Exogenic carbon cycle

Figure 10.12 illustrates the carbon and sulfur isotopic compositions of a variety of materials. For both carbon and sulfur, there is an important fractionation that obtains when organic processes are involved. Organic material is depleted in 3c, and sulfide produced from bacterial reduction of sulfate is depleted in 34s. In the exogenic carbon cycle, there are two principal reservoirs of carbon the oxidized inorganic carbon reservoir, which is mostly carbonate... 

The two dominant exogenic reservoirs of carbon are carbonate rocks and organic matter in sediments. They are linked in the carbon cycle via atmospheric CO2 and the carbon species dissolved in the hydrosphere. The for the total... 

Based on the hrst simplihcation listed above, the exogenic cycles of carbon and sulfur are regarded as closed systems. As such, the bulk isotopic composition of average exogenic carbon and sulfur do not vary through time. We can write a rate equation for the rate of change in (mass X isotopic composition) of each reservoir in Figure 14 to reflect this isotope mass balance. For example ... 

If we assume that the entire carbon cycle is maintained in a steady state condition (Siever, 1968 Garrels and Mackenzie, 1972 Garrels et al., 1975, 1976), it follows that the amounts of carbon present at any time in the biosphere, atmosphere and hydrosphere remain relatively small. Thus this carbon circulates at relatively high rates, and has a short residence time within each of these three pools. Most of the carbon is stored in sediments and is thus removed from the exogenic circulation for millions of years. This carbon returns slowly via the endogenic cycle (Fig. 2.1.5). 

Although as autotrophs they are not dependent on exogenous carbon sources for energy metabolism, they possess a large capacity for the degradation of many of the amino acids to intermediates of the TCA cycle. 

Fig. 2.1.3. Schematic presentation of the exogenic and endogenic cycling of inorganic carbon.

Fig. 2.1.5. The relative cycling frequency of an average carbon atom within the atmosphere, biosphere and hydrosphere (exogenic cycles) relative to the cycling in the litho-spere (endogenic cycles).

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