Carbon is an abundant element on Earth, particularly in its crust, surface waters, atmosphere, and biota. Carbon occurs in many different chemical combinations, including calcium carbonate (CaCO ₃), carbon dioxide (CO₂), methane (CH ₄), and a huge diversity of organic compounds (including hydrocarbons and biochemicals). The storage of carbon in the various compartments of Earth and its biosphere, and the many transfers occurring among them, is referred to as the carbon cycle.

The most abundant mineral forms of carbon in crustal rocks and soil are limestone (CaCO₃) and dolomite (Ca,MgCO ₃). These mostly occur in sedimentary rocks, which were formed in ancient marine environments through biological influences that resulted in the precipitation of limestone and dolomite from ions of calcium (Ca²⁺), magnesium (Mg²⁺), and bicarbonate (HCO ₃^-) dissolved in water. The amount of carbon stored in sedimentary rocks has not yet been accurately estimated, but is thought to...