Carbon is the key element of life on Earth and exists in more than a million compounds (Holmén, 2000; Berner, 2004). The unique covalent long-chained and aromatic carbon compounds form the basis of organic chemistry and the “roadmap” for understanding life from the cellular to the ecosystem level. The oxidation states of C atoms range from +IV to −IV; methane (CH4) is the most reduced form of C (−IV), with CO2 and other carbonate forms existing in the most oxidized state (+IV). The major reservoirs of C are stored in the Earth’s crust, with much of it as inorganic carbonate and the remaining as organic C (e.g., kerogen) (figure 13.1; Sundquist, 1993). The global C cycle can be divided into short- and long-term cycles based on the vast differences in the turnover times of different C pools (Berner, 2004). The carbonate reservoir can be divided into two primary subreservoirs: (1) dissolved inorganic carbon (DIC) in the ocean (H2CO3, HCO3−, and CO32−), and (2) solid carbonate minerals [CaCO3, CaMg(CO3)2, and FeCO3] (Holmén, 2000). While the global C cycle is quite complex, it is perhaps the best understood of all the bioactive element cycles. In fact, there have been numerous review papers on this cycle (e.g., Keeling, 1973; Degens et al., 1984; Siegenthaler and Sarmiento, 1993; Sundquist, 1993; Schimel et al., 1995; Holmén, 2000). Much of the interest in the global C cycle in recent years stems from linkages with environmental issues concerning carbon-based greenhouse gases (e.g., CO2 and CH4) and their role in global climate change (Dickinson and Cicerone, 1986). As described in chapter 8, short-term controls on the C cycle are largely a function of the uptake of inorganic C by autotrophs to fuel fixation in photosynthesis, and the utilization of organic carbon as a food resource by heterotrophs recycling inorganic C back into the system. This short-term cycle, which allows for the transfer of C between the lithosphere, hydrosphere, biosphere, and atmosphere over periods of days to thousands of years, is relatively short in comparison to the more than 4 billion year age of the Earth.
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