ABSTRACT
Carbon is one of the less abundant elements in the crust (320 ppm) (Weast and Astle, 1982), but it is central to all life on Earth. Organisms contribute to the carbon cycle on Earth’s surface principally by xing inorganic carbon (CO2) to form organic carbon and remineralizing organic carbon back to inorganic carbon (Figure 10.1). The burial of organic matter removes a small portion of the organic carbon from the carbon cycle. Buried organic matter may be gradually transformed into coal, petroleum, bitumen, kerogen, organic matter in shales, natural gas (see Chapter 23) or, rarely,
graphite or diamonds. The carbon in these substances reenters the carbon cycle through human exploitation of fossil fuels or the weathering of organic-rich rocks.
