Inorganic carbon, atmospheric

The strong increase in atmospheric concentrations of carbon dioxide [ 127] has generated considerable interest in the global carbon cycle [ 128-130]. Techniques for determining the components of the carbonate system have been refined, new techniques have been developed, or both. Among the four measurable parameters (total inorganic carbon), pH, pC02, and total alkalinity... 

Isotope equilibrium exchange reactions within the inorganic carbon system atmospheric CO2 - dissolved bicarbonate - solid carbonate lead to an emich-ment of 13C in carbonates. 

There may, however, be some cancellation of errors. For example, the concentration of atmospheric C02 ([ref], in Eq. (T)) depends in a nonlinear fashion on the amount of total dissolved inorganic carbon in the ocean surface layer because of the equilibria with water (see Chapter 8.B) so that relatively less atmospheric C02 can be taken up by the oceans as its atmospheric concentrations increase. This would leave relatively more C02 in the atmosphere, increasing its greenhouse effect. On the other hand, since the strongest infrared absorption bands of C02 are already saturated (vide supra), the radiative forcing (at-(), in Eq. (T)) decreases as its concentrations increase. 

Due to bioproductivity processes, carbon is transported from the surface to deep layers of the ocean, where it is re-mineralized. This process maintains the inorganic carbon concentration gradient and preservation of C02 concentration in the atmosphere at a level which is (100-200) 106 lower than it would be without bioproductivity. 

From Figure 9.1, it can be seen that the major form of carbon in the atmosphere is C02(g), constituting over 99% of atmospheric carbon. Carbon dioxide makes up 0.035% by volume of atmospheric gases, or 350 ixatm = 350 ppmv. The atmosphere has a mass of CO2 that is only 2% of the mass of total inorganic carbon in the ocean, and both of these carbon masses are small compared to the mass of carbon tied up in sediments and sedimentary rocks. Therefore, small changes in carbon masses in the ocean and sediment reservoirs can substantially alter the CO2 concentration of the atmosphere. Furthermore, there is presently 3 to 4 times more carbon stored on land in living plants and humus than resides in the atmosphere. A decrease in the size of the terrestrial organic carbon reservoir of only 0.1% y-1 would be equivalent to an increase in the annual respiration and decay carbon flux to the atmosphere of nearly 4%. If this carbon were stored in the atmosphere, atmospheric CO2 would increase by 0.4%, or about 1 ppmv y-l. The... 

Raymond, P.A, Bauer, J.E., and Cole, J.J. (2000) Atmospheric CO2 evasion, dissolved inorganic carbon production, and net heterotrophy in the York River estuary. Limnol. Oceanogr. 45, 1707-1717. 

Alkalinity is an important parameter in assessing the elfects of environmental change on aqueous systems (see Section 3.3.4.1). It is also important to understand that, by definition, alkalinity (Equation (3.61)) is independent of addition or removal of CO2 (or H2CO3) from the system (c/ Equation (3.62) - H2CO3 does not appear in the charge balance expression). This can be very useful in the determination of the concentration of dissolved inorganic carbon species in aqueous systems that are in equilibrium with an atmosphere containing C02(g) (Example 3.7). 

The initial two moles of organic carbon are both transformed into inorganic carbon. One mole is precipitated as a mineral, with carbon being trapped inside the calcium carbonate crystals, i.e. stored on a long-term time scale. The other mole can be released into the atmosphere and reused for phototrophy. Therefore, the oxalate-carbonate pathway constitutes a true carbon sink because one out of two moles of organic carbon is stored in a mineral state with a long residence time, whereas the other one returns to the atmospheric reservoir. 

The natural rates of carbon cycling in oceans and land are nearly in a steady state that is, the rates of movement of carbon between the atmosphere and trees, or between algae and the inorganic carbon dissolved in the ocean do not change measurably from year to year and tend to balance each other. However, human activities have recently introduced changes that will be addressed in Chapter 8. 

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