones. On average, volcanoes release between 130 and 380 million metric tons of \(\text{CO}_2\) per year. This flux balances the carbon removed by silicate weathering, preventing all carbon from being permanently locked in the lithosphere, and maintaining the planet’s long-term atmospheric \(\text{CO}_2\) concentration and climate stability.