Simplified carbon cycle

An important example of non-linearity in a biogeochemical cycle is the exchange of carbon dioxide between the ocean surface water and the atmosphere and between the atmosphere and the terrestrial system. To illustrate some effects of these non-linearities, let us consider the simplified model of the carbon cycle shown in Fig. 4-12. Ms represents the sum of all forms of dissolved carbon (CO2, H2CO3, HCOi" and... 

Fig. 4-12 Simplified model of the biogeochemical carbon cycle. (Adapted from Rodhe and Bjdrkstrdm (1979) with the permission of the Swedish Geophysical Society.)...

To begin preparing for the Unit 4 Issue, make a simplified sketch of the carbon cycle,... 

The diagram shown below is a simplified version of the carbon cycle. 

Fig. 12.8. Simplified sketch showing main relationships inside the coupled calcium and carbon cycles of the oxalate-carbonate pathway in a hypothetical ecosystem. Plants and fungi are oxalate producers. Oxalotrophic bacteria (in the soil or animal guts) use oxalate as carbon, energy and electron sources, leading to CO2 and calcium carbonate production. Calcium carbonate can accumulate inside the soils. Because the carbon of the carbonate originates from organic carbon, its fossilization in the soil constitutes a carbon sink.

Methanol and ethanol have been considered as promising fuels for generating H2, especially for on-board fuel cell applications due to their easy availability, ability to transport, and reaction simplicity.52 121 159 169 For example, both alcohols have high H2-to-carbon ratio (H/C) of 4 and 3, respectively (Table 2.1). They could be synthesized from renewable sources such as biomass and thus the ability to close the carbon cycle.161 166 Unlike hydrocarbon fuels, methanol and ethanol are free from sulfur, and this avoids additional sulfur removal step in the fuel processing. In addition, methanol can be reformed at a lower temperature, around 300 °C, and this makes the fuel processing relatively simple and less complicated. Furthermore, unlike natural gas, which produces primarily syngas, reforming of methanol and ethanol can in principle produce a mixture of H2 and C02, and this would also simplify the downstream CO cleanup for fuel cells such as PEMFCs where CO is a poison. 

A simplified version of the carbon cycle is given in Fig. 7.9. By far the largest reservoir is in marine sediments and sedimentary materials on land (20000000 GtC), mainly in the form of CaC03. However, most of this material is not in contact with the atmosphere and cycles through the solid Earth on geological timescales (see Section 4.1). It therefore plays only a minor role in the short-term cycle of carbon considered here. The next largest reservoir is seawater (about 39 000 GtC), where the carbon is mainly in the dissolved form as HC03 and C03 . However, the deeper parts of the oceans, which contain most of the carbon (38 100 GtC), do not interact with the atmosphere at all rapidly, as discussed in... 

Fig. 7.9 A simplified version of the global carbon cycle for the 1980s. The numbers in boxes indicate the reservoir size in GtC. Arrows represent fluxes and the associated numbers indicate the magnitude of the flux in GtC r. After IPCC (2001). With permission of the Intergovernmental Panel on Climate Change.

Figure 2. Simplified illusiration of the interactions between major players of the carbon cycle (Engel and Macko, 1993).

Fig. 6.1 Simplified summary of the preindustrial global carbon cycle, showing approximate sizes of the main reservoirs (variously shaped boxes) and annual fluxes (arrows) in Gt (1015g) of carbon (after several sources, including Bolin et al. 1979, 1983 Kempe 1979 Mopper Degens 1979 DeVooys 1979 Siegenthaler Sarmiento 1993 Sundquist 1993 Arthur 2000 Falkowski et al. 2000). Reactive sediments are capable of exchanging material with the water column.

Fig. 6.7 Much simplified carbon cycle showing exchanges between the surface, crust and mantle compartments.Approximate relative sizes of reservoirs within each compartment are shown together with average 513C values (after Schidlowski 1988 Hoefs 1997).

The first strand of evidence for the global ocean uptake comes from models of the ocean carbon cycle. These generally combine descriptions of the relevant carbonate chemistry with some representation of tracer transport in the ocean. As well they need a model (usually highly simplified) of the biological processes in the ocean and finally some parameterization of the surface fluxes of... 

Organic residues added to the soil contain about 50% carbon which is eventually converted to carbon dioxide. Soil microorganisms are responsible for the evolution of about 95% of the gas which is then fixed mainly by green plants. A simplified representation of the carbon cycle is given in Fig. 4.8. 

A simplified diagram representing the various reservoirs and transport mechanisms and pathways involved in the cycles of nutrient elements at and above the surface of the Earth is given in Eigure 1. The processes are those considered to be the most important in the context of this article, but others of lesser significance can be postulated. Eor some of the elements, notably carbon, sulfur, chlorine, and nitrogen, considerable research has been done to evaluate (quantitatively) the amount of the various elements in the reservoirs and the rates of transfer. 

Wachtcrshauser s prime candidate for a carbon-fixing process driven by pyrite formation is the reductive citrate cycle (RCC) mentioned above. Expressed simply, the RCC is the reversal of the normal Krebs cycle (tricarboxylic acid cycle TCA cycle), which is referred to as the turntable of metabolism because of its vital importance for metabolism in living cells. The Krebs cycle, in simplified form, can be summarized as follows ... 

Acetogenins. Acetogenins are produced upon chain elongation with activated acetate units (or malonate followed by loss of carbon dioxide). A simplified sketch of this sequence is given in Fig. 1. During the first steps, a Claisen-type condensation of two acyl precursors yields a (3-ketoacyl intermediate A. Upon reduction to B and dehydration to C, followed by hydrogenation to D and hydrolysis, the chain elongated fatty acid E is produced. The next cycle will add another two carbons to the chain. Similarly, a reversed sequence leads to chain... 

Figure 13.2 Transfer of carbon in aerobic estuarine environments. The boxes are reservoirs and the arrows are fluxes the inorganic C component of this cycle is simplified in this figure. (Modified from Valiela, 1995.)...

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