Temperature dependence, carbon cycle

The conversions conducted in both steps are currently based on empirical relationships that are more or less robust. For example, the relationship between the chlorophyll and carbon content in an average phytoplankton cell is dependent on factors that influence cell metabolism, including nutrient arailability, temperature, and light. The temperature dependence of photosynthesis is associated with an enzyme-mediated step in the Calvin cycle (Figure 7.6a). 

Thermodynamic data on hydrocarbons show that at temperatures above 1300°C. ethylene, acetylene, and carbon are more stable than methane or other saturated hydrocarbons however, although data of this kind will indicate the species likely to be formed at high temperatures, the amounts of unsaturated hydrocarbons and carbon produced will depend on the rates of the reactions involved and on the temperature cycle used. 

Fig. 11.2 Temperature dependence of the gas pressure in a preliminarily evacuated volume (left vertical scale) and its recalculation into the amount of hydrogen evolved from the sample (right scale) upon heating at a rate of 20 K/min for single-walled carbon nanotubes (SWNTs) and graphite nanofibers (GNFs, two heating cycles) saturated with hydrogen at a pressure of 9 GPa and temperatures up to 450°C...

Once CO2 dissolves in water, an equilibrium is established among dissolved C02 (C02 H20), bicarbonate ions (HCOJ), and carbonate ions (C03 ) (see Chapter 7). Thus, while the sea-to-air fluxes F2i and F31 can be linearly related to dissolved C02 (C02 H20), they are not linearly related to the total dissolved carbon (M2 and M3), which is the sum of C02 H20, HCO, and C03. This relationship depends on seawater temperature and pH, the latter of which depends in a complex way on salinity and concentrations of dissolved salts. Seawater pH varies between 7.5 and 8.4, and therefore most of the C02 that dissolves in the ocean is not in the C02 H20 form (Chapter 7). For the purpose of a global carbon cycle model, one wishes to avoid the complication of an explicit ocean chemistry model to relate F21 and F3J to M2 and M3, respectively. The following empirical relationship has been developed (Ver et al., 1999)... 

In an attempt to reconcile the two approaches, Frangois and Walker proposed in 1992 the addition of a new CO2 consumption flux to the carbon cycle, identified as the precipitation of abiotic carbonates within the oceanic crust, subsequent to its alteration at low temperature. This flux is directly dependent on deep water temperature, which has decreased by 8°C over the Cenozoic. An increase in the continental weathering rate might be compatible with a constant degassing rate, since the sink of carbon through low-temperature alteration of the oceanic crust is decreasing. The balance between input and output is thus still in place. However, this additional sink of carbon is poorly constrained. The present-day consumption of carbon is estimated to be about 1.4 X 10 mol y, but the kinetics of the process is essentially unknown. This attractive hypothesis still needs experimental verification. 

Graphitization. During graphitization, the parts are heated up to 3000°C (see Ch. 4, Sec. 3). The temperature cycle is shorter than the carbonization cycle and varies depending on the size of the parts, lasting from as short as a few hours to as long as three weeks. It is usually performed in a resistance furnace (the original Acheson cycle) or in a medium-frequency induction furnace. 

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